Aucun titre de diapositiveAims
To describe the main components of the Communications satellite payload and explain how designs are impacted by the changing needs of the user
October 2004
Receive
Antenna
Transmit
Antenna
Repeater Functions
Power Amplification
Frequency Conversion
October 2004
With Gregorian or Cassegrain Sub –reflector
Gridded Reflectors for Polarisation Discrimination
Dual Gridded Assemblies for dual plane polarisation
Direct Radiating Phased Arrays
Generation of Multiple Beams from the same Aperture
Reflectors with De-focused Feed Arrays
October 2004
Provide minimum C/No degradation
Downconvert Frequency for re-transmission
Provide high reliability in functionality
Beam-to-beam interconnectivity
Functional re-configurability
Elimination of Alias Bands before and after Sampling Processes
Partitioning of Spectrum to allow Channelised Amplification
Partitioning of Spectrum for usage by Different Services
Partitioning of Spectrum for use on Different Routes
October 2004
What is special about Communications Satellites?
Inaccessibility of the orbits used
LEO – Generally highly inclined
GEO – High altitude means: High potential energy AND High kinetic energy
Either way large high energy launch vehicles required
Very expensive to launch in the first place
Inaccessible to astronauts or remote control vehicles
Repair by external intervention virtually impossible
The design must be tolerant of internal failures
October 2004
Payload Constraints
Physical size, must fit on spacecraft platform, compatibility with launch vehicle fairing
Thermal Dissipation
Mass
Power consumption
Thermal Control
Received Noise
Thermal noise
Transmitter Noise
October 2004
Quality of the Receive System – G/T
The quality of the satellite receive system, in terms of its ability to receive a given signal with a high signal to noise ratio is usually expressed as:
Ga/ Ts
Where:
Ga = Antenna Gain (Relative numerically to that of an isotropic radiator and referenced to an arbitary interface at the output of the antenna)
Ts = The Noise Temperature of the complete System (Referenced to the same interface at the output of the antenna)
October 2004
Noise Temperature
T4 / (G1.G2.G3) ……...
October 2004
October 2004
Payload Constraints
Spurious Products
Intermodulation products: Non linearity in active devices
Passive intermodulation products (PIMP): Transmit chain, post High Power Amplification
In Band: Directly impacts C/N0
Out of Band: Interference to other transponders or systems
October 2004
October 2004
Sout = aSin
Memoryless Non-linear devices can be approximated over a limited signal range by a polynomial relationship such as:
Sout = a1Sin + a2Sin2 + a3Sin3 + a4Sin4 + …
If 2 signals are applied such that:
Sin = Asinω1t + Bsinω2t
Then Sout is found to contain frequency components as follows:
ω1, ω2, (ω1 - ω2), (ω1 + ω2), 2ω1, 2ω2, (2ω1 - ω2), (ω1 - 2ω2), 3ω1, 3ω2…
October 2004
Intermodulation Products (2)
Order of a product is m = n + k for frequency nf2 - kf1 for 2 carriers
For many closely spaced carriers, IMPs are distributed contiguously
3rd order products most important in band
(C/I3) multi-carrier = (C/I3) 2carrier - 8 dB
October 2004
N=5
N=10
2F1 – F2
0.5 N(N-1)(N-2)(N-3)(N-4)
Reasons for filtering after the High Power Amplifiers
To reject Out Of Band Spurious (which might adversely affect other systems)
To reject Intermodulation Noise which would fall in adjacent channels
To reject transmit noise which would fall in receive bands on the same satellite
To provide theoretically loss less recombination of amplification channels into a single signal path prior to transmission
This is achieved using an Output Multiplexer(OMUX)
October 2004
Payload Constraints
Transmit Characteristics
AM/PM conversion
AM/PM transfer
AM modulation of one carrier transferred to PM modulation of another
October 2004
Gain v Frequency Slope followed by AM to PM Transfer
Results in Intelligible Cross Talk
Group Delay v Frequency Slope followed by AM to PM Transfer
Similar effects
October 2004
Gain Slope
October 2004
Reliability, R, defined as: (Number of Success)/(Number of Trials)
For a single mission R = Probability of the success of the mission
Failure Rate, λ, measured in failure instances in 109 hours (FITS)
For a single mission of duration of t hours:
Reliability, R, is found to be:
R = e- λT
where T = t/109
For items in a functional chain (where each link must succeed for overall success):
Failure rates add to give total failure rate
Reliabilities multiply to give overall reliability
October 2004
Probability of mission failure of an equipment is (1-R)
If a system uses 2 identical equipments in parallel, the probability of failure is the probability of both failing. This is (1-R)2
Reliability of the system is the probability of one or none failing.
This is is 1 – (1-R)2 = 2R – R2
“Cold” Redundancy
If an equipment is switched off, λ typically decreases by a factor of ten
Thus if non-active equipments are switched off reliability can be improved further
In such a situation with a choice of 1 from 2,
then RT = 11R – 10R1.1
-100 dBW
1.5 dB
+14 dBW
0.5dB
3nS
1nS
10dB
3
4
5
5
6
6
1
October 2004
Difficult to make “Future Proof”
Should not do processing onboard which could be done on the ground by reconfiguring the overall system
October 2004
Uplink to Downlink frequency mapping flexibility
Channel Bandwidth flexibility
Independent optimisation of uplink and downlink access, modulation and coding
Link advantage through isolation of uplink and downlink noise and interference effects
Data rate conversion and signal reformatting
Packet level switching
Typical Digital Processor Architecture
Rx AAF A/D DEMUX LC DBFN SWITCH FRC MUX D/A AIF SSPA
D/C D/C U/C
C-Band
Downlink
156
156
C-Band
Payload
Transmit
Section
2
12
12
2
October 2004
Payload Equipment – Dual Travelling Wave Tube Amplifier (TWTA) Direct Thermally Radiating Type
October 2004
October 2004
October 2004
October 2004
8
9
10
11
12
13
14
15
1E-7
1E-6
1E-5
1E-4
1E-3
1E-2
4-PSK
16-QASK
8-DPSK
16-PSK
0
0.1
0.2
0.3
0.4
0
0.5
1
1.5
2
4-PSK
8-DPSK
16-QASK
16-PSK
type