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Transcript of Satellite Bus Platform Subsystems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 1Rev -, July 2001
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Section 2.4
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 2Rev -, July 2001 Vol 2: Communication Satellites
Sec 4: Satellite Bus/Platform Subsystems
2.4.1: Introduction
2.4.1.1 What Is a Bus
2.4.1.2 Major Subsystems
2.4.1.3 Typical Spin Stabilized Spacecraft
2.4.1.4 Typical 3-axis Spacecraft
2.4.1.5 Historical Trends [Anik Spacecraft]
2.4.1.6 RF and Array Power Trend
2.4.1.7 TO Mass and Lifetime Trend
2.4.1.8 Technology Trends
Outline of This Part
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 3Rev -, July 2001
What is a Bus?The bus is the platform that supports the payload and maintains
the satellite’s position in orbit.
The bus also provides the interface with the launch vehicle.
Bus Subsystems
Structure Electrical Power
Attitude Determination and Control
Telemetry &
Command
Propulsion Thermal Control
Subsystem
Mechanisms
2.4.1.1 Bus Subsystem
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 1: Introduction
2.4.1.1: What is a Bus?
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 4Rev -, July 2001
Major Subsystemsprovides “real estate” for mounting all bus and payload units and the interface with the launch vehicle
provides electrical power to the payload and bus units
provides the control for achieving and maintaining orbit and pointing
provides the propulsive power for achieving and maintaining orbit
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 1: Introduction
2.4.1.2: Major Subsystems
Structure
Electrical Power
Attitude Determination and Control
Propulsion
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 5Rev -, July 2001
Major Subsystem
controls the spacecraft and monitors its health
maintains a benign operating environment
provides the means for deploying appendages which must be stored for launch, and the means to adjust appendages
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 1: Introduction
2.4.1.2: Major Subsystems
Telemetry & Command
Thermal Control Subsystem
Mechanisms
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 6Rev -, July 2001
Typical Spin Stabilized Spacecraft
2.4.1.3 Typical Spin Stabilized Spacecraft
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 1: Introduction
2.4.1.3: Typical Spin Stabilized Spacecraft
Image Courtesy Image Courtesy of Boeing of Boeing
Satellite SystemsSatellite Systems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 7Rev -, July 2001
Typical 3-Axis Spacecraft
2.4.1.4 Typical 3-axis Spacecraft
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 1: Introduction
2.4.1.4: Typical 3-Axis Spacecraft
Image Courtesy of Image Courtesy of Boeing Satellite Boeing Satellite SystemsSystems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 8Rev -, July 2001
Historical Trends [Aniks]
Anik A Anik B Anik C Anik D Anik E Nimiq Anik F1Prime Contractor Hughes RCA Hughes Spar Spar LM HughesSatellite Type HS-333 RCA-2000 HS-376 HS-376 GE-S5000 A2100AX HS-702Number 3 1 3 2 2 1 1Launch Vehicle Delta Delta STS/PAM-D Delta/STS Ariane Proton ArianeLaunch Date(s) 1972-75 1978 1982-85 1982-84 1991 1999 2000Transfer Orbit Mass (kg) 560 920 1140 1217 2930 3590 4600Array Power (W) 235 620 800 800 3900 8800 15000Life (years) 7 7 10 10 12 15 15Stabilization Spin 3-axis Spin Spin 3-axis 3-axis 3-axisTotal no. Channels 12 18 16 24 40 32 84Channels (C/Ku) 12/- 12/6 -/16 24/- 24/16 -/32 36/48HPA Power (W) (C/Ku) 5/- 10/20 -/15 11/- 12/50 -/120 40/115Total RF Power (W) 60 240 240 264 1088 3840 6960
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 1: Introduction
2.4.1.5: Historical Trends (Anik Spacecraft)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 9Rev -, July 2001
0
4000
8000
12000
16000
1972 1982 1991 1999 2000
Arra
y Po
wer
0
1000
2000
3000
4000
5000
6000
7000
1972 1982 1991 1999 2000
RF P
ower
RF & Array Power Trend
2.4.1.6 RF & Array Power Trend
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 1: Introduction
2.4.1.6: RF & Array Power Trend
(Wat
ts)
(Wat
ts)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 10Rev -, July 2001
0
1000
2000
3000
4000
5000
1972 1982 1991 1999 2000
Mas
s [k
g]T.O. Mass & Lifetime Trend
0
4
8
12
16
1972 1982 1991 1999 2000
Life
[yrs
]
2.4.1.7 T.O. Mass & Lifetime Trend
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 1: Introduction
2.4.1.7: T.O. Mass & Lifetime Trend
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 11Rev -, July 2001
Technological Trends
2.4.1.8 Technological Trends
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 1: Introduction
2.4.1.8: Technology Trends
Image Courtesy of Telesat CanadaImage Courtesy of Telesat Canada
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 12Rev -, July 2001
Sec 4: Satellite Bus/Platform Subsystems
Vol 2: Communication Satellites
Electrical Power SubsystemPart 2
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 13Rev -, July 2001
Outline of This Part
2.4.2.1 Introduction
2.4.2.2 Solar Arrays
2.4.2.3 Batteries
2.4.2.4 Power Electronics
2.4.2.5 Typical Failure Modes
2.4.2.6 Solar Array Analysis and Prediction Methods
2.4.2 Electrical Power Subsystem (EPS)
Vol 2: Communication Satellites
Sec 4: Satellite Bus/Platform Subsystems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 14Rev -, July 2001
IntroductionA spacecraft power subsystem is designed to provide sufficient power to operate the spacecraft equipment over the life of the spacecraft.
For the majority of the mission, the source of electrical power is the solar arrays.
In eclipse, which occurs daily during 44-day periods twice a year for GEO spacecraft, electrical power is provided by batteries.
Regulation (control) of the variable source power is achieved with power interface electronics.
2.4.2.1 Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 15Rev -, July 2001
Typical EPS Configuration
SADM = Solar Array Drive Mechanism
Solar
Array (N)
Solar Array (S)
Power Control
Electronics
Battery
SADM
SADM
Fuse Box
Pyro Box
To spacecraft loads
To spacecraft Pyros
Battery discharge path
Battery charge
path
2.4.2.1 Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.1 Typical EPS Configuration
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 16Rev -, July 2001
CylindricalSolarArray
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2a Cylindrical Solar Array
Image Courtesy Image Courtesy of Boeing of Boeing
Satellite SystemsSatellite Systems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 17Rev -, July 2001
Planar SolarArray
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2b Planar Solar Array
Picture Courtesy of Telesat Canada
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 18Rev -, July 2001
Solar Wing Assembly
Solar PanelSolar
cells Hinge
Yoke
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2c Solar Wing Assembly
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 19Rev -, July 2001
Solar Cell ConnectionsSolar cell assemblies are electrically configured on the panels in strings and circuits.
Strings consist of a number of solar cells in series to provide the required voltage.
Circuits are composed of a number of strings connected in parallel to provide the required current.
A solar cell assembly consists of a solar cell (silicon or gallium arsenide on germanium) and a cover glass bonded to its front surface.
Cell StringCircuit
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2d Solar Cell Connections
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 20Rev -, July 2001
Solar Panel Connections
Solar Panel
String
Cell
Circuit
+vBus
-vBus
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2e Solar Panel Connections
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 21Rev -, July 2001
ElectricalLoad
Collision with N-Type atom dislodgeselectron and results in electron migrationto negative terminal
-
+
-
+
e-
n
e-
Photon
N-TypeLayer
P-Type Layer e- Electron
Flow
Collision with P-Type atom dislodges electroncreating a vacancy spot called a hole whichwill migrate to positive thermal to accept electron
Solar Cell Operations
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2f Solar Cell Operations
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 22Rev -, July 2001
Current source
Rs
RloadRl
Rs = Source resistanceRl = Leakage resistanceRload = External load
Solar Cell Electrical Model
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2g Solar Cell Electrical Model
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 23Rev -, July 2001
Voltage
Current
Short circuit current (Isc)
Open Circuit voltage (Voc)
Voltage at Max Power Point (Vmp)
Current at Max Power Point (Imp)
Max Power Point
Constant current part of curve
Constant voltage part of curve
Solar Cell I-V Curve
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2h Solar Cell I-V Curve
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 24Rev -, July 2001
I = Isc * (1 - C1 * {exp[V / (C2 * Voc)] - 1})
Where:V is the bus voltage at which the current is to be calculated and C1 & C2 are constants as calculated below:C1 = [1-(Imp/Isc)] * {exp[-Vmp/(C2*Voc)]}C2 = [(Vmp/Voc) - 1] / [ln(1-Imp/Isc)]
Cell Voltage
Cell Current
Isc
VocVmp
Imp
Icell
Vcell
The Basic Cell Equation
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2i Solar Cell I-V Curve
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 25Rev -, July 2001
Voltage
Current
Short circuit current (Isc)
Open Circuit voltage (Voc)
BOL
EOL
Operating point
Array I-V Curve
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2j Array I-V Curve
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 26Rev -, July 2001
0.00
0.10
0.20
0.30
0.40
0.50
0 40 80 120 160 200
Panel Voltage, volts
Pan
el C
urre
nt, a
mps
Max Pow er PointPmax = 53.57 WImaxp = 0.3747 AVmaxp = 143.0 V
I-V Curve
Power Curve (I*V)
Max Power Point
I shunt
I load
Vref
Vbus
Iload
Iupper
array
Ishunt
Error amp
Ilower
array
Iload
Ireturn
Shunt eleme
nt
Spacecraft load
Upper section of solar array
Lower section of solar array
IshuntIload + Ishunt
Main bus
Shunt bus
Return bus
Shunt Regulator Operation
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2k Shunt Regulator Operation
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 27Rev -, July 2001
Array Output vs. Time
2.4.2.2 Solar Arrays
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.2l Array Output vs. Time
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 28Rev -, July 2001
Batteries store electrical power for use during an eclipse or those short periods of time when there may not sufficient array power to support the full spacecraft load.
Battery types used for commercial spacecraft include:
• Nickel Cadmium (older spacecraft)
• Nickel Hydrogen (modern spacecraft)
• Lithium Ion (next generation spacecraft)
Spacecraft Batteries
2.4.2.3 Batteries
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 29Rev -, July 2001
A voltaic cell is the basic device for converting chemical energy into electrical energy.
It consists of two different metal plates immersed in a solution.
The metal plates are called positive and negative electrodes and the solution is called the electrolyte.
- +
Sulfuric acid
electrolyte
Zinc electrode
Copper electrode
External loadExternal current
flow
Basic Battery Chemistry
2.4.2.3 Batteries
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.3a Basic Battery Chemistry
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 30Rev -, July 2001
Charge
Positive electrode: Ni(OH)2 + OH- NiOOH + H2O + e-
Negative electrode: H2O + e- ½H2 + OH-
Overcharge
Positive electrode: 2OH- ½O2 + H2O + 2e-
Negative electrode: 2H2O + 2e- 2OH- + H2
Recombination: ½O2 + H2 H2O
Chemical Equation For Nickel Hydrogen Cell
2.4.2.3 Batteries
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 31Rev -, July 2001
Discharge
Positive electrode: NiOOH + H2O + e- Ni(OH)2 + OH-
Negative electrode: ½H2 + OH- H2O + e-
Chemical Equation For Nickel Hydrogen Cell
2.4.2.3 Batteries
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 32Rev -, July 2001
A battery assembly consists of a number of battery cells connected in series, where the potentials of the individual cells add to give the total battery potential.
The chassis mechanically fixes the cells as well as provides a thermal conductive path for the heat generated by the cells.
Satellite batteries also include electrical heaters and cell bypass circuitry.
The electrical heaters maintain the battery cells at the desired temperature during the endothermic (heat absorbing) charge phase.
Battery
2.4.2.3 Batteries
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 33Rev -, July 2001
Individual cell bypass circuitry provides an alternate conductive path should the associated cell fail open. This is simple circuitry consisting, typically, of diodes.
Most battery cells in use today are Nickel Hydrogen (NiH2) technology. This technology provides a significant improvement in cycle life and energy density compared to Nickel Cadmium.
A NiH2 battery cell is comprised of a stack assembly of “pineapple slice” electrodes, separators, gas screens and insulator rings mechanically fixed on a central core with end plates, a sealed cylinder pressure vessel with electrical axial electrical terminals, and an electrolyte solution.
Battery
2.4.2.3 Batteries
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 34Rev -, July 2001
2.4.2.3 Batteries
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.3b Superbird 83-Ah Nickel-Hydrogen Cell
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 35Rev -, July 2001
Battery Temperature Profile
Time (hours)
Temp
0 12 24
Charge Cool down Discharge
Heaters ‘on’Heaters ‘on’
2.4.2.3 Batteries
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.3c Battery Temperature Profile
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 36Rev -, July 2001
The power control electronics regulates the power bus voltage during sunlight conditions by controlling the amount of solar array power that is passed to the spacecraft loads.
A control signal, which is generated by a comparison of the actual and desired power bus voltages, controls how much of the excess solar array power is redirected through the shunt switches.
The power control electronics may contain a dedicated shunt switch module for each solar array circuit. Contained in each module is a shunt switch and an isolation diode to protect the power bus from being short circuited when this switch is operational.
Power Control Electronics
2.4.2.4 Power Electronics
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 37Rev -, July 2001
Battery discharging, during eclipse or periods of limited solar array power availability, is achieved via the battery discharge electronics. This circuitry is designed to operate over a large range of battery input voltages and provides a regulated output voltage.
Activation of the circuitry is controlled by a voltage comparison control signal.
The power control electronics shunt switch modules are designed such that capability exists to disable a given module and permit continued use of the associated solar array circuit.
The charge, discharge, and control electronics consists of a number of independent modules that permit the loss of a single module without any operational impacts.
Power Control Electronics
2.4.2.4 Power Electronics
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 38Rev -, July 2001
Fuse BoxThe fuse box typically consists of dedicated fuses for each spacecraft load.
Redundant parallel-configured fuses are sometime used for each spacecraft load. The idea here is that if one fuse blows because of an internal defect rather than because of load current draw, the other fuse will remain intact and power to the load will not be interrupted. A real load failure, however, or current surge, will cause both fuses to blow, thus protecting the load.
Pyro BoxA pyro box typically consists of redundant transistor switches that permit firing of the pyrotechnic devices: hold-down straps and bolts that are fired to permit deployment of arrays and antennas.
Dedicated separation switches may be allocated to the primary and redundant pyros of some separation mechanisms.
2.4.2.4 Power Electronics
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 39Rev -, July 2001
Slip Ring Assembly
Solar Array harness Drive
MotorGear
Assembly
Slip Ring Assy
Spacecraft harness
2.4.2.4 Power Electronics
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.4 Slip Ring Assembly
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 40Rev -, July 2001
Broken solar cells: A broken cell contributes to an overall loss of power. Accelerated life testing should quantify expected failure rates; the acceptance test program, and visual inspection, should identify failures that occur on the ground. Power subsystem design should tolerate a number of broken cells in orbit.
Battery cell short or open: Again, this results in a loss of power. Life testing should demonstrate robust design, the acceptance test program should identify cell short failures on the ground, and power subsystem design should tolerate a limited number of in orbit battery cell failures.
Typical Failure Modes
2.4.2.5 Typical Failure Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 41Rev -, July 2001
Power control electronics module failure: This should have little or no impact for a single in orbit failure because of the built-in redundancy. The acceptance test program should identify a component failure on the ground prior to launch.
Blown fuse: Parallel redundant fuses insure that a single defective fuse will not prevent powering of associated equipment. Blowing both fuses is a strong indication of a problem with the associated equipment and, thus, performs the intended function of protecting the spacecraft power bus.
Typical Failure Modes
2.4.2.5 Typical Failure Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 42Rev -, July 2001
Array Performance
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6a Array Performance
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 43Rev -, July 2001
Electrical Configuration
Solar Panel
String
Cell
Circuit
+v Bus
-v BusCircuits 2..N
Panel voltage ~ number of cells in series (Ns x Vcell)
Panel current ~ number of strings in parallel (Np x Icell)
Circuit 1
Circuits diode ‘ored’
Isolating diodes
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6b Electrical Configuration
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 44Rev -, July 2001
Typical IV Curve
0.00
0.10
0.20
0.30
0.40
0.50
0 40 80 120 160 200
Panel Voltage, volts
Pan
el C
urre
nt, a
mps
Max Pow er PointPmax = 53.57 WImaxp = 0.3747 AVmaxp = 143.0 V
I-V Curve
Power Curve (I*V)
Max Power Point
Current available at operating voltage• Short circuit
current, Isc
• Open circuit voltage, Voc
• Current at max power point, Imp
• Voltage at max power point, Vmp
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6c Typical IV Curve
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 45Rev -, July 2001
Factors Affecting the Current
Sun intensity ƒ(orbit, t)
Panel assembly factors
Cell characteristicsVoc, Isc, Vmp, Imp ƒ(rad, temp)
Temperature effects ƒ(t)
Solar Panel Configuration Iarray = ƒ(Ns, Np)
Radiation ƒ(t)
Current vs timeCell Equation
Iarray = ƒ(N1.. Nn)
Solar Array Program
(SAP)
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6d Factors Affecting the Current
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 46Rev -, July 2001
The Basic Cell Equation
Cell Voltage
Cell Current
Isc
VocVmp
Imp
Icell
Vcell
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6e Solar Cell I-V Curve
I = Isc * (1 - C1 * {exp[V / (C2 * Voc)] - 1})
Where:V is the bus voltage at which the current is to be calculated and C1 & C2 are constants as calculated below:C1 [1-(Imp/Isc)] * {exp[-Vmp/(C2*Voc)]}C2 = [(Vmp/Voc) - 1] / [ln(1-Imp/Isc)]
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 47Rev -, July 2001
Modeled Vs Measured
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6f Modeled Vs. Measured
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 48Rev -, July 2001
Array Output Over Mission LifeArray current is typically calculated once a day.
Sun intensity is cyclic over the year and can be modeled.
Cell temperature is usually given at each of the 4 seasons at BOL & EOL and curve-fitted over mission life.
Cell degradation due to radiation effects is given with respect to 1 MeV electron fluence. Degradation over time is calculated as the product of the flux per day and the time on-orbit, which gives the fluence at that point in time.
Solar flares can be included as an SF-dose linearly applied over the mission, over part of the mission, or as discrete events specified by the designer or user.
SF Alphas are taken as 5% of the SF protons.
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 49Rev -, July 2001
Sun Intensity
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6g Sun Intensity
Sun
Inte
nsity
Fac
tor
April 1st
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 50Rev -, July 2001
IV Curves for the 4 Seasons
Array Working Point
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6h IV Curves for the 4 Seasons
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 51Rev -, July 2001
Isc FactorsThe short circuit current (Isc) factors affecting the performance of a solar array are listed below:
Solar intensity - variation due to varying sun angle and distance from from the sun. Normalized to 135.5 mW/cm^2.
Coverglass transmission loss - caused by glassing of the solar cell and UV degradation when on-orbit.
Assembly loss - measurement error and scattering due to glassing and interconnects.
Isc temperature coefficient - change in cell current due to temperature variations.
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 52Rev -, July 2001
Voc FactorsThe open circuit voltage (Voc) factors affecting the performance of the solar array are listed below:
Assembly loss - series resistance of cell interconnections and weld resistance that depresses the knee of the IV curve
Voc temperature coefficient - change in cell voltage due to temperature variations. Magnitude affected by radiation.
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 53Rev -, July 2001
The ProcessThe process of performing a solar array prediction involves gathering specific data about the solar array to be analyzed. Such data includes:
• Cell front and back shielding values from the physical characteristics of the panel, materials, cell, cover etc.
• Reducing the radiation environment (electron, proton and solar flare) to the equivalent 1 MeV electron fluence
• Curve-fitting the cell degradation factors affected by radiation
• Developing computer code for that particular cell
• Determining the rest of input data such as panel configuration, seasonal temperatures, losses etc.
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 54Rev -, July 2001
Concept of 1 MeV FluenceThe concept of damage-equivalent, normally-incident (DENI) mono-energetic 1 MeV fluence was developed by the solar cell industry to determine the degradation effects of a radiation environment with various energies and incident angles.
In this normalizing calculation, the actual damage due to electrons of various energies is related to the damage produced by 1 MeV electrons by the damage coefficients for electrons.
Likewise, proton damage is related to 10 MeV protons, which in turn is related to the damage produced by 1 MeV electrons.
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 55Rev -, July 2001
Concept of 1 MeV FluenceOne 10 MeV proton does approximately the same damage as 3000 electrons of 1 MeV energy.
By combining the electron, proton and SF fluences, a single value of equivalent 1 MeV fluence can be used to determine cell degradation in a complex radiation environment.
Trapped protons in geostationary orbit are not modeled because their energies are low enough so that they are absorbed by the coverglass.
Typical solar cell IV curves before and after exposure to a heavy dose (1X1015 e/cm2) of 1 MeV electrons are shown on the next slide.
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 56Rev -, July 2001
Temperature & Radiation Effects*Typical solar cell IV characteristics before and after irradiation.
Temperature effects are also shown.
With higher temperatures, current increases while voltage decreases
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6i Temperature and Radiation Effects
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 57Rev -, July 2001
1 MeV Fluence vs Cover Thickness*Shielding effectiveness changes with incident radiation, particle type and particle energy.
Figure 2.4.2.6j is a useful graph for estimating 1 MeV fluence for a given shielding.
PICTURE
Fluencevs
CoverThickness
2.4.2.6 Solar Array Analysis and Prediction Methods
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 2: Electrical Power Subsystems (EPS)
2.4.2.6j 1 MeV Fluence vs Cover Thickness
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 58Rev -, July 2001
Sec 4: Satellite Bus/Platform Subsystems
Vol 2: Communication Satellites
Telemetry, Tracking & Command Subsystem
Part 3
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 59Rev -, July 2001
Sec 4: Satellite Bus/Platform Subsystems
2.4.3: Telemetry, Tracking & Command (TT&C) Subsystem
Vol 2: Communication Satellites
Outline of This Part2.4.3.1 TT&C Key Requirements
2.4.3.2 TT&C Equipment
2.4.3.3 TT&C Key Items
2.4.3.4 Command System Block Diagram
2.4.3.5 Command Format
2.4.3.6 Telemetry System Block Diagram
2.4.3.7 Telemetry Format
2.4.3.8 Creation of an 8-Bit Telemetry Word
2.4.3.9 Data Encoding
2.4.3.10 Failures, Degradations & Margins
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 60Rev -, July 2001
• Receive, decrypt, authenticate, and process commands.• Collect, format, encrypt, and transmit satellite telemetry.• Support satellite control functions.
• Attitude determination and control• Battery charge management, solar array pointing• Autonomous configuration management
• Support range determination from ground station(s). • Provide antenna coverage for transfer & drift orbit operations
and during on-orbit attitude anomalies.• Be designed without any single point-of-failures.
2.4.3.1: TT&C Key Requirements
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
A TT&C System must:
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 61Rev -, July 2001
Typical TT&C Subsystem
TT&C RF Equipment Flight Software TT&C Baseband
Equipment
- CMD Receivers- CMD Horn Antenna(s)- TLM Horn Antenna(s)- CMD & TLM Omni Antenna- MISC RF H/W and Cabling
- CMD & TLM Database
- AD&C Software (Flight S/W)
- Encoder/Decoder Units- Remote Terminal Units
- Payload - Bus
- Computers- Harnesses
2.4.3.2: TT&C Equipment
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
Figure 2.4.3.2 TT&C Equipment
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 62Rev -, July 2001
TT&C• CMD Uplink 500 bps
• TLM Downlink 4 kbps
• Encryption, Decryption
• Spacecraft Ranging
TT&C Omni Antenna
Arabsat 3A
TT&C On-station Antenna
2.4.3.3: TT&C Key Items
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
Figure 2.4.3.3 TT&C AntennasDrawings Used by Permission
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 63Rev -, July 2001
Commanded functions include unit configuration, gain settings, redundancy settings, jet firings etc. The red and blue lines indicate main redundancy paths, while the black lines indicate redundancy switching options.
Command Receiver
Command Receiver
Command Decoder
Command Decoder
Remote Terminals
Remote Terminals
H
Ranging signal to tlm tx
Ranging signal to tlm tx
2.4.3.4: Command System Block Diagram
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
Figure 2.4.3.4 Command System Block Diagram
Cross Strapping
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 64Rev -, July 2001
SYNCH/ADDRESS EXEC OP-CODE DATA WORD PARITY
SPACECRAFT COMMAND WORD
Commands are validated on-board prior to execution. Validation criteria are:
• Synchronization pattern
• Spacecraft address
• Command length
• Command segment order & content
• Parity
2.4.3.5: Command Format
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 65Rev -, July 2001
Telemetered signals include unit status, temperatures, voltages, currents, register contents, etc.
Sensors
Signal Conditioning
Telemetry Tx
Telemetry Encoder
Telemetry Encoder
Remote Terminal
Unit
Sensors
Signal Conditioning
Telemetry Tx
H
Ranging signal from cmd rx
Ranging signal from cmd rx
2.4.3.6: Telemetry System Block Diagram
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
Figure 2.4.3.6 Telemetry System Block Diagram
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 66Rev -, July 2001
Frame Synch SCID
Telemetry Words
TLM Mode Format ID Fixed
Variable Telemetry Words
Variable Telemetry Words
Variable Telemetry Words
Variable Telemetry Words
Variable Telemetry Words Frame Count Checksum
SPACECRAFT TELEMETRY MINOR FRAME
2.4.3.7: Telemetry Format
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
• Telemetry transmission is composed of major and minor frames.• A major frame is a complete set of telemetry data.• The major frame is made up of a number of minor frames.• Each minor frame carries a number of Telemetry Words.
Framing
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 67Rev -, July 2001
8-bit Digital Coding
2.4.3.8: Creation of an 8-Bit Telemetry Word
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 68Rev -, July 2001
00 001 110 016 Your message00 110 110 066 Secret key11 000 111 307 Coded message
11 000 111 307 Coded message00 110 110 066 Secret key00 001 110 016 Your message
When:2 bits are the same, cipher text = 12 bits are different, cipher text = 0
2.4.3.9: Data Coding
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
Simple Example: The Exclusive “OR” Function
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 69Rev -, July 2001
Typical TT&C designs offer low risk configurations :• No deployable antennas for transfer orbit operations• No RF switches in the command path(s)• Redundancy and cross-strapping of CMD/TLM/RNG signals• Multiple modes of operation, i.e. High & Low Power Transmitter
outputs• Positive RF link margins for CMD/TLM/RNG
On-orbit problems are generally due to H/W failures or degradation.
Operational recovery is achieved by a combination of cross-strapping signal paths and redundant equipment selection.
In a loss of earth-lock, Flight Software (FSW) typically reconfigures TLM transmission to high power, wide angle coverage to facilitate S/C recovery attempts.
2.4.3.10: Failures, Degradations & Margins
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 3: Telemetry, Tracking & Command (TT&C)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 70Rev -, July 2001
Sec 4: Satellite Bus/Platform Subsystems
Vol 2: Communication Satellites
Attitude Control SubsystemsPart 4
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 71Rev -, July 2001
Sec 4: Satellite Bus/Platform Subsystems
2.4.4: Attitude Control Subsystems
Vol 2: Communication Satellites
Outline of This Part• Introduction• ACS Principles and Design• Sensors• Actuators• Spacecraft Processors• Operating Modes• Reliability and Risk• ACS Testing
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 72Rev -, July 2001
The Attitude of a Spacecraft is its orientation in space.
Position and Velocity describe the translational motion of the center of mass of the spacecraft. Translational motion is motion from one location to another.
Attitude and attitude motion describe the rotational motion of the body of the spacecraft about the center of mass.
How is Attitude determined, and how is it controlled?
2.4.4.1: Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
Initial Definitions
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 73Rev -, July 2001
Attitude Determination is the process of computing the orientation of the spacecraft relative to a point of reference such as the Earth. This typically involves the use of several types of sensors and a means to process the resulting data.
Attitude Control is the process of orienting the spacecraft in a predetermined direction. This consists of stabilization, maintenance of an existing orientation, maneuver control, and controlling the reorientation of the spacecraft from one attitude to another.
Both of these functions are performed by the Spacecraft’s Attitude Control Subsystem (ACS).
2.4.4.1: Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.1.1: Functional Definitions
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 74Rev -, July 2001
Nominally box : + 0.05o longitude
+ 0.05o inclination
Worst case: + 0.1o for each
At 35,786 Km:
1o = 624 Km
0.05o =31Km
+ 0.05o = box 62 Km square
Station Keeping Box Maintain satellite in an orbit position so it always in the FOV of a non-tracking Earth Station
Stationkeeping box(0.05º or 0.1º)
Earth Station Beam Width
2.4.4.1: Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
Figure 2.4.4.1.1 Station Keeping Box
2.4.4.1.1: Functional Definitions
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 75Rev -, July 2001
Sensors(Side 1)
• Earth• Sun• Gyro
Sensors(Side 2)
• Earth• Sun• Gyro
Processor 1
Processor 2
Actuators(Side 1)
• Thrusters• Wheels
Actuators(Side 2)
• Thrusters• Wheels
2.4.4.1: Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.1.2: Typical ACS Configuration
Figure 2.4.4.1.2 Typical ACS Configuration
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 76Rev -, July 2001 Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
2.4.4.1.3: Definition of Axes
2.4.4.1: Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
YAW
Z
ROLL
X
PITCHY
EARTHSENSOR
THREE-AXIS (BODY STABILIZED
EARTH
Figure 2.4.4.1.3a 3-Axis
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 77Rev -, July 2001
PITCH
(East-West)
2.4.4.1: Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.1.3: Definition of Axes
Figure 2.4.4.1.3b Pitch
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 78Rev -, July 2001
ROLL
(North-South)
2.4.4.1: Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.1.3: Definition of Axes
Figure 2.4.4.1.3c Roll
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 79Rev -, July 2001
YAW
(Beam Rotation)
2.4.4.1: Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.1.3: Definition of Axes
Figure 2.4.4.1.3d Yaw
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 80Rev -, July 2001
Center of Mass (c.o.m.) is the point where the satellite mass is considered to be “concentrated”. It is known as “that point at which the entire mass of an object may be considered to be located for purposes of understanding the object's motion.”1
Center of Gravity is the point where the force of gravity is considered to be acting. It may be different than the center of mass when mass distribution is not equidistant from the source of gravitational attraction.
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.1: Centers of Mass and Gravity
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 81Rev -, July 2001
Moment of Inertia is a measure of resistance to change in rotational speed. It is a way of specifying the mass “distribution” about a certain axis.
Product of Inertia is a measure of the influence of an object’s geometry on its rotation. It is a way of defining the symmetry of an object about a plane defined by two axes.
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.2: Moments and Products of Inertia
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 82Rev -, July 2001
Angular Momentum is a property of a rotating body,
H = [I] wIt is representative of a body’s moment of inertia [I], and rotation rate, w, and is usually measured in Newton-meter-seconds.
Angular momentum is a vector value, i.e., it has both magnitude and direction.
“Torque” is an external influence caused by forces acting about the center of mass, and is usually measured in N-m. It is caused by rotational devices (motors, shafts) and will affect the body’s angular momentum.
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.3: Angular Momentum and Torque
EQ. 2.4.4.2.3 Angular Momentum
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 83Rev -, July 2001
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.4: Torque and Moment Arm
Fo rce F
C e n te ro f
M a s s
r
P e r p e n d ic u la rd is ta n c e
o rM o m e n t Ar m
Figure 2.4.4.2.4 Torque and Moment Arm
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 84Rev -, July 2001
A “system” is a grouping of 2 or more “bodies.”
In the instance of a spacecraft, we have the spacecraft body itself and each of the momentum and reaction wheels.
Each “body” can rotate, and has its own angular momentum.
Individual “bodies” can affect the rotational state of each other (torque one another) and exchange angular momentum.
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.5: System Angular Momentum
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 85Rev -, July 2001
System Angular Momentum is the “vector sum” of all “body” contributions.
It is affected only by torques “external” to the system. These torques can be interactions between the spacecraft and its environment, such as solar pressure on the solar panels.
It is not affected by torques internal to the system. Internal effects, such as antenna deployments, stay inside the system.
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.5: System Angular Momentum
Gyroscopic Stiffness Effect demonstrates how a rotating body tends to stay rotating in the same state (unless acted upon).
A spin axis will remain pointing in the same direction, this is a consequence of Newton’s laws.
2.4.4.2.6: Gyroscopic Stiffness Effect
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 86Rev -, July 2001
Gyroscopic Stiffness is beneficial. It provides stability in orientation, reduces effects caused by external disturbances and it is the result of having angular momentum.
Spacecraft solutions that emerge as a result are:
• Spinning the satellite (“spin-stabilized”)
• Spinning wheels within satellite (“3-axis stabilized”)
• 3-Axis stabilization, but without a momentum bias
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.6: Gyroscopic Stiffness Effect
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 87Rev -, July 2001
• Solar radiation pressure
• Gravity gradient and other gravitational sources
• Earth’s magnetic field
• Micro-meteoroid impacts
• Thrusters
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.7: Disturbance Sources and their Effects
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 88Rev -, July 2001
Attitude effects resulting from disturbance torque include:
Precession is the rate of change in the direction of the angular momentum vector. This is caused by a torque acting over time.
If precession is slow and not corrected for, the whole satellite will drift in its orientation.
Nutation appears as coning type of motion when the spacecraft is disturbed from its equilibrium state; whenever precessional torques are applied the mode will be activated. The coning motion centers around the original direction of angular momentum.
It is the result of having too much rotational kinetic energy, which can be damped out actively (on-board controller), or passively (naturally)
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.7: Disturbance Sources and their Effects
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 89Rev -, July 2001
In order to counteract disturbances, the ACS system includes:
• Devices that interact with the external environment, therefore affecting the system angular momentum. These include thrusters and magnetic torquers.
• Devices that act inside the spacecraft to redistribute the angular momentum within the spacecraft. Momentum/reaction wheels do this. Wheel “saturation”, however, requires momentum unloading.
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.8: Ways to Counteract Disturbances
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 90Rev -, July 2001
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.9: Spin-Stabilized Satellites
H
SINGLE-SPIN(SPINNERS)
w
Figure 2.4.4.2.9a Single Spin (Spinners)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 91Rev -, July 2001
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.9: Spin-Stabilized Satellites
H
NONSPINNING
SPINNINGwFigure 2.4.4.2.9b Spin Stabilized Satellites
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 92Rev -, July 2001
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.10: Three-Axis Stabilized Satellites
H
MOMENTUM-BIASED
w
Figure 2.4.4.2.10a 3-Axis Stabilized Satellite
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 93Rev -, July 2001
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.10: Three-Axis Stabilized Satellites
Xs Zs
Ys
CE N T RA L R IG IDBO D Y
SO LA R P A NE L
M O M EN TU M W H E ELS
M O M EN TU M -B IAS ED (W IT H RE A C TIO N W H E E LS IN 3 AXE S)
Figure 2.4.4.2.10b Momentum-Biased
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 94Rev -, July 2001
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.10: Three-Axis Stabilized Satellites
ZERO-MOMENTUM(W ITH REACTION W HEELS IN 3 AXES)
Figure 2.4.4.2.10c Zero-Momentum
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 95Rev -, July 2001
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.10: Three-Axis Stabilized Satellites
Figure 2.4.4.2.10d Zero-Momentum (Gravity Gradient Stabilized)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 96Rev -, July 2001
Disturbances from Environment
Spacecraft Dynamics
ACS Subsystem
Sensors SCP Actuators
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.11: ACS Analytical Design
Figure 2.4.4.2.11 ACS Analytical Design
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 97Rev -, July 2001
Pointing error budgets analyze the temporal behavior of the various error sources, usually divided into four categories:
• Constant
• Long term (longer than one day)
• Diurnal (one day)
• Short term (less than 10 minutes)
An addition of the above categories represents a conservative assessment.
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.12: Pointing Budgets
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 98Rev -, July 2001
Typical error sources are:
• Thermal Distortion
• Disturbance Torques
• Misalignment Errors
• Structural Hysteresis
• Orbital Effects
• Sensor Noise
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.2.12: Pointing Budgets
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 99Rev -, July 2001
Source of Error Normal Mode Stationkeeping Mode Roll Pitch Yaw Roll Pitch Yaw
Constant (or Fixed bias) ErrorsEarth sensor alignment 0.022 0.022 0.022 0.022Sun sensor alignment 0.015 0.015 0.015 0.015Antenna characterization errors 0.032 0.032 0.021 0.032 0.032 0.021Root Sum Square (RSS) of Constant Errors 0.041 0.041 0.021 0.041 0.041 0.021In-orbit calibration residual of Constant Errors 0.016 0.016 0.021 0.016 0.016 0.021Long-term ErrorsSensor long-term degradation 0.011 0.009 0.021 0.011 0.009 0.021Sensor seasonal variations 0.007 0.007 0.007 0.007Structure seasonal thermal distortions 0.017 0.017 0.008 0.017 0.017 0.008Orbital variations (East/West and North/South) 0.011 0.014 0.015 0.011 0.014 0.015Root Sum Square (RSS) of Long-term Errors 0.024 0.025 0.027 0.024 0.025 0.027Diurnal ErrorsGyro drift 0.105 0.105Sensor diurnal errors 0.033 0.039 0.045 0.033 0.039 0.045Ephemeris Error 0.002 0.004 0.018 0.002 0.004 0.018Structure diurnal thermal distortions 0.012 0.012 0.012 0.012Root Sum Square (RSS) of Diurnal Errors 0.035 0.041 0.115 0.035 0.041 0.115Short-Term ErrorsSensor noise 0.004 0.002 0.015 0.004 0.002 0.015Actuator transients 0.007 0.007 0.007 0.018 0.018 0.025Solar array tracking torque 0.002 0.004 0.002 0.002 0.004 0.002Maximum uncorrected disturbance torque 0.022 0.022 0.013 0.029 0.029 0.017Root Sum Square (RSS) of Short-term Errors 0.023 0.025 0.021 0.034 0.035 0.034Total Error (arithmetic sum of RSS terms) 0.123 0.132 0.184 0.134 0.142 0.197Total Error (arithmetic sum after in-orbit calibration) 0.098 0.107 0.184 0.109 0.117 0.197
2.4.4.2: ACS Principles and Design
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems2.
4.4.
2.12
: Poi
ntin
g B
udge
ts
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 100Rev -, July 2001
In basic ACS systems, onboard control logic responds to two-axis attitude sensing while it provides three-axis control.
In more advanced systems, the three axes are being sensed and controlled directly and the total angular momentum is maintained near a defined nominal value.
Typical geosynchronous systems use sensors such as:• Earth sensors, Horizon sensors
• Sun sensors, Star trackers
• Gyros
• Accelerometers, magnetometers
• RF Beacons
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
Introduction
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 101Rev -, July 2001
Types of Earth Reference sensors are:
Infrared detectors (CO2 spectral band)– Detects Earth’s horizon
– Reference for roll, pitch, and spin rate
Beacon Tracking– RF pointing system that uses a beacon (RF signal generated from
the ground) to point the satellite antenna at the Earth
– Usually for geosynchronous orbit
– Requires cooperative Earth station
Magnetometer– Senses Earth’s magnetic field, requires ephemeris knowledge
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.1: Earth Reference Sensors
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 102Rev -, July 2001
• Spinners use the rotation of the satellite as a timebase to detect the difference between the IR value of deep space and that of Earth.
• This change in IR level is used by the ACS Electronics to point the antennas towards the Earth.
• Three-axis satellites use torsional or vibrating mirrors to set-up an artificial timebase.
Deep space
Deep space
IR Radiation from Earth
North sensors scan “North” of equatorSouth sensors scan “South” of equator
NES
SES
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.1: Earth Reference Sensors
Sensors
Figure 2.4.4.3.1a Earth Reference Sensors
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 103Rev -, July 2001
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.1: Earth Reference Sensors
NORMAL
PITCHERROR
YAW ERROR
ROLL ERROR
N
S
N
S
N
S
N
S
ERROR DETECTION
Figure 2.4.4.3.1b Error Detection
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 104Rev -, July 2001
Oscillating Earth Sensor Static Earth Sensor
Earth Sensor output is proportional to area on Earth. Electronics use this to point the satellite body.
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.1: Earth Reference Sensors
Figure 2.4.4.3.1c Earth Sensor Positioning
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 105Rev -, July 2001
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.2: Beacon Sensors
PLANAR ARRAYANTENNA
ANTENNASYSTEM S SSM A
2 - 85 Hz TO NES
2 - 85 Hz TO NES
TCR#1
DAzDEL
TCR#2
DAzDEL
DAzDEL
DAzDEL
SCP#1
SCP#2
SH IELDED T W ISTEDPAIRS W ITH RETU RNS
TTC&R RFSUBSYSTEM
ATTITUDE CONTROLSUBSYSTEM
ANTENNASUBSYSTEM
Figure 2.4.4.3.2 Beacon Sensors
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 106Rev -, July 2001
Sun Sensor
– For sun acquisition and tracking
– Good second reference for earth satellite
– For single-axis updating (yaw), or recalibration of gyro
Star Trackers (many varieties)
– Gimballed tracker (mechanically complex)
– Strapped-down mapper (substantial data processing required)
– Electronic tracker (image dissector)
– Provides a high-accuracy reference
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.3: Stellar Reference Sensors
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 107Rev -, July 2001
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.3: Stellar Reference SensorsSUNLIGHT
AIR VACUUM
PATTERN MASK
PHOTOCEL
SLIT MASK
COARSE ANALOGSUN SENSOR
(CASS)
Figure 2.4.4.3.3a Stellar Reference Sensors
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 108Rev -, July 2001
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.3: Stellar Reference Sensors
Figure 2.4.4.3.3b Cass Output
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 109Rev -, July 2001
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.3: Stellar Reference Sensors
BIT 4F
BIT 3F
BIT 2F
BIT 1F
QUADRATUREPATTERNS (FINE BITS)(GRAY CODED BITS 1
THROUGH 3)
BIT 6CBIT 5C
BIT 4C
BIT 3C
BIT 2C
BIT 1C
COARSE BITS(GRAY CODED BITS
4 THROUGH 9)
ATA
PHOTOCELLS
DIGITAL SUN SENSOR (DSS) Figure 2.4.4.3.3c Digital Sun Sensor
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 110Rev -, July 2001
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
1/8°
2°
32°GRAY CODED
BIT 1
BIT 2
BIT 3
BIT 4
BIT 5
BIT 6
BIT 7
BIT 8
BIT 9
*AVERAGE VALUE
±1/8
±1/4
±1/2
±1
±2
±4
±8
±16
±32
DSS OUTPUTFI
NE
BIT
SC
OA
RSE
BIT
S2.4.4.3.3: Stellar Reference Sensors
Figure 2.4.4.3.3d DSS Output
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 111Rev -, July 2001
Gyroscope
• Provides gyrocompass reference
• Provides a reference for active nutation damping
• 3-axis determination (position and rate)
• Requires independent updating to compensate for drift
Accelerometer
• Guidance reference to boost phase
• Phase reference for active nutation damping
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.4: Inertial Reference Sensors (Gyros)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 112Rev -, July 2001
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
AZ
(YAW)
AZ
(SKEW )
AZ
(PITCH)
AZ
(ROLL)
GYRO ASSEBBLIESARE FREQUENTLY
ARRANGED ASSINGLE UNITS
CONTAINING 3 OR 4INDEPENDENT
GYROS, ARRANGEDSO AS TO PROVIDE
POSITION AND RATEINFORMATION
ABOUT ANY AXIS.
a
b
a = 45°b = 35.3°90° - b = 54.7°AS = 1 (X+Y+Z)
3
A A A
2.4.4.3.4: Inertial Reference Sensors (Gyros)
Figure 2.4.4.3.4 Gyros
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 113Rev -, July 2001
Accelerometers
• Accelerometers typically respond to linear acceleration.
• A miniature servo system responds to input acceleration along its sensitive axis.
• The movement is detected by a position error detector.
• An amplifier sends a feedback current via a restoring coil in a magnetic field. This applies a restoring force on the seismic system, returning it to its original position, nulling the position error detector.
• An analog voltage proportional to the input acceleration is measured and decoded into counts for the processor.
2.4.4.3: Sensors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.3.4: Inertial Reference Sensors (Gyros)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 114Rev -, July 2001
Typical modern geosynchronous systems use actuators such as: • Two gimbaled momentum wheels / four reaction wheels in a
pyramid, all with momentum unloading (there are many variations)
• Propulsion torquers
• Magnetic actuators
The actuator (and sensor) controls, are performed through: • Central processor units (sensor processing, actuator drivers)
• Remote control units (sensor and actuator interfaces)
• Flight software (controls, service and fault protection algorithms)
2.4.4.4: Actuators
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
Introduction
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 115Rev -, July 2001
Momentum and Reaction Wheels (MW, RW) are:• Used to stabilize the satellite throughout its orbital mission life
• Minimize roll and pitch errors
• Damp roll / yaw nutation
• Can provide gyroscopic stiffness
Three wheel assemblies are comprised of:• Two “momentum” wheels (primary mode)
• One “reaction” wheel (used in secondary mode)
• Each with its own wheel drive electronics (WDE)
2.4.4.4: Actuators
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.4.1: Momentum and Reaction Wheels
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 116Rev -, July 2001
Four-wheel assemblies compromise of:
– Four momentum wheels in a pyramid configuration, all operating concurrently
Only three actually needed to meet the momentum storage requirements of the spacecraft, fourth wheel provides redundancy.
In all cases, momentum exchange permits cancellation of cyclic torques, primarily solar pressure torques, without employing attitude jets.
2.4.4.4: Actuators
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.4.1: Momentum and Reaction Wheels
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 117Rev -, July 2001
2.4.4.4: Actuators
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.4.1: Momentum and Reaction WheelsFigure 2.4.4.4.1 Momentum and Reaction Wheel
Imag
e Co
urte
sy o
f Tel
esat
Can
ada
Imag
e Co
urte
sy o
f Tel
esat
Can
ada
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 118Rev -, July 2001
As we have seen, apogee burns require a specific motor, the Liquid Apogee Engine (LAE).
Other, smaller thrusters are used for angular momentum unloading, stationkeeping, and large angle rotations.
Thrusters usually come in two (or more) redundant sets, with each set capable of providing torque about all 3 axes.
Each set is referred to as a “string” (A or B).
Each thruster has its redundant equivalent.
Each has its own latch valve (to isolate it).
2.4.4.4: Actuators
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.4.2: Thrusters
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 119Rev -, July 2001
Three categories of thrusters are common:
• Solid propellant
• Liquid propellant
• Electric propulsion
2.4.4.4: Actuators
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.4.2: Thrusters
Figure 2.4.4.4.2 A Thruster at Work
Photo Courtesy of General Dynamics
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 120Rev -, July 2001
Magnetic TorquerMagnetic coils—or electromagnets—are used to generate magnetic dipole moments for attitude and angular momentum control. These are also used to counteract residual spacecraft biases and attitude drift due to environmental disturbance torques.
A magnetic torquer consists of a simple coil which produces a magnetic dipole when current flows around the loop.
The magnetic dipole’s strength is proportional to the ampere-turns and area enclosed by the coil, and the direction is normal to the plane of the coil.
The torque acting on the spacecraft is the cross-product of the magnetic dipole of the coil and the Earth’s magnetic field. The direction of the control torque can be reversed by changing the direction of the current in the coils.
2.4.4.4: Actuators
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.4.3: Magnetic Actuators
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 121Rev -, July 2001
2.4.4.4: Actuators
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.4.4: Solar Wing Positioners and Drivers
Zm
Xm
YmSOLAR RADIATION
PRESSURE MOMENT
EARTH
a
X
YZ
CM
CPr S
Figure 2.4.4.4.4a Solar Radiation Pressure Moment
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 122Rev -, July 2001
2.4.4.4: Actuators
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
SPECU LAR YR EFLEC T ED
PH O T O N S
IN C O M IN GPH O T O NS
S
Fn n
SU R FAC E AR EA = ASO LAR PR ESSU R E = P
FT
SOLAR RADIATION FORCEON SURFACE
2.4.4.4.4: Solar Wing Positioners and Drivers
Figure 2.4.4.4.4b Solar Radiation Force on Surface
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 123Rev -, July 2001
As in any feedback implementation, the sensed data has to be processed to establish deviations from the desired attitude and generate the necessary commands to the actuators.
Such computation, involving logic switching, control law implementation, and dynamic compensation, can be accomplished by unique electronic circuitry (analog and digital), a general purpose spacecraft computer, or by dedicated microprocessors.
Typically, modern spacecraft include a flight processor that is used for many spacecraft functions, not only ACS.
2.4.4.5: Spacecraft Processors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
Introduction
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 124Rev -, July 2001
2.4.4.5: Spacecraft Processors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.5.1: Typical ACS Hardware Architecture
GYROS
SUN SENSORS
TW O-AXISEARTH SENSORS
ATTITUDEREFERENCE SENSORS
PROCESSOR
CONTROLW HEELS
THRUSTERS
TORQUE ACTUATORSSCE
DCU-B
Figure 2.4.4.5.1 Typical ACS Hardware Architecture
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 125Rev -, July 2001
2.4.4.5: Spacecraft Processors
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.5.2: Typical ACS Software Design
EphemerisEphemerisPredictionPrediction
EarthEarthSensorSensorControlControl
SunSunSensorSensorControlControl
GyroGyroControlControl
AttitudeAttitudePredictionPrediction
ThrusterThrusterControlControl
SolarSolarWing DriveWing Drive
ControlControl
MomentumMomentumWheelWheel
ControlControl
AttitudeAttitudeDeterminationDetermination
AttitudeAttitudeControlControl
ModeModeControlControl
GainGainSelectSelect
Figure 2.4.4.5.2 Typical ACS Software Design
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 126Rev -, July 2001
ACS operation must consider spacecraft operation during all aspects of its mission, including:
• Launch/Ascent
• Transfer Orbits
• Sun/Earth Acquisitions
• In-Orbit Testing
• On-station Operations
• Station Keeping and Momentum Dumping
• Safing
2.4.4.6: Operating Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
Introduction
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 127Rev -, July 2001
2.4.4.6: Operating Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.6.1: Launch and AscentDuring launch and ascent, the spacecraft ACS will typically be inactive. The launch vehicle has its own ACS equipment.
Separation from the launch vehicle is passive: the launch vehicle will orient the satellite with the sun line incident on the stowed solar panels.
The launch vehicle may also impart a slow spin to the satellite to maintain it in this orientation. This spin is usually imparted either by rotating the launch vehicle stage prior to release, or by ejecting the satellite with the springs set to uneven tensions.
While the inactive attitude control is being performed, attitude determination is usually obtained through the gyros.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 128Rev -, July 2001
2.4.4.6: Operating Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.6.2: Transfer OrbitsAfter launch, the spacecraft will undergo a series of thruster firings to switch through one or more transfer orbits. These raise the orbit and then circularize it into a geosynchronous location.
The thrusters involved include the Liquid Apogee Engine and various Reaction Control thrusters to maintain the satellite in the desired orientation throughout the procedure.
In some cases, electric propulsion thrusters are also used to gradually circularize the orbit.
Also, typically in the course of transfer orbit operations, antenna and solar array deployments are performed.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 129Rev -, July 2001
2.4.4.6: Operating Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.6.3: Sun/Earth AcquisitionsThe final stage of transfer orbit operations, prior to arrival on-station, is to perform Sun and then Earth acquisition.
Sun acquisition is initiated by slewing the wings (using solar wing drive mechanisms) to search for the sun, using the solar panels’ output current as an indicator.
Once this is accomplished, the spacecraft is then slewed to put the sun in the field of view of the sun sensor (usually digital, for higher accuracy).
Spacecraft slews are then performed within the sun sensor field of view to more accurately position the spacecraft.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 130Rev -, July 2001
2.4.4.6: Operating Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.6.4: In-Orbit TestingOnce the spacecraft is on-station, in-orbit testing is performed to ensure the spacecraft is completely operational. As part of this testing, antenna pattern measurements, sometimes called antenna cuts, are performed.
This is done by using the momentum wheels to slew the spacecraft in pitch and then roll while a ground station measures the strength of a signal it is transmitting. This verifies the pattern edges and beam shape.
Once the antenna pattern measurements have been completed, a bias value can be obtained to alter the spacecraft orientation in pitch and roll to maximize antenna performance over the desired footprint.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 131Rev -, July 2001
2.4.4.6: Operating Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.6.5: On-Station OperationsIn its normal, on-station mode, the spacecraft usually uses data from the Earth sensor or, in some cases, from RF beacon sensors, to maintain highly accurate pointing of the antennas over the coverage area.
The Earth sensor provides pitch and roll information, while the gyros are used to provide yaw data. The gyros are recalibrated once per orbit using the sun sensor, when the sun is in the proper position. The solar wing drive mechanisms are used to allow the solar panels to follow the sun.
Momentum buildup due to disturbances is stored in the momentum wheels, which maintain the desired angular momentum direction, or in a zero-momentum system, as the case may be.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 132Rev -, July 2001
2.4.4.6: Operating Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.6.6: Station Keeping and Momentum DumpingSince the momentum wheels store momentum resulting from disturbance torques, a momentum dumping maneuver must be periodically performed in order keep the wheels within the linear region of their spinning characteristics.
This is generally done by firing thrusters, and allowing the wheels to spin down at the same time.
Often, momentum dumping is performed in conjunction with stationkeeping maneuvers in order to save thruster propellant.
However, momentum dumping may be needed more frequently if momentum buildup is faster than the station keeping interval.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 133Rev -, July 2001
2.4.4.6: Operating Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.6.6: Station Keeping and Momentum DumpingStation keeping is generally performed by firing the appropriate combination of thrusters to correct drifts in East/West or North/South position.
Propellant thrusters are often used for this purpose, but electrical propulsion thrusters are also common.
The interval between station keeping maneuvers may range from one day to two weeks.
Often, the maneuvers are performed automatically, especially for the more frequent cases.
During the maneuver, momentum wheels maintain the spacecraft’s pointing accuracy.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 134Rev -, July 2001
2.4.4.6: Operating Modes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.6.7: SafingThe spacecraft must also include a safing mode that activates in the case of failures of equipment, incorrect thruster firings, or other unexpected events.
The safing mode will vary depending on spacecraft design, but as a rule it must be entered automatically from any operating mode of the ACS, during transfer orbits and on-station. Its top priorities must be to maintain communication with Earth.
The safe mode will allow the spacecraft to stay in a benign state, where fault propagation is minimized, telemetry continues to be provided to the ground, and where ground command capability is available.
In the case that Earth pointing is lost, the safing mode will immediately attempt to reacquire Earth in the shortest possible time.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 135Rev -, July 2001
ACS reliability is typically 98% after 15 years.
For both actuators and sensors, unit and subsystem integrity depends on:
• Unit redundancy and subsystem cross-strapping• Design lifetime• Operational range of the units• Fields of View of the sensors• Interference to the sensors (Sun, Moon, Spacecraft reflections)• Controlled backup modes in the event of a failure• Workmanship (why we test)• Alignment and offset of the units• End to end polarity of the sensors to actuators
2.4.4.7: Reliability and Risk
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.7.1: Reliability
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 136Rev -, July 2001
Typical Maneuvers (Control processor applies to all cases)
Transfer Orbit GNC Units RiskSpin Up / Down Thrusters / Sensors (1st) Med
Precession for Apogee burns Thrusters / Wheels / Sensors (1st) Med
Orbit Raising Single Engine High
Nutation Control Thrusters / Wheels / Pivot (1st) Med
Reorientation Thrusters / Wheels / Sensors Low
Solar Array/Ant. Deployments Mechanisms (1st) High
Sun / Earth AcquisitionsThrusters / Wheels / Sensors Med
2.4.4.7: Reliability and Risk
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.7.2: Risk
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 137Rev -, July 2001
Typical Maneuvers (Control processor applies to all cases)
Mission Operations GNC Units RiskRoll / Yaw / Pitch Control Wheels / Sensors Low
North / South Stationkeeping Thrusters / Wheels / Sensors Low
East / West Stationkeeping Thrusters / Wheels / Sensors Low
Momentum Unloading Thrusters / Wheels / Sensors Low
OtherIn-orbit Test All (1st) Med
Antenna Pattern Mapping Thrusters / Wheels / Sensors Med
2.4.4.7: Reliability and Risk
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.7.2: Risk
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 138Rev -, July 2001
The qualification of an ACS design entails analysis, simulation and testing on several different levels.
• Unit Testing
• Simulations
• Bench-level Validation
• Satellite-level Testing
• In-Orbit Testing
2.4.4.8: ACS Testing
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
Introduction
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 139Rev -, July 2001
2.4.4.8: ACS Testing
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.8.1: Unit TestingAll of the ACS units have their own unit-level test programs.
Newly-designed units undergo qualification testing.
Units with minor design modifications (from the qualified article) are subjected to protoflight testing.
Units with no changes from qualified designs are acceptance tested.
Each of these programs entails a suite of environmental and performance tests. Testing range and extent depend on whether the unit is undergoing qualification, protoflight, or acceptance testing.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 140Rev -, July 2001
2.4.4.8: ACS Testing
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.8.2: SimulationsAt the spacecraft level, simulations are performed to support mission and failure mode analysis and to determine whether the ACS’s design is sound from a system point of view.
The simulations are performed in software running on a high performance platform.
Flight dynamics simulators are run in conjunction with mathematical models of the various sensors and actuators, with the control algorithms which reside in the spacecraft control processor.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 141Rev -, July 2001
2.4.4.8: ACS Testing
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.8.3: Bench-Level ValidationOnce flight control software has been written and tested at the module level, validation testing needs to be performed at the bench level.
This is usually performed through closed-loop testing, in which a real spacecraft control processor is interfaced with simulators for sensors and actuators.
In this case, the real communications interfaces between the different spacecraft units are also tested.
This is done by ensuring that the sensor/actuator simulators are implemented in hardware platforms which replicate the real spacecraft’s electrical interfaces.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 142Rev -, July 2001
2.4.4.8: ACS Testing
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.8.4: Satellite-Level TestingSatellite-level testing for the ACS is at the level of checkouts rather than design verification.
Functional checks are performed on the various units before, during, and after each phase of spacecraft testing.
In addition to this, end-to-end polarity checks, unit alignments, and offset verifications are performed in order to verify that all equipment has been properly installed.
All redundant units are switched in and out of the operational configuration to verify the cross-strapping.
In particular, it is important to recheck alignments following major environmental tests, such as shock, vibration, and thermal vacuum, to ensure that the units are still within their fixed error tolerances.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 143Rev -, July 2001
2.4.4.8: ACS Testing
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 4: Attitude Control Subsystems
2.4.4.8.5: In-Orbit TestingFinally, with regard to in-orbit testing, most tests are functional in nature.
Of course, since most ACS capabilities need to have been exercised in order to reach the on-station location, the in-orbit ACS test is relatively brief.
Cross-strapping capability is once again verified, and alignments of the various units are derived.
Antenna pattern measurements provide fixed error values at the antenna for which the ACS can then perform a bias correction.
After several weeks to a few months of observation, it is usually possible to derive values for key ACS characteristics, such as overall pointing performance and momentum buildup, in order to compare them to the values which were predicted by analysis.
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 144Rev -, July 2001
Part OutlineIntroduction
Rocket Science
Design Approach
Technology
Risk Assessment
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 145Rev -, July 2001 Vol 2: Communication Satellites
Sec 4: Satellite Bus/Platform Subsystems
2.4.5: Propulsion
2.4.5.1 Introduction
2.4.5.2 Functional Description
2.4.5.3 Rocket Equation
2.4.5.4 Typical Propellant Budget
2.4.5.5 Design Parameters and Processes
2.4.5.6 Typical Propulsion Subsystem
2.4.5.7 Electro-Magnetic Thrusters
2.4.5.8 Electro-Static Thrusters
2.4.5.9 Electro-Thermal Thrusters
2.4.5.10 Risk Areas, Impact, and Mitigation Plans
Outline of This Part
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 146Rev -, July 2001
Part 5: Propulsion
2.4.5.1 Introduction
IntroductionIn Conjunction with the Attitude Control Subsystem, the Propulsion Subsystem provides for control of:
• Orbit Insertion
• Orbit Maintenance
• Orbit Attitude Control
• Station Relocations
• End Of Life Deorbiting
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 147Rev -, July 2001
Typical Transfer Orbit Functions Typical On Station Functions
2.4.5.1 Introduction
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.1a Typical Transfer Orbit Functions* Figure 2.4.5.1b Typical On Station Functions**
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 148Rev -, July 2001
Functional DescriptionOrbit Insertion functions consist of:• Satellite attitude, stability and orbit adjustment during transfer
orbit.
• Corrections for reorientation, spin rate, orbit dispersion (an error associated with achieving the desired orbits), East and West drift, eccentricity and inclination during drift orbit and station acquisition.
Orbit Maintenance & Attitude Control consists of: • Orbit inclination, longitudinal position, drift, eccentricity, angular
momentum and Roll and Yaw attitude corrections on station.
2.4.5.2. Functional Description
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 149Rev -, July 2001
EQ. 2.4.5.3a, Thrust
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Rocket ScienceThe basic rocket theory is based on Newton’s Third Law: “Every action has an equal and opposite Reaction”.
The two major Rocket Performance Parameters are the Thrust (F) and The Specific Impulse (Isp) .
• The Thrust is the amount of Force applied to the rocketF m.Ve [N]
m is Mass flow rate of the propellant [kg/sec]
Ve is Propellant Exhaust velocity [m/sec]
• The Specific Impulse is the ratio of the Thrust to the Weight flow rate of the propellant:
Isp F/m.g [sec]
g is 9.807 [m/sec2]
2.4.5.3. Rocket EquationPart 5: Propulsion
EQ. 2.4.5.3b, Specific Impulse
Figure 2.4.5.3a Thrust*
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 150Rev -, July 2001
Rocket Science
2.4.5.3. Rocket Equation
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.3b Rocket Science, Propellant System*
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 151Rev -, July 2001
An Example of a Spacecraft Propellant BudgetAssumed Dispersion Magnitude = 3.00 SIGMA Calculated On Orbit Operational Lifetime= 10.34 yearsAssumed Initial orbit Parameters : Inclination = 25.70 degrees; Radius of Apogee = 27437.16 km; Radius of Perigee = 6544.00 km
Kinematics Burn Efficiency Effective Change in mass or Propellant RemainingManeuver Description Delta V Length Isp Propellant Used Remaining Mass
[m/sec] [min] [0] [sec] [Kg] [Kg] [Kg]Payload Lift off Mass 2215.80 3621.6
Spacecraft Separated Weight: (Payload Mass - LV adapter) 61.00 2215.80 3560.6ORBIT INSERTION FUNCTIONS
1st Sub-synchronus Attitude Control 2.30 2213.50 3558.3Liquid Apogee Motor Firing 1 16.18 1.95 0.99 311 18.99 2194.51 3539.3Liquid Apogee Motor Firing 2 175.9 21.07 0.9758 311 203.20 1991.31 3336.1Post Firing Attitude Control 1.60 1989.71 3334.52nd Sub-synchrous Attitude Control 1.00 1988.71 3333.5Liquid Apogee Motor Firing 3 175.9 19.81 0.9778 311 191.00 1797.71 3142.5Post Firing Attitude Control 1.20 1796.51 3141.3Transfer Orbit Attitude Control 1.40 1795.11 3139.9Liquid Apogee Motor Firing 4 960.7 88.49 0.9938 311 853.41 941.70 2286.5Post Firing Attitude Control 2.40 939.30 2284.1Transfer orbit Attitude Control 1.00 938.30 2283.1Liquid Apogee Motor Firing 5 744.06 51.38 0.9979 311 495.57 442.73 1787.5Post Firing Attitude Control 5.90 436.83 1781.6Transfer Orbit Attitude Control 1.00 435.83 1780.6Liquid Apogee Motor Firing 6 71.03 4.25 1 310 41.17 394.66 1739.5Post Firing Attitude Control 0.80 393.86 1738.7PRE ON-STATION Attitude Control 2.30 391.56 1736.4Liquid Apogee Motor Firing 7 3.57 2.52 0.9334 292 2.32 389.24 1734.0ON STATION MAINTENACE and ATTITUDE CONTROL FUNCTIONS
DISPERSION CORRECTION 60.30 328.94 1673.7In Orbit Test Attitude Control 0.80 328.14 1672.9In Orbit Tests Station Keeping (SK) 4.6 3.1 0.9439 295 2.82 325.33 1670.1North South SK FIRST SIX YEARS 258.06 165.84 0.9439 295 151.06 174.27 1519.1East West SK FIRST SIX YEARS 6 3.71 0.9334 292 3.41 170.86 1515.7
2.4.5.4. Typical Propellant Budget
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 152Rev -, July 2001
List aplicablespacacraft Functions
Orbit InsertionOrbit mainteneceAttitude Control
Determine TotalImpulse for Attitude
control.
Determine delta Vand Thrust level
constraints for orbitinsertion andMaintenace
Determine Thrustlevels for control
authority, duty cyclesand mission life
requirements
Determine propulsionsubsystem options
- combined or separate- Low or High thrust- Liquid, solid, electricor plasma
Deterime level ofredundancy and
overall configurationfor each option
Estimate Key parameters foreach option
- Effective Isp and Thrust fororbit and attitude control- Propellant mass andPressurant Volume
Estimate total massand power for each
option
Qualify hardware atcomponent andsubsytem level
Finalize design andprocure/manufacture
equipment
Inetgrate intospacecraft system
level AssemblyIntegration and test
Program
Are requirementsmet? Yes
NoNo
No
Propulsion Subsystem Design Process
2.4.5.5. Design Parameters & Process
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.5a Propulsion Subsystem Design ProcessImage Courtesy of Telesat Canada
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 153Rev -, July 2001
2.4.5.5. Design Parameters & Process
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.5b Design Parameters & Process*
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 154Rev -, July 2001
Chemical ThrustersChemical Spacecraft Thrusters could be either mono propellant or bi-propellant.
Bipropellant thrusters provide higher Isp than mono-propellant thrusters.
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.6b Liquid Apogee Engine
Figure 2.4.5.6a Magellan Rocket Engine Module
Photo Courtesy of Telesat Canada
Courtesy of General Dynamics (Space
Systems)
2.4.5.6. Typical Propulsion Subsystem
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 155Rev -, July 2001
2.4.5.6. Typical Propulsion Subsystem
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.6.1a ACE Propulsion System
2.4.5.6.1: Typical Bipropellant System
Courtesy of General Dynamics (Space Systems)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 156Rev -, July 2001
2.4.5.6. Typical Propulsion Subsystem
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.6.1b Typical Bipropellant System*
2.4.5.6.1: Typical Bipropellant System
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 157Rev -, July 2001
2.4.5.6.2: Schematic• Fuel, oxidizer and
pressurant are loaded individually
• Fuel and oxidizer are hypergolic (i.e. they burn when mixed)
• Cannot launch with system pressurized because of tank design.
2.4.5.6. Typical Propulsion Subsystem
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.6.2 Schematic of Galileo Propulsion System*
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 158Rev -, July 2001
2.4.5.6.3: Monopropellant Vs Bipropellant Systems
2.4.5.6. Typical Propulsion Subsystem
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.6.3a Typical Monopropellant System* Figure 2.4.5.6.3b Typical Bipropellant System**
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 159Rev -, July 2001
2.4.5.6.4: Pressurant SystemsLiquid Apogee Motors operate in a regulated constant pressure mode to maintain high efficiency.
On-orbit chemical thrusters operate in a blow-down mode.
2.4.5.6. Typical Propulsion Subsystem
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.6.4 Pressurant Systems*
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 160Rev -, July 2001
Electro-Magnetic Thrusters
2.4.5.7. Electro-Magnetic Thrusters
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.7 Hall Current Thruster
Courtesy of General Dynamics (Space Systems)
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 161Rev -, July 2001
Electro-Static Thrusters
2.4.5.8. Electro-Static Thrusters
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.8a Electro-Static Thruster Schematic
Image Courtesy of the Associated
Plasma Laboratory (LAP), National
Space Research Institute (INPE),
Brazil
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 162Rev -, July 2001
Ion Engine
2.4.5.8. Electro-Static Thrusters
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Electro-static thrusters are also known as Ion Engines.
Figure 2.4.5.8b Ion Engine Functional Diagram
Image Courtesy of the Associated Plasma Laboratory (LAP), National Space Research Institute (INPE), Brazil
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 163Rev -, July 2001
Ion Engine
2.4.5.8. Electro-Static Thrusters
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.8c Ion Engine Firing
Image Courtesy of the Associated Plasma Laboratory (LAP), National Space Research Institute (INPE), Brazil
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 164Rev -, July 2001
Electro-Thermal Thrusters
2.4.5.9. Electro-Thermal Thrusters
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Figure 2.4.5.9bEHT(Electrically Heated
Thrusters) SystemFigure 2.4.5.9aMR-510 Arcjet System with Power Processor, 4 arcjets, and 4 cables
Two sub types of commercially available Electro-Thermal thrusters.
Courtesy of General Dynamics (Space
Systems)
Courtesy of General Dynamics (Space Systems)
Courtesy of General Dynamics (Space Systems)
Figure 2.4.5.9c MR-510 Arcjet Firing
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 165Rev -, July 2001
Risk Areas, Impact & Mitigation Plans• Rupture of fuel systems
• Impact: Catastrophic failure• Mitigation Plans: Rigorous Qualification & Test Plan
• Liquid apogee motor under-performance• Pyro valve failures• Latch valve leakage• Thruster valve failures• Thruster failures
For the last five risk areas• Impact: Degraded mission• Mitigation Plans: Rigorous Qualification & Test Plan and stringent
workmanship processes
2.4.5.10. Risk Areas, Impact and Mitigation Plans
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 5: Propulsion
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 166Rev -, July 2001
Sec 4: Satellite Bus/Platform Subsystems
Vol 2: Communication Satellites
Mechanical SubsystemsPart 6
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 167Rev -, July 2001 Vol 2: Communication Satellites
Sec 4: Satellite Bus/Platform Subsystems
2.4.6: Mechanical Subsystems
2.4.6.1 Structure
2.4.6.2 Mechanisms
Outline of This Part
Slide Number 167Rev -, July 2001
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 168Rev -, July 2001
2.4.6.1 Structure
2.4.6.1.1: IntroductionThe Mechanical Subsystem provides stable mechanical support to other subsystems and components.
Sustains all loads and environments during:• Fabrication & Transportation• Launch & Transfer Orbit• On orbit control maneuvers • Attitude control failures and
recoveries
The Mechanical Subsystem maintains dimensional stability for sensitive payload equipment.
Figure 2.4.6.1.1a Shuttle Launch
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 6: Mechanical Subsystems
Figure 2.4.6.1.1b FLTSATCOM Structure*
Photo Courtesy of Telesat Canada
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 169Rev -, July 2001
2.4.6.1.2: Design Parameters & Process
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Figure 2.4.6.1.2 Design Process
Part 6: Mechanical Subsystems
2.4.6.1 Structure
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 170Rev -, July 2001
2.4.6.1.3: Risk Areas, Impact & Mitigation PlansRisk Areas, Failures Occur due to:
• Inadequate design margins
• Poor workmanship
• Poor processes
• Overload situations
Impact of Failures & Mitigation Plans:• Mission Catastrophic Failures:
• As when the primary structure is affected, such as the main thrust tube or the launch vehicle interface region
Mitigating Plan: Robust Design margins, Rigorous Validation Plan
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 6: Mechanical Subsystems
2.4.6.1 Structure
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 171Rev -, July 2001
Impact of Failures & Mitigation Plans (cont):• Performance Degradation Type Failures
• Reduced RF performance when payload appendages are affected
Mitigating Plan: Adequate Redundancy, Rigorous Validation Plan
• Shorter life when bus appendages are affected
Mitigating Plan: Adequate Redundancy, Rigorous Validation Plan
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
2.4.6.1.3: Risk Areas, Impact & Mitigation Plans
Part 6: Mechanical Subsystems
2.4.6.1 Structure
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 172Rev -, July 2001
2.4.6.2.1: IntroductionLaunch envelope constraints and environments dictate the need for deployment mechanisms.
Mechanism & actuator key requirements:• Minimum torque margin of 200 percent (3 to 1 ratio)
• Operational capability at least 1.5 times the number of cycles anticipated through ground test and on-orbit life
• Failure tolerant (w.r.t. primary initiator actuation)
• Capability to allow for end-to-end verification in a 1 g environment
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
2.4.6.2 MechanismsPart 6: Mechanical Subsystems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 173Rev -, July 2001
2.4.6.2.2: Design ApproachDesign considerations include:
• Minimize static and dynamic disturbances during launch, deployments and on-orbit operations
• Redundancy (primarily initiators and electronics)
• Clearance and accessibility issues due to tolerance stack-up (the incremental buildup of dimensional errors as parts are assembled), thermal environments and local obstructions
• Containment of any solid debris resulting from actuation and operation
• Operational life (brushes, seals, labyrinths, lubricants)
• Verification of design and workmanship process
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
2.4.6.2 MechanismsPart 6: Mechanical Subsystems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 174Rev -, July 2001
2.4.6.2.3: TechnologyMechanism technology falls in two categories:
• Mechanized; motorized drives generally used for highly-cyclical applications such as:
• Antenna Pointing Mechanisms
• Solar Array Pointing and Tracking
• Attitude Control Reaction Wheels
• Gimbals for Propulsion units
• Passive; Springs and hinges with dampers, generally used for low cyclical applications such as:
• Antenna and Solar Array retention
• Antenna and Solar Array deployments
• Spacecraft launch vehicle separation systems
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
2.4.6.2 MechanismsPart 6: Mechanical Subsystems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 175Rev -, July 2001
2.4.6.2.4: Risk, Failures & Mitigation PlansDeployable assembly “Hangs Up”:
• Impact: Can be a catastrophic failure
• Mitigation Plans: Rigorous qualification, robust torque margins and meticulous workmanship
Continuous use over single point or thermal design limitation:• Impact: Degraded mission
• Mitigation Plans: Rigorous qualification & test plans and stringent workmanship processes
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
2.4.6.2 MechanismsPart 6: Mechanical Subsystems
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 176Rev -, July 2001
Sec 4: Satellite Bus/Platform Subsystems
Vol 2: Communication Satellites
ThermalPart 7
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 177Rev -, July 2001 Vol 2: Communication Satellites
Sec 4: Satellite Bus/Platform Subsystems
2.4.7: Thermal
2.4.7.1 Introduction
2.4.7.2 Thermal Environment
2.4.7.3 Thermal Design Process
2.4.7.4 Thermal Design Approach
2.4.7.5 Thermal Key Items
2.4.7.6 Thermal Hardware
2.4.7.7 Thermal Equipment
2.4.7.8 Thermal Controller Functions
2.4.7.9 Failures, Degradation and Margins
Outline of This Part
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 178Rev -, July 2001 Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
2.4.7.1. IntroductionPart 7: Thermal
Figure 2.4.7.1 Thermal System
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Drawing Drawing Courtesy TelesatCourtesy Telesat
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 179Rev -, July 2001
2.4.7.2. Thermal Environment
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 7: Thermal
Figure 2.4.7.2 Thermal Environment*
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 180Rev -, July 2001
2.4.7.3. Thermal Design Process
Radia tion P rope rtyVa lues
Spacecra ft O rien ta tionand A ttitude
Spacecra ft G eom etry
E lectrica l P ow erD issipa tion
T herm ophyscia l P roperty Va lues
Requ irem ents- T em pera ture L im its- S urvivab ility
Rad ia tion C om puter P rogram
Therm al Ana lyzer C om puterP rogram
(S pacecra ft The rm al M ath M odel)
The rm al C ontro l H /W E lem ents(R adia to rs , Louvers, H eaters,
T herm al B lankets)
P red ic ted T herm a l P erform ance
Com parison
R adia tion E xchange Factorsand V iew Facto rs
Rad ia tion Absorbed onExterna l Surfaces
Com ponent-Leve l T ests
System -Leve l T ests
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 7: Thermal
Figure 2.4.7.3 Thermal Design Process
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 181Rev -, July 2001
Part 7: Thermal
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
2.4.7.4. Thermal Design Approach
Figure 2.4.7.4 Thermal Design Approach
Slide Number 181Rev -, July 2001
Drawing Drawing Courtesy TelesatCourtesy Telesat
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 182Rev -, July 2001Figure 2.4.7.5 Thermal Key Items
Slide Number 182
Drawing Drawing Courtesy TelesatCourtesy Telesat
2.4.7.
5.
Therm
al Key
Items
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 183Rev -, July 2001
Thermal Hardware
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 7: Thermal
2.4.7.6. Thermal Hardware
Figure 2.4.7.6 Two Views of the LM A2100 S/C Photos Provided by Lockheed Martin
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 184Rev -, July 2001
• Heat pipe network embedded in equipment panels• North, South, Earth
• Enhanced thermal conductance joint between North-South panels
• OSR radiator surfaces on North and South panels
• Standard multi-layer insulation blankets• Black Kapton outer layer
• Germanium shields for RF sensitive items
• Heater controllers regulate temperatures
Thermal Equipment
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 7: Thermal
2.4.7.7. Thermal Equipment
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 185Rev -, July 2001
Computer controlled heaters:• Set points are ground commandable
• Temperature sensor groupings are ground changeable
• Control type (min., max., duty cycle, difference) are ground changeable
• Total ground override capability exists
• Controller commands every heater on/off state, if enabled
Bimetallic thermostats switch heater circuits on and off in response to temperature changes.
Thermal Controller Functions
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 7: Thermal
2.4.7.8. Thermal Controller Functions
Technical Introduction to Geostationary Satellite Communication Systems Original Prepared by Telesat Canada
Slide Number 186Rev -, July 2001
• Maintain all component temperatures within operational limits throughout mission life as thermal surface properties degrade, potentially resulting in failure.
• Fully redundant heater circuits with single-point-failure tolerance
• Bimetallic thermostats are configured in dual or quad redundant configurations
• 25% margin on heater output capability, or a 10oC heater sizing margin
• 5oC uncertainty margin added to raw temperature predictions
Failures, Degradation & Margin
Vol 2: Communication Satellites, Sec 4: Satellite Bus/Platform Subsystems
Part 7: Thermal
2.4.7.9. Failures, Degradation & Margin