Aviation Communication, Navigation, and Surveillance (CNS)
description
Transcript of Aviation Communication, Navigation, and Surveillance (CNS)
Aviation Communication, Navigation, and Surveillance (CNS)
Instructor: Dr. George L. Donohue
Prepared by: Arash Yousefi Spring 2002
Summary Chapter 1: Introduction to
CNS Chapter 2: The
Navigation Equations Chapter 3: Terrestrial
Radio-Navigation Systems Chapter 4: Satellite Radio
Navigation Chapter 5: Terrestrial
Integrated Radio Communication-Navigation Systems
Chapter 6: Air-Data Systems
Chapter 7: Attitude and Heading References
Chapter 8: Doppler and Altimeter Radars
Chapter 9: Mapping & Multimode Radars
Chapter 10: Landing Systems
Chapter 11: Data Links and digital communication
Chapter One
Introduction
Definitions Navigation: the determination of the position and
velocity of a moving vehicle. The process of measuring and calculating state vector onboard
Surveillance or Position Reporting: the process of measuring and calculating state vector out side the vehicle
Navigation sensor: may be located in the vehicle, in another vehicle, on the ground , or in space
Vx
Vy
Vz
ZY
X
Six- component state vector
zv
yv
xv
V
z
y
x
Definitions Automatic Dependent
Surveillance(ADS): reporting of position, measured by sensors in an aircraft, to a traffic control center.
Guidance: handling of the vehicle. Two Meanings;1. Steering toward a destination of known
position from the aircraft’s present position2. Steering toward a destination without
explicitly measuring the state vector (mostly military arcfts)
Categories of Navigation1. Radio Systems: consist of a network of
transmitters(sometimes also receivers) on the ground, satellite or on other vehicle.
2. Celestial Systems: compute position by measuring the elevation and azimuth of celestial bodies relative to the navigation coordinate frame at precisely known times.
3. Mapping Navigation Systems: observe images of the ground, profile of altitude, or other external features.
Dead-reckoning navigation systems Derive their state vector fro, a
continuous series of measurements relative to an initial position. Two kinds:
1. Acft heading & either speed or acceleration. Gyroscopes or magnetic compassesheading Air-data sensors or Doppler radar speed Inertial sensorsvector acceleration
2. Emissions from continues-wave radio stations
Create ambiguous “lanes” that must be counted to keep track of coarse position
The Vehicle (1)
1. Civil Aircraft: mostly operate in developed areas(Ground-based radio aids are plentiful)
Air Carriers: large acft used on trunk routes and small acft used in commuter service.
General Aviation(GA): range from single-place crop dusters to well-equipped four-engine corporate jets.
The Vehicle (2)
2. Military Aircraft Interceptors & combat air patrol: small, high-climb-
rate protecting the homeland Close-air support: mid-size to deliver weapons in
support of land armies Interdiction: mid-size and large acft to strike behind
enemy lines to attack ground targets Cargo Carriers: same navigation requirements as civil
acft Reconnaissance acft: collect photograph Helicopter & short take of and landing(STOL) vehicle Unmanned air vehicle
The Vehicle (3)
Fig 1.1
Avionics Placement on multi-purpose transport
Phases of Flight Takeoff Terminal Area En-Route Approach Landing Surface Weather
Navigation Phases
Navigation Phases
Picture courtesy of MITRE Corporation
Takeoff Navigation From taxiing into runway to climb out Acft is guided along the runway centerline
by hand-flying or a coupled autopilot based on steering signals
Two important speed measurements are made on the runway The highest ground speed at which an aborted takeoff is
possible pre-computed and compared, during the takeoff run, to the actual ground speed as displayed by navigation system
The airspeed at which the nose is lifted is pre-calculated and compared to the actual airspeed as displayed by the air-data system
Terminal Area Navigation1. Departure: begins from maneuvering out the
runway, ends when acft leaves the terminal-control area
2. Approach: acft enters the terminal area, ends when it intercepts the landing aid at an approach fix
Standard Instrument Departure (SIDs) & Standard Terminal Approach Route (STARs)
Vertical navigation Barometric sensors Heading vectors Assigned by traffic controller
En Route Navigation Leads from the origin to the destination and
alternate destinations Airways are defined by navaids over the land
and by lat/long over water fixes The width of airways and their lateral separation
depends on the quality of the navigation system From 1990s use of GPS has allowed precise
navigation In the US en-route navigation error must be less
than 2.8 nm over land & 12 nm over ocean
Approach Navigation Begins at acquisition of the landing aid until the
airport is in sight or the acrft is on the runway, depending on the capabilities of the landing aid
Decision height (DH): altitude above the runway at which the approach must be aborted if the runway is not in sight The better the landing aids, the lower the the DH DHs are published for each runway at each airport An acrft executing a non precision approach must
abort if the runway is not visible at the minimum descent altitude (typically=700 ft above the runway)
Landing Navigation Begins at the DH ends when the acrf
exits the runway Navigation may be visual or
navigational set’s may be coupled to a autopilot
A radio altimeter measures the height of the main landing gear above the runway for guiding the flare
The rollout is guided by the landing aid (e.g. the ILS localizer)
Missed Approach Is initiated at the pilot’s option or at the
traffic controller’s request, typically because of poor visibility. And alignment with the runway
The flight path and altitude profile are published
Consists of a climb to a predetermined holding fix at which the acrf awaits further instructions
Terminal area navaids are used
Surface Navigation Acrf movement from the runway to
gates, hanger Is visual on the part of the crew,
whereas the ground controller observes acrf visually or with surface surveillance radar
GPS reports from acrfs that concealed in radar shadows reduce the risk of collision
Weather Instrument meteorological
conditions (IMC) are weather conditions in which visibility is restricted, typically less than 3 miles
Acft operating in IMC are supposed to fly under IFR
Design Trade-Offs (1)
1. Cost Construction & maintenance of transmitter
stations Government Concern Purchase of on-board HW/SWUser Concern
2. Accuracy of Position & velocity Specified in terms of statistically distribution
of errors as observed on a large # of flights Civil air carrier Based on the risk of collision
Landing error depends on runway width, acft handling characteristics, flying weather
)10( 9
Design Trade-Offs (2)
3. Autonomy:The extent to which the vehicle determines its own position & velocity without external aids. Subdivided to; Passive self-contained systems neither receive nor
transmit electromagnetic signals (dead-reckoning systems such as inertial navigators
Active self-contained systems Radiate but do not receive externally generated signals(radars, sensors). Not dependent on existence of navigation stations
Design Trade-Offs(3) (continue form
previous slide)
Natural radiation receivers i.e. magnetic compasses, star trackers, passive map correlators
Artificial radiation receivers measure electromagnetic radiation from navaids(earth or space based) but do not transmit (VOR, GPS)
Active radio navaidsexchange signals with navigation stations(i.e. DME, collision-avoidance systems). The vehicle betrays its presence by emitting & requires cooperative external stations. The least autonomous of navigation systems
Design Trade-Offs (4)
Latency Time delay in calculating position & velocity,
caused by computational & sensor delays Can be caused by computer-processing
delays, scanning by a radar beam, or gaps in satellite coverage
Geographic coverage Terrestrial radio systems operating below
approximately 100 KHz can be received beyond line of sight on earth; those operating above 100 KHz are confirmed to line of sight
Design Trade-Offs (5) Automations
The crew receive a direct reading of position, velocity, & equipment status, without human intervention
Availability The fraction of time the system is usable Scheduled maintenance, equipment failure, radio-
propagation problems i.e 0.99 HRS Outage/YR for voice communication
System capacity Reliability Maintainability
Design Trade-Offs (6)
Ambiguity The identification, by the navigation system,
of two or more possible positions of the acft, with no indication of which is correct
Integrity Ability of the system to provide timely
warning to acft when its error are excessive For en-route an alarm must be generated
within 30sec of the time a computed position exceeds its specified error
Evolution of Air Navigation
1922 ATC begins
1930 Control Tower
1935, an airline consortium opened the first Airway Traffic
Control Station
Airway Centers
1940s Impact of radar
1960s & 70s
ADS-B GPS
Page 11-15 Katon, Fried
Integrated Avionics Subsystems (1)
1. Navigation2. Communication
intercom among the crew members & one or more external two-way voice & data links
3. Flight control Stability augmentation & autopilot The former points the airframe & controls its oscillations The latter provides such functions as attitude-hold,
heading-hold, altitude hold
4. Engine control The electronic control of engine thrust(throttle
management)
Integrated Avionics Subsystems (2)
5. Flight management Stores the coordinates of en-route
waypoints and calculates the steering signals to fly toward them
6. Subsystem monitoring & control Displays faults in all subsystems and
recommends actions to be taken
7. Collision-avoidance Predicts impending collision with other acft
or the ground & recommends an avoidance maneuver
Integrated Avionics Subsystems (3)
8. Weather detection Observes weather ahead of the acft so
that the route of flight can be alerted to avoid thunderstorms & areas of high wind shears
Sensors are usually radar and laser
9. Emergency locator transmitter(ELT) Is triggered automatically on high-g
impact or manually Emit distinctive tones on 121.5, 243, and
406 MHz
Architecture (1)
Displays; Present information from avionics to the pilot Information consists of vertical and horizontal
navigation data, flight-control data (e.g. speed and angle of attack), and communication data (radio
frequencies)
Architecture (2) Flight controls;
The means of inputting information from the pilot to the avionics
Traditionally consists of rudder pedals and a control-column or stick
Switches are mounted on the control column, stick, throttle, and hand-controllers
Architecture (3)
Computation; The method of processing sensor data Two extreme organizations exist:
1. Centralized; Data from all sensors are collected in a bank of central computer in which software from several subsystems are intermingled
2. Decentralized; Each traditional subsystem retains its integrity
Architecture (4)
Data buses Copper or fiber-optics paths among sensors,
computers, actuators, displays, and controls Safety partitioning
Commercial acft sometimes divide the avionics to;1. Highly redundant safety-critical flight-control system2. Dually redundant ,mission-critical flight-management
system3. Non-redundant maintenance system
Military acrft sometimes partition their avionics for reason other than safety
Architecture (5)
Environment Avionics equipment are subject to;
acft-generated electricity-power transient, whose effects are reduced by filtering and batteries,
externally generated disturbances from radio transmitters, lightening, and high-intensity radiated fields
The effect of external disturbances are reduced by
shielding metal wires and by using fiberoptic data buses
add a Faraday shielding to meal skin of the acft
Architecture (6)
Standards Navaid signals in space are standardized by
ICAO Interfaces among airborne subsystems, within
the acft, are standardized by Aeronautical Radio INC. (ARINC), Annapolis Maryland, a nonprofit organization owned by member airlines
Other Standards are set by: Radio Technical Commissions for Aeronautics,
Washington DC European Organization for Civil Aviation Equipment
(EUROCAE) etc.
Human Navigator Large acft often had (before 1970) a third
crew member, flight engineer: To operate engines and acft subsystems e.g.
air conditioning and hydraulics) Use celestial fixes for positioning
Production of cockpits with inertial, doppler, and radio equipments facilitated the automatically stations selection, position/waypoint steering calculations and eliminated the number of cockpit crew to two or one.
Chapter Two
The Navigation Equations
Data resources The navigation equations
describe how the sensor outputs are processed in the on-board computer in order to calculate the position, velocity, and attitude.
contain instructions & data and are part of the airborne software. The data is stored in read-only (ROM) at the time of manufacturing
Mission-dependent data (e.g. waypoints) are either loaded from cockpit keyboard or a cartridge (data-entry device)
Acrft navigation system The system utilizes three types of sensor
information1. Absolute position data from radio aids, radar
checkpoints, and satellites2. Dead-reckoning data, obtained from inertial,
Doppler, or air-data sensors, as a mean of extrapolating present position
3. Line-of-sight directions to stars, which measure a combination of position & attitude errors
The navigation computer combines the sensor information to obtain an estimate of acft’s position, velocity, and attitude.
System Hierarchy
Time to go
Range, bearing to displays, FMS
Steering signals to autopilot
Star line of sight
Dead-reckoning
computations
Positioning computatio
ns
Celestial equations
•Positioning sensors
•Radio(VOR, DME, Loran, Omega)
•Satellite (GPS)
•Radar
•Inertial air data
•Doppler
Most probable position
computation
Course
computations
Heading attitude
Way points
Position data
•Position
•Velocity
•Attitude
Position
Velocity
To map display
To weapon computers
To cockpit display pointing sensorAttitud
e
Block diagram of an aircraft navigation
system
Geometry of The Earth (1)
Apparent gravity field g = the vector sum of the gravitational and centrifugal fields
G = Newtonian gravitational attraction of the earth = inertial angular velocity of the earth(15.04107 deg/hr
g = apparent gravity field
)R(ΩΩGg
Ω
Geometry of The Earth (2)
For navigational purposes, the earth’s surface can be represented by an ellipsoid of rotation around the Earth’s spin axis
The size & shape of the best-fitting ellipsoid is chosen to match the sea-level equipotential surface.
Geometry of The Earth (3)
Fig 2.2
Median section of the
earth, showing the reference ellipsoid &
gravity field
Coordinate Frames (1)
The position, velocity and attitude of the aircraft must be expressed in a coordinate frame.
Navigation coordinate
frame
1. Earth-centered, Earth-fixed (ECEF): The basic coordinate frame for navigation near the Earth
Origin is at the mass center of earth y1, y2 Lie in True equator y2 Lies in the Greenwhich meridian y3 Lies along the earth’s spin axis
2. Geodetic spherical coordinates: Spherical coordinates of the normal to the reference ellipsoid.
Z1 longitude Z2 geodetic latitude Z3 altitude h above the reference ellipsoid This system is used in maps and mechanization of dead-
reckoning and radio navigation systems.
Coordinate Frames (2)
Coordinate Frames (3)
3. Geodetic wander azimuth: Locally level to the reference ellipsoid
Z3 is vertical up Z2 points at an angle , west of true north. Z1 points at an angle , north of true east Most commonly used in inertial navigation
Dead-Reckoning Computation (1)
DR is the technique of calculating position from measuring of velocity.
It is the means of navigation in the absence of position fixes and consists in calculating the position (the zi-coordinates) of a vehicle by extrapolating (integrating) estimated or measured ground speed.
Prior to GPS, DR computations were the heart of every automatic navigator.
Dead-Reckoning Computation (2)
In simplest form, neglecting wind:
Where:
dtVxxwVV
dtVyywVV
t
eastTgeast
t
northTgnorth
0
0
0
0
,sin
,cos
T
T
g
W
V
xxyy
00 , east & north distances traveled during the measurement interval
Ground speed
True heading
Angle between acft heading and true north
Dead-Reckoning Computation (3)
Fig 2.4
Dead-Reckoning Computation (4)
In the presence of a crosswind the ground-speed vector does not lie along the acft’s center line but makes an angle with it
The drift angle can be measured with a Doppler radar or a drift sight (a downward-pointing telescope whose reticle can be rotated by the navigator to align with the moving ground)
Dead-Reckoning Computation (5)
In the moving air mass:
Where:
Then:
eastwindTTASeast
northwindTTASnorth
VVV
VVV
)sin()sin(
)cos()cos(
TASV
The pitch angle
True airspeed
Sideslip angle
dtVxx
dtVyy
t
east
t
north
0
0
0
0
Positioning (1)
Radio Fixes: There are five basic airborne radio measurements:
1. Bearing: The angle of arrival, relative to the airframe, of a radio signal from an external transmitter. It is measured by difference in phase or time of arrival at multiple sensors
2. Phase: The airborne receiver measures the phase difference between continuse-wave signals emitted by two stations using a single airborne antenna
Positioning (2) (Radio Fixes Cont.)
3. Time difference: The airborne receiver measures the difference in time of arrival between pulses sent from two stations.
4. Two-way range: The airborne receiver measures the time delay between the transmission of a pulse and its return from an external transponder at a known location
5. One-way range: The airborne receiver measures the time of arrival with respect to its own clock
Positioning (3) Line-of-Sight distance measurements
Acft near the surface of the earth at and a radio station that may be near the surface or in space, at The slant range, | |from the acft to the station could be measured by one-way or two-way ranging
0RsiR
0RR si
Positioning (4) Assume an
acft position
Calculate the exact distance and azimuth to each radio transmitter using ellipsoid
Earth equation
Calculate the predicted propagation time & time of
arrival
The probable position is the assumed position, offset by the
vector sum of the time difference, each in the direction
of its station, converted to distance
Calculate the difference between the measured and predicted time of arrival to
each station
Measure the time of arrival using the acft’s own clock Assume a new acft position and
iterate until the residual is within the allowed error
Ground-Wave One-Way Ranging: Loran and Omega waves propagate along the curved surface of the earth. With a sensor, an acft can measure the time of arrival of the navigation signal from two or more two or more station & compute its own position
Positioning (5) Ground wave Time-differencing: An acft can
measure the difference in time of arrival of Loran & Omega signals from two or more station
Assume an acft position
Calculate the exact range and azimuth from the assumed position to each observed
radio station using ellipsoid Earth equation
Calculate the predicted propagation time allowing fir
the conductivity of the intervening Earth’s surface and
the presence of the sunlight terminate between the acft and
the station
Subtract the measured and predicted time differences to
the two stations
Measure the difference in time of arrival of the signals from
the two stations
Subtract the times to two station to calculate the predicted difference in
propagation time
Calculate The time-difference gradients from which is
calculated the most probable position of the acft after the
measurements
Iterate until the residual is smaller than the allowed error
Positioning (6)
Terrain-Matching Navigation: These nav. sys. obtain occasional updates when the acft over flies a patch of a few square miles, chosen for its unique profile. A digital map of altitude above sea level, is
stored for several parallel tracks The acft measures the height of the terrain
above sea level as the difference between barometric altitude and radar altitude.
Each pair of height measurements & the dead-reckoning position are recorded & time taged
sh
Positioning (7) (Terrain-Matching
Navigation) After passing over the patch, acft uses its
measured velocity to calculate the profile as a function of distance along track between the measured and stored profile and calculates the cross-correlation function between the measured and stored profiles
)(xhm
)(ms
Terrain-Matching Navigation (1)
Fig 2.6
Parallel tracks
through terrain patch
Terrain-Matching Navigation (2)
Fig 2.7
dxxhxhnA
smms )()()(0
Where: A= length of map patch, the integration is long enough (n>1),
radarbaros hhh
Measurement of terrain
altitude
Course Computation (1)
Range & Bearing Calculation: is to calculate range and bearing from an acft to one or more desired waypoints, targets, airports, checkpoints, or radio beacons.
Best-estimate of the
present position of
acft
Course computatio
n
Computed range & bearing to other
vehicle subsystems
Course Computation (2)
Fig 2.8
t
tT yy
xxB
yyxxD
arctan
)()( 21
21
Course Computation (3)
Airway Steering: It calculates a great circle from the takeoff point(or from a waypoint) to the destination (or another waypoint).
The acft steered along this great circle by calculating the lateral deviation L from the desired great circle and commanding a bank angle:
The bank angle is limited to prevent excessive control commands when the acft is far of course. Near the destination, the track is frozen to prevent erratic steering
As the acft passes each waypoint, a new waypoint is fetched, thus selecting a new desired track. The acft can then fly along a series of airways connecting checkpoints or navigation station
LdtKLKLKc 321
Course Computation (4)
Area Navigation: Between 1950-1980, acft in developed countries
flew on airways, guided by VOR bearing signals Position along the airway could be determined at
discrete intersections using cross-bearings to another VOR( )
In 1970s DME, collocated with VOR, allowed acft to determine their position along the airway continuously. Thereafter authorities allowed them to fly anywhere with proper clearance a technique called RNAV (random navigation) or area navigation
Course Computation (5) Area Navigation
Plan view of area-navigation fix
Measure ρ1, ρ2 (distances to DME stations V1, V2)
Triangle P1V1V3 Position
P1
Course Computation (6)
Area Navigation RNAV uses combinations of VORs and DMEs
to create artificial airways either by connecting waypoints defined by lat/long or by triangulation or tri-lateration to VORTAC stations(doted lines to A1)
The on-board flight-management or navigation computer calculates the lateral displacement L from the artificial airway and the distance D to the next waypoint A1 along the airway
Course Computation (7)Assume P1
based on prior nav. information
Calculate ρ1, ρ2 using the range equation
Correct the measures ranges for the altitudes of acft and
DME station
End
Subtract the measured & calculated ranges
)()(
)()(
333
111
calculatedmeasured
calculatedmeasured
Estimate ρ1 along the vector
whose components along and are and
k
3131
i Is small
enough
i Is not
small enough
Area Navigation: An artificial airway is defined by the points A1 and A2. D and L are found interatively:
Digital Charts1. Visual charts: Showing terrain, airports, some
navaids and restricted areas.2. En-route instrument chart: Showing airways,
navigation aids, intersections, restricted areas, and legal boundaries of controlled airspace.
3. Approach plates, SIDs and STARs: Showing horizontal and vertical profile of pre-selected paths to and from the runway, beginning or ending at en-route fixes. High terrain and man-made obstacles are indicated. Missed approach to a holding fix are described visually
Chapter Three
Terrestrial Radio-Navigation Systems
General Principles
1. Radio Transmission and Reception
If an antenna with length of L is placed in space and excited with an alternating current with wave length of λ and;
If L=λ /2 then almost all the applied AC power
will be radiated into space Modular Transmitt
erReceive
rProcesso
r
Display of data bus interface
Elementary radio-navigation system
Radio Frequencies
Name Abbreviation
Frequency
Frequency Wave length
Very low VLF 3 to 30 kHz 100 to 10km
Low LF 30 to 300 kHz 10 to 1km
Medium MF 300 to 3000 kHz 1km to 100 m
High HF 3 to 30 MHz 100 to 10m
Very high VHF 30 to 300 MHz 10 to 1cm
Ultrahigh UHF 300 to 3000 MHz 1m to 10cm
Super high
SHF 3 to 30 GHz 10 to 1cm
Extremely high
EHF 30 to 300 GHz 10 to 1mm
Free Space Rules (1)
Regardless of frequency, the following rules apply in free space.
1. The propagation speed of radio waves in a vacuum=speed of light (300k km/sec)
2. The receiver energy is a function of the area of the receiving antenna. R=the range between antenna in the same units as for antenna area 2R4
area antennaReceiver power dTransmitte
powerReceiver
Free Space Rules (2)
3. Multiple antennas may be used at both ends of the path to increase the effective antenna area. Increase in area produce an increase in directivity or gain and result in more of the transmitted power reaching the receiver.
gain(G) in the direction of maximum response=directivity(D) * efficiency
Maximum effective aperture=effective area of an antenna=
A transmitter of power P & antenna gain G has effective radiated power (ERP) of PG along its axis of maximum gain
4/D
uency)light/freq of speed (thenght wavele
antennasbetween range
antenna ing transmittof area effective
antenna receiving of area effective
power dTransmittepower Received
22
R
A
AR
AA
t
r
tr
Free Space Rules (3)
4. The minimum power that a receiver can detect is referred to as its sensitivity. Where unlimited amplification is possible, sensitivity is limited by the noise existing at the input of receiver. Noise types;
1. External. Due to other unwanted transmitters, electrical-machinery interference, atmospheric noise
2. Internal. Depending on the state of the art and approaching, as a lower limit, the thermal noise across the input impedance of the receiver
Free Space Rules (4)
5. The minimum bandwidth occupied by the system is proportional to the information rate.
1. To assess the free-space range of a radio system, it is necessary to have at least the following facts:
1. Transmitter power and antenna gain2. Receiver antenna gain and noise figure3. The effective bandwidth of the system4. The effect on the system performance of external
or internal noise
Free Space Rules (5) Required radio transmitter power of a radio system as a
function of key system parameters
loss antennapath n propagatio
gain antennareceiver
gain antennamitter trans
)modulationfrequency (e.g. spreadingbandwidth and
mehod modulation toduefactor t improvemen noise
figure noisereceiver
receiverin ratio noise-to-signal required )/(
receiverin power noise
poweritter transm
(dB) log10
P
R
T
N
REQ
N
T
NRTPREQN
T
L
G
G
F
NF
NS
P
P
FGGNFLN
S
P
P
It is assumed that the polarization of the transmitting & receiving antenna are the same
Free Space Rules (6)
The radiation pattern from half-wave wires is a maximum along their perpendicular bisectors & a minimum along the axis of the wirethe equisignal pattern forming a “doughnut”
Propagation & noise characteristics
In free space, all radio waves, regardless of frequency, are propagated in straight lines at the speed of light.
Along the surface of the earth: About 3 MHz appreciable amount of energy follows the
curvature of the earth. Ground wave Up to about 30 MHz, appreciable energy is reflected
from the ionosphere. Sky wave
Ground Wave Normally received when listening to a
standard AM broadcast transmitter Dependent on several factors:
1. Conductivity and dielectric constant of the earth 2. At low frequencies, it is physically difficult to
construct a vertical transmitting antenna large enough to be half a wavelength
3. In most parts of the world & at most times of the years, atmospheric noise at low frequencies is so much greater than receiver noise that additional transmitter power must be used
Ground Wave(continue from previous slide)
4. A characteristic of ground waves is that their propagation velocity is not entirely constant
5. At low frequencies they offer the only long-range radio communication means to vehicle that are not dependent on the ionosphere or airborne or satellite-borne relay station
Sky Wave (1) Ionosphere:
between 50 & 500 km above the earth’s surface, radiation from the sun produces a set of ionized layers
Acts as a refractive medium; when the refractive index is high
At A: the radio wave strikes the refractive layer at too steep angle and continues to space
At B:the radio wave strikes at a oblique, is bent sufficiently and travels somewhat parallel to the earth
At C: the wave arrives at the refractive layer with glancing incidence & immediately returns to earth
At D: the refractive index is too low in relationto frequency to seriously deflect the radio wave. Travels on out to space& happens at frequencies above 30 MHz
Transmitter Receiver
Skip distance at F1
IonosphereA B DC
F2
F4
F3
F1
Sky Wave (2)
Maximum usable frequency: maximum frequency that for a given distance & degree of ionization a signal returns to earth
Skip distance: The distance that a given signals returns to earth
If more than one ionizing layer are present, there may be various skip distances for the same frequency
Sky wave vs. Ground wave At those frequencies and distances where
ionospheric reflection occurs, the attenuation of the radio signals is only that due to the spreading out of the power over the surface of the earth and is, consequently, proportional to distance.
Ground wave attenuation is very much greater, except at the lowest frequencies
Distance from transmitter
Rece
ived s
ignal
stre
ngth
Sky waveGround wave
How the signal level produced at the receiver by the two types of transmitter is
look like, at the frequencies around 1 MHz
Line-of-Sight Waves (1) Above approximately 30 MHz, propagation follows
the free-space laws. The transmission path is predictable, and the wavelengths are so short as to readily permit almost any desired antenna structure
From approximately 100 MHz to 3 GHz, the transmission path is highly predictable and is unaffected by the time of the day, season, precipitation, or atmospherics.
Above 3 GHz, absorption & scattering be precipitation & by the atmosphere begin to be noticed, and become limiting factors above 10 GHz
Line-of-Sight Waves (2)
A receiver, at a point in space, receives a direct ray from transmitter and a reflected ray from the ground
Because of the short wavelength, the path difference is sufficient to cause addition or cancellation as the receiver moves up & down in elevation.
Deep nulls, of vertically zero signal strength, are produced at those vertical angles at which the direct wave path & the reflected wave path differ by exactly an odd multiple of half-wavelength
Direct
signal
With
counterp
oise
Wit
hout
counte
rpois
e
Vertical reflection path
Null
Lobe
With counterpois
e
Without counterpois
e
Line-of-Sight Waves (3) Maxima of signal strength occur where the two
path lengths produce in-phase signal The number of nulls per vertical degree of
elevation increases with the height of the antenna & frequency
Line-of-sight systems on the earth are subject to the limitations of the horizon
Beyond the line of of sight, signal strength at these frequencies drops off almost as suddenly as does visible light when passing from day to night. Very large powers & antenna gains are needed and such systems don’t have much value in aircraft CNS systems
Line-of-Sight Waves (4)
Line-of-sight range
Position determination methods
Fig4.8
Common geometric position fixing
scheme
Direction finding (1) Ground-based direction-finders: Take bearings
on airborne transmitter & then advise the acft of its bearing from the ground station. The operation is time cumbersome & time-
consuming, and requires an airborne transmitter & communication link
Airborne direction-finders & homing adaptors: Take bearings on ground transmitter and typically can afford only the simplest of systems and must tolerate large errors. Direction-finding continues to be used as a backup
aid to more accurate systems
Direction finding (2) Loop antenna Direction-Finder Principles: No longer
in production but is principles still apply to the current generation of equipments Measures the differential distance to a transmitter from
two or more known points Is a rectangular loop of wire whose inductance is resonated
by a variable capacitor to the frequency to be received The signal is assumed to be vertically polarized & it
induces voltage in the arms AB & CD Currents in AB&CD are equal in amplitude & phase when
the plan of the loop is 90deg to the direction of arrival of the signal (null position)
Physically rotating the loop to the null position indicates the direction to the transmitting station
Loop Antenna Direction Finding Fig 4.9-
Direction finding
loop
Airborne VHF/UHF Direction-Finder Systems VHF equipment used by Coast Guard for air-sea
rescue on the 225 to 400 MHz communication band on the distress frequency of 343 MHz Equipment designed only for hominguse a fixed-
antenna system that generates two sequentially switched cardioid patterns whose equisignal crossover direction is found by turning the acft toward transmitting station
Equipment designed for both direction finding and hominguses a rotating antenna that generates a similar pair of cardioid patterns, whose equisignal crossover direction is found
Civil-aviation communication118-156 MHz, Military-aviation communication 225-400 MHz
Non directional Beacons Aircraft use radio beacons to aid in finding the initial
approach point of an instrument landing system as well as for nonprecision or precision approach systems
Operating in the 200 to 1600 kHz, they have output power ranging from as low as 20 watts up to several kilowatts
They are connected to a single vertical antenna & produce a vertical pattern
Cone of silence
Nondirectional beacon, vertical pattern
Marker Beacons (1)
Each beacon generates a fan-shaped pattern, the axis of the fan being at right angles to the airway
Operate at 75 MHz & radiate a narrow pattern upward from the ground, with little horizontal strength, so that interference between marker beacons is negligible
Fig 4.12
Fan-marker pattern
Marker Beacons (2)
Fig 4-13
Fan-marker pattern
VHF Omnidirectional Range(VOR) (1) Adopted for voice communication & navigation The VOR operates in 108 to 118 MHz band,
with channels spaced 100 kHz apart The ground station radiates a cardioids pattern
that rotates at 30rps, generating a 30 Hz sine wave at the airborne receiver. Ground station also radiates an omnidirectional signal, which is frequency modulated with a fixed 30 Hz reference tone. There is no sky-wave contamination at very high frequency & no interference from stations beyond the horizon, performance is relatively consistent
VHF Omnidirectional Range(VOR) (2)
Transmitter Characteristics VOR adapted horizontal polarization, even though
acft VHF communication uses vertical polarization. Each radiator in the ground station transmitter is an Alford loop. The Alford loop generates a horizontally polarized signal having the same field pattern as a vertical dipole
Fig 4.14
Alford loop
VOR Block Diagram Fig 4.15
VHF Omnidirectional Range(VOR) Receiver characteristics
The airborne equipment comprises a horizontally polarized receiving antenna & a receiver. This receiver detects the 30 Hz amplitude modulation produced by the rotating pattern & compares it with
the 30 Hz frequency-modulated reference. Fig 4.16
Doppler VOR Doppler VOR applies the principles of wide antenna aperture to
the reduction of site error The solution used in US by FAA involves a 44-ft diameter circle
of 52 Alford loops, together with a single Alfrod loop in the center
Reference phaseThe central Alford loop radiates an omnidirectional continuous wave that is amplitude modulated at 30 Hz
The circle of 52 Alford loops is fed by a capacitive commutator so as to simulate the rotation of a single antenna at a radius of 22ft
Rotation is at 30rps, & a carrier frequency 9960 Hz higher than that in the centeral antenna is fed to the commutator
With 44-ft diameter & a rotation speed of 30 rps, the peripheral speed is on the order of 1400 meters per second, or 480 wavelengths per second at VOR radio frequencies
Distance-Measuring Equipment (DME) (1)
DME is a internationally standard pulse-ranging system for acft, operating in the 960 to 1215 MHz band. In the US in 1996, there were over 4600 sets in use by scheduled airlines and about 90,000 sets by GA DME
Operation
Distance-Measuring Equipment (DME) (2)
The acft interrogator transmits pulses on one of 126 frequencies, spaced 1 MHz apart, in the 1025 to 1150 MHz band. Paired pulses are used in order to reduce interference from other pulse systems. The ground beacon(transponder) receives these pulses & after a 50 sec fixed delay, retransmits them back to the acft. The airborne automatically compares the elapsed time between transmission and reception, subtracts out the fixed 50 sec delay, & displays the result ona meter calibrated in nautical miles.
Hyperbolic Systems Named after the hyperbolic lines of
position (LOP) that they produce rather than the circles Loran-C Omega Decca Chayka
Measure the time-difference between the signal from two or more transmitting station
Measure the phase-difference between the signal transmitted from
pairs of stations
Long-Range Navigation(Loran) A hyperbolic radio-navigation system
beginning before outbreak of WW II1. Uses ground waves at low frequencies, thereby
securing an operating range of over 1000 mi, independent of line of sight
2. Uses pulse technique to avoid sky-wave contamination
3. A hyperbolic systemit is not subject to the site errors of point-source systems
4. Uses a form of cycle (phase) measurements to improve precision
All modern systems are of the Loran-C variety
Long-Range Navigation(Loran-C) (1) Is a low-frequency radio-navigation aid operating in the
radio spectrum of 90 to 110 kHz Consists of at least three transmitting stations in groups
forming chains Using a Loran-C receiver, a user gets location information
by measuring the very small difference in arrival times of the pulses for each Master -Secondary pair
Each Master-Secondary pair measurement is a time difference. One time difference is a set of points that are, mathematically, a hyperbola. Therefore, position is the intersection of two hyperbolas. Knowing the exact location of the transmitters and the pulse spacing, it is possible to convert Loran time difference information into latitude and longitude
Loran-C (2)
Signal shapeSignal shape
Position Position determinationdetermination
Loran-C (2)
Omega (1) Eight VLF radio navigation transmitting stations trough
out the world1. Continuous-wave (CW) signals transmitted on four common
frequencies, and 2. One station unique frequency
Sub-ionosphere They are propagated between the earth’s surface and the
ionosphere VLF signal attenuation is low Omega signals propagate
to great ranges (typically 5000 to 15,000 nmi Primary interest to navigation users is the signal phase Primary interest to navigation users is the signal phase
which provides a measure of transmitter-receiver distancewhich provides a measure of transmitter-receiver distance
Omega (2)
Omega receiver provides an accuracy of 2 to 4 nmi 95% of the time for navigation purposes When a receiver utilizes Omega signal phase corrections
transmitted from nearby monitor stationposition accuracy comes down to 500 meters
Thus the resulting system has an accuracy that is comparable to the high-accuracy navigation aid
Commonly used in oceanic civil airline configurations, combined with an inertial navigation system, so that the Omega system error effectively ‘bounds” the error of the inertial system
Omega (3)
Fig. 4.34
Omega station configuration
Omega (4)
Important features of omega signals
1. Four common transmitted signal frequencies: 10.2, 11 1/3, 13.6, and 11.05 kHz
2. One unique signal frequency for each station
3. A separate interval of 0.2 sec between each of the eight transmissions
4. Variable-length transmission periods
Omega (5)
Fig. 4.35
Omega system signal
transmission format
Omega (6)
Position determination Fig 4.37
Hybrid geometry for phase-difference
measurements
Decca Developed by British and used during World War II. Based on the measurment of differential arrival time(at the
vehicular receiver) of transmissions from two or more synchronized stations (typicaly 70 mi apart)
i.e two stations (A,B) 10 mi apart and each radiating synchronized radio-frequency carries of 100 kHz Wave length=3000 m, ~2 mi On a line between the stations the movement of a vehicle D one
mile from the other station will cause the vehicle to traverse one cycle of differential radio-frequency phase
10 places along the line AB where the signals from the twp stations will be in phase one
As the vehicle moves laterally away from this line, isophase LOPs can be formed with the stations and BD-AD as a constatnt for each LOP
Chayka A pulse-phase radio-navigation system
similar to the Loran-C system Used in Russia and surrounding territories By using ground waves at low
frequencies, the operating range is over 1000 mi; by using pulse techniques, sky-wave contamination can be avoided
Designed to provide both a means of determining an accurate user position and source of high-accuracy time signals
Chapter Four
Satellite Radio Navigation
Introduction (1)
Since the 1960s, the use of satellites was established as an important means of navigation on earth
Equipped acft receiving satellite transmitted signals can derive their 3D position and velocity.
There are two main satellite navigation systems The U.S. Department of Defense’s NAVSTAR Global
Positioning System (GPS) and The Russian Federation’s Global Orbiting Navigation
Satellite System(GLONASS)
Introduction (2)
ICAO & RTCA have defined a more global system that includes these two systems , geostationary overlay satellite, along with any future satellite navigation systems
The advantage of satellite navigation is that they provide an accurate all-weather worldwide navigation capability
The major disadvantages are that they can be vulnerable to international or uninternational interference and temporary unavailability due to the signal masking or lack of visibility coverage
System Configuration Consists of three segments
Space segment Control segment User segment
Space Segment The space segment is comprised of the satellite
constellation made up of multiple satellites. The satellite provides the basic navigation frame of reference and transmit the radio signals from which the user can collect measurements required for his navigation solution
Knowledge of the satellites’ position and time history (ephemeris and time) is also required for the user’s solutions.
The satellite also transmit that information via data modulation of the signals
•CDMA @ 1.2 to 1.5 GHz
•LB and “P” “C”
•Very accurate atomic clocks ~< nanosecond
Control Segment Consists of three major elements
Monitor stations that track the satellites’ transmitted signals & collect measurements similar to those that the user collect for their navigation
A master control station that uses these measurements to determine & predict the satellites’ ephemeris & time history and subsequently to upload parameters that the satellite modulate on the transmitted signals
Ground station antennas that perform the upload control of the satellite
User Segment Is comprised of the receiving
equipment and processors that perform the navigation solution
These equipments come in a variety of forms and functions, depending upon the navigation application
Basics of Satellite Radio Navigation (1) Different types of user equipments solve a basic set of
equations for their solutions, using the ranging and/or range rate (or change in range) measurements as input to a least-squares, or a Kalman filter algorithm.
Fig 5.2
Ranging satellite radio-navigation
solution
Basics of Satellite Radio Navigation (2) The measurements are not range & range rate (or change in
range), but quantities described as pseudorange & pseudorange rate (or change in pseudorange). This is because they consisits of errors, dominated by timing errors, that are part of the solution. For example, if only ranging type measurments are made, the actual measurement is of the form
is the measured peseudorange from satellite i is the geometric range to that satellite, is the clock
error in satellite i, is the user’s clock error, c is the speed of light and is the sum of various correctable or uncorrectable measurements error
iPRusiii tctcRPR iPR
iR situt
iPR
Basics of Satellite Radio Navigation (3)
Neglecting for the moment the clock and other measurement errors, the range to satellite i is given as
are the earth-centered, earth fixed (ECEF) position components of the satellite at the time of transmission and are the ECEF user position components at that time
222usiusiusii ZZYYXXR
sisisi andZYX ,
uuu andZYX ,
Atmospheric Effects on Satellite Communication Ionosphere:
Shell of electrons and electrically charged atoms & molecules that surrounds the earth
Stretching from 50km to more than 1000km Result of ultraviolet radiation from sun Free electrons affect the propagation of radio
waves At frequency below about 30 MHz acts like a
mirror bending the radio wave to the earth thereby allowing long distance communication
At higher frequencies (satellite radio navigation) radio waves pass through the ionosphere
NAVSTAR Global Positioning System GPS was conceived as a U.S. Department of
Defense (DoD) multi-service program in 1973, bearing some resemblance to & consisting of the best elements of two predecessor development programs: The U.S. Navy’s TIMATION program The U.S. Air Force’s program
GPS is a passive, survivable, continuous, space-based system that provides any suitably equipped user with highly accurate three-dimensional position, velocity, and time information anywhere on or near the earth
Principles of GPS & System Operation GPS is basically a ranging system, although precise
Doppler measurements are also available To provide accurate ranging measurements, which are
time-of-arrival measurements, very accurate timing is required in the satellite. (t<3 nsec) GPS satellite contain redundant atomic frequency standards
To provide continues 3D navigation solutions to dynamic users, a sufficient number of satellite are required to provide geometrically spaced simultaneous measurements.
To provide those geometrically spaced simultaneous measurements on a worldwide continues basis, relatively
high-altitude satellite orbits are required
GPS System Configuration Fig 5.8
General System Characteristics The GPS satellites are in approximately 12
hour orbits(11 hours, 57 minutes, and 57.27 seconds) at an altitude of approximately 11,000 nmi
The total number of satellite in the constellation has changed over the years ~24
Each satellite transmits signals at two frequencies at L-Band to permit ionosphere refraction corrections by properly equipped users
System Accuracy GPS provides two positioning services, the Precise
Positioning Service (PPS) & the Standard Positioning Service (SPS)
The PPS can be denied to unauthorized users, but SPS is available free of charge to any user worldwide
Users that are crypto capable are authorized to use crypto keys to always have access to the PPS. These users are normally military users, including NATO and other friendly countries. These keys allow the authorized user to acquire & track the encrypted precise (P) code on both frequencies & to correct for international degradation of the signal WAAS < 3 m horizontal < 7.5 m vertical GPS 15m
The GPS segmentsSegments
Input Function Product
Space Satellite commandsNavigation messages
Provide atomic time scaleGenerate PRN RF signals Store & forward navigation message
PRN RF signalsNavigation messageTelemetry
Control PRN RF signals TelemetryUniversal coordinatedTime(UTC)
Estimate time & ephemerisPredict time & ephemerisManage space assets
Navigation messageSatellite commands
User PRN RF signalsNavigation messages
Solve navigation equations
Position, velocity, & time
Wide Area Augmentation System(WAAS) Developed by the FAA in parallel with European
Geostationary Navigation Overlay Service (EGNOS) & Japan MTSAT Satellite-Based Augmentation System
A safety-critical system consisting of a signal-in-space & a ground network to support en-route through precision approach air navigation
The WAAS augments GPS with three services all phases of flight down to category I precision approach
1. A ground integrity broadcast that will meet the Required Navigation Performance (RNP)
2. Wide area differential GPS (WADGPS) corrections that will provide accuracy for GPS users so as to meet RNP accuracy requirements
3. A ranging function that will provide additional availability & reliability that will help satisfy the RNP availability requirements
WAAS Concept (1)
Fig 5.34
WAAS Concept (2)
Fig 5.35
Inmarsat-3 four ocean-region deployment showing 5deg elevation
contours
WAAS Concept (3)
Uses geostationary satellite to broadcast the integrity & correction data to users for all of the GPS satellites visible to the WAAS network
A slightly modified GPS avionics receiver can receive these broadcasts
Since the codes will be synchronized to the WAAS network time, which is the reference time of the WADGPS corrections, the signals can also be used for ranging
WAAS Concept (4)
A sufficient number of GEOs provides enough augmentation to satisfy RNP availability & reliability requirements
In the WAAS concept, a network of monitoring stations (wide area reference stations, WRSs) continuously track the GPS (&GEO) satellite & rely the tracking information to a central processing facility
# Geo 2 minimum & 4 desired
WAAS Concept (5)
The central processing facility (wide area master station, WMS)m in turn, determines the health & WADGPS corrections for each signal in space & relays this information, via the broadcast messages, to the ground earth station (GESs) for uplink to the GEOs
The WMS also determines & relays the GEO ephemeris & clock state messages to the GEOs
Chapter Five
Terrestrial Integrated Radio Communication-Navigation
Systems
Introduction (1)
Since 1970s, same portion of the frequency spectrum & common technology has been use for communication & navigation
Integrated relative & absolute communication-navigation systems provide both digital communication & navigation functions using same wave form
1. Digital Communication
2. Navigation functionsContent of the
digital data & time of arrival of the
message measured by
receiver
Introduction (2) Integrated relative & absolute
communication-navigation systems1. Decentralized (node-less): The operation is not
dependent on any central site or node. Each user determines its own position
2. Centralize: The operation is dependent on a central site (node) Frequently it is desired to have the position of large
number of users known & tracked at a central site (i.g. military/civil command & control system)
Users may obtain their positions by automatic, periodic, or occasional requests from the central nodenodal system
3. Hybrid: Contain both nodal and node-less systems
Joint Tactical Information Distribution System Relative Navigation (JTIDS Rel Nav)
Decentralized position location & navigation system Mostly military used Each user determines its position,
velocity, and altitude from data received from other users
~900-~1200 MHz Spread spectrum
Chapter Six
Air-Data Systems
Introduction (1)
An air-data system consists of aerodynamic & thermodynamic sensor & associated electronics
The sensors measure characteristics of the air surrounding the vehicle and convert this information into electrical signals that are subsequently processed to derive flight parameters including Calibrated airspeed,true airspeed, mach number, free-
stream static pressure, pressure altitude Baro-corrected altitude, free-stream static pressure, pressure altitude, baro-corrected altitude, free-stream outside air temperature, air density, angle of attack, angle of sideslip
Introduction (2) Measured information is used for flight displays,
autopilots, weapon-system fire-control computation, and for the control of cabin-air pressurization systems
Since 1990s, all computations & data management are digital & based on microprocessor technology. New avionics architectures are incorporating air-data functions into other subsystems such as inertial/GPS navigation units or are packaging the air-data transducers into the flight-control computers
Each type pf acft has unique challenges, primarily in regard to the accuracy of measuring the basic aerodynamic phenomena
Air-data Measurements (1)
All of the air-data parameters that are relevant to flight performance are derived by sensing the pressure, temperatures, and flow direction surrounding the vehicle
Because air is moving past the acft, the pressure at various places on the acft’s skin may be slightly higher or lower than free streamAirborne
Sensors •Pressure
•Temperature
•Flow direction
Air-data parameters
relevant to flight
performance
Air-data Measurements (2)
The probes deployed around the skin of acft, sample the static pressure (via static ports), total pressure (via the pitot tube), total temperature (via the temperature probe), and local flow direction (via the angle-of-attack & sideslip vanes)
All of these sensing elements, except for the flush-mounted static port, are intrusive because they disturb the local airflow
Air-data System
Typical nose-mounted air-data
boom with pressure probes &
flow-direction
vanes
Probes & vanes
in acft body
Air-data (1) Static pressure is the absolute pressure of the still air
surrounding the acft. To obtain a sample of static air in a moving acft, a hole
(static port) or series of holes are drilled in a plate on the side of the fuselage or on the side of the pilot tube probe which extends into the free air stream
Total pressure refer to the pressure sensed in a tube that is open at the front & closed at the rear
),,(
/
2/1 2
TfC
CVm
Tf
VPP ST
Air-data (2)
Outside air temperature, referred to as static air temperature and is required for the computation of true airspeed, air density (which is required for some types of fire-control aiming solutions)
Angle of attack is the angle, in the normally vertical plane of symmetry of the acft, at which the relative wind meets an arbitrary longitudinal datum line in the fuselage
Chapter Seven
Attitude and Heading References
Introduction (1) Heading references is required for steering &
navigation Simplegravity-leveled magnetic compass Elaborateinertial navigator
Attitude references Simplevisible horizon Elaborateattitude reference instruments in poor
weather An automatic pilot requires measurements of body
rates & attitude Attitude & rate instruments stabilize other avionic
sensors(I.e. doppler radar, navigation radars, weapon delivery systems)
Introduction (2)
Cockpit displays Inexpensive acftself-contained vertical &
directional gyroscope that are viewed directly by the crew
Complex acft attitude driven from remotely located sensors &
are displayed on glass instruments Vertical situation driven by the level-axis outputs
of an inertial navigator Complex acft usually carry at least one set
of self-contained vertical & directional gyroscopes for emergencies
Electronic display Fig 9.1 a,b
Basic Instruments Gyroscope
A spinning wheel(source of angular momentum) that will retain its direction in inertial space in the absence of applied torques section 7.3.4.
Gravity Sensors Simple pendulums with
electromagnetic pickoffs
Vertical References (1) Basic reference is the earth’s gravitational
field that stationary platformcan be sensed with great
accuracy by a simple pendulum, spirit level, or accelerometer
Moving platformall the devises indicate the vector sum of vehicle acceleration & local gravity. δ=angle between the true & apparent vertical is
V
H
ag
a
tan
)ft/sec (32.2gravity todueon accelarati:g
acft ofon accelerati vertical,horizontal:,2
VH aa
Vertical References (2)
Geometry of vertical determination
Heading References The best heading references are inertial
navigators Less expensive, smaller, & less accurate
heading references are Those that depend on the earth’s magnetic
field”magnetic compass” Those that depend on the use of gyroscope
to retain a preset azimuth”directional gyroscope”
Those that use sub-inertial gyroscopes to maintain a three-axis reference
Chapter Eight
Doppler and Altimeter Radars
Doppler Radars (Functions & Applications)
The primary function is to continuously determine the velocity vector of an acft with respect to the ground
Doppler Radars (Advantages)
Advantages over other methods of velocity measurements Velocity is measured with respect to the
earth’s surface. Unlike; Air data systemwith respect to the air mass Terrestrial radio navigation systemmeasurements
are based on differencing of successive position measurements
Self-contained; it requires no ground-based stations or satellite transmitters
Extremely small airborne transmitter power requirements
Doppler Radars (Advantages)
Narrow radar beams pointed toward the ground at steep anglelow detect ability
All-weather system Operates over both land terrain & water Extremely accurate average velocity
information No required international agreement No required pre-flight alignment & warm-
up
Doppler Radars (Disadvantages)
Requires an external airborne source of heading information (I.e. gyro-magnetic compass, attitude-heading reference for autonomous dead-reckoning navigation
Requires either internal or external vertical reference for conversion of velocity info to earth referenced
Position info derived from Short term velocity info is not as accurate as the
average velocity For over-water operation, accuracy is degraded
due to backscattering characteristics
Functionalities Fig 10.1
Doppler navigation system
Principles & Design Approach Doppler effect: change (Doppler shift) in
observed frequency when there is relative motion between a transmitter & a receiver If the relative velocity is much smaller than
speed of light:
ion transmissofth waveleng
receiver &ansmitter between tr velocity relative
light of speed
ion transmiss theoffrequency
shiftDoppler
fc
V
c
f
v
V
c
fVv
R
RR
If the value of λ is known
& v is measured, the relative velocity can
be calculated
Doppler Radar Beam GeometryBasic Doppler Radar beamgeometry
centroid beam
thealongr unit vecto centeroid beam the
and Vvector velocity ebetween th angle
cos2
cos2
2cos2
b
b
V
c
Vfv
or
VVVR
•Also used for ground proximity warning system.
• Combine with GPS digital terrain database for enhanced ground proximity monitoring
Three beam Doppler Radar To measure all three orthogonal components
of velocityThree-beam lambdaDoppler radar configuration
The Doppler Spectrum Fig 10.6
Chapter Nine
Mapping & Multimode Radars
Introduction Developed in World War II for
bombing through clouds at night Perform two navigation functions
Permitted acft to find its way over enemy terrain, without ground navigation aids or sight of the ground
Provide precise navigation during the bombing run by use of cursors set on the target point in a display
Chapter Ten
Landing Systems
Introduction Every successful flight culminates in a landing.
Although the majority of landings are conducted solely with visual cues, acft must frequently land in weather that requires electronic assistance to the pilot or the autopilot
On the vicinity of the destination the acft begins its decent & intercepts the projected runway center line, then makes a final approach & landing with position errors of a few feet in each axis at touchdown
The catastrophic accidents occur during these flights phases of which two-thirds are attributed to errors made by the flight crew
Low-Visibility Operations (1)
Considerable interference to civil & military operations result due to reduced visibility in terminal areas
i.e the visibility at London’s Gatwick Airport requires Category II operational capabilities for 115 hours per year & Category III capabilities for 73 hours per year during primary operating hours
Low-Visibility Operations (2) While the successful landing of acft depends on many
factors other than ceiling & visibility, such as crosswinds & storm activity, the term all-weather operations often refers only to operations in condition of reduced visibility
Instrument meteorological conditions (IMC) are times in which visibility is restricted to various degrees defined by regulations in certain countries
Acft operating in IMC are supposed to fly under Instrument Flight Rules also defined by regulations
During a landing, the decision height (DH) is the height above the runway at which the landing must be aborted if the runway is not in sight. The better the electronic aids, the lower is the DH
Visibility Categories (by ICAO) (1)
Category I Decision height not lower than 200 ft; visibility
not less than 2600 ft, or Runway Visual Range (RVR) not less than 1800 ft with appropriate runway lighting.
The pilot must have visual reference to the runway at the 200ft DH above the runway or abort the landing.
Acft require ILS and marker-beacon receiver beyond other requirements for flights under IFR.
Category I approaches are performed routinely by pilots with instrument ratings
Visibility Categories (by ICAO) (2)
Category II DH not lower than 100 ft & RVR not less than
1200 ft (350m) The pilot must see the runway above the DH or
abort the landing Additional equipment that acft must carry
include dual ILS receivers, either a radar altimeter or an inner-marker receiver to measure the DH, an autopilot coupler or dual flight directors, two pilots, rain-removal equipment (wipers or chemicals), and missed-approach attitude guidance. An auto-throttle system also may be required
Visibility Categories (by ICAO) (3)
Category III subdivided into IIIA. DH lower than 100 ft and RVR not less
than 700 ft (200m)-sometimes called see to land: it requires a fail-passive autopilot or a head-up display
IIIB. DH low than 50 ft & RVR not less than 150 ft (50m)-sometimes called see to taxi; it requires a fail-operational autopilot & an automatic rollout to taxing speed
IIIC. Zero visibility. No DH or RVR limits. It has not been approved anywhere in the world
Decision Height Acfts are certified for decision heights,
as are crews When a crew lands an acft at an airport,
the highest of the three DHs applies. An abort at the DH is based on visibility Alert height is the altitude below which
landing may continue in case of equipment failure Typical Alert height is 100 ft
Standard lighting Pattern Airports at which Category II landings are permitted
must be equipped with the standard lighting pattern
Category III runway
configuration
The Mechanics of Landing (1)
1. The approach Day & night landings are permitted under
visual flight rules (VFR) when the ceiling exceeds 1000 ft & the horizontal visibility exceeds 3 mi, as juged by the airport control tower
In deteriorated weather, operations must be conducted ubder Instrument Flight Rules (IFR) An IFR approach is procedure is either non-precision
(lateral guidance only) or precision (both lateral & vertical guidance signals) Category I, II, and III operations are precision-approach
procedures
The Mechanics of Landing (2) An afct landing under IFR must transition
from cruising flight to the final approach along the extended runway center line by using the standard approach procedures published for each airport
Approach altitudes are measured barometrically, and the transition flight path is defined by initial & final approach fixes (IAF & FAF) using VOR, VOR/DME
Radar vectors may be given to the crew by approach control
The Mechanics of Landing (3)
From approximately 1500 ft above runway, a precision approach is guided by radio beams generated by ILS. Large acft maintain a speed of 100 to 150 knots during descent along the glide path beginning at the FAF (outer marker)
The glide-path angle is set by obstacle-clearance and noise-abatement considerations with 3 deg as the international civil standard
The sink rate is 6 to 16 ft/sec, depending on the acft’s speed & on headwinds
The Mechanics of Landing (4)
The ICAO standard: glide path will cross the runway threshold at a height between 50 & 60 ft. Thus, the projected glide path intercepts the runway surface about 1000 ft from the threshold.
Fig 13.3Wheel path
for instrument landing of a
jet acft
The Mechanics of Landing (5)
2. The flare Maneuver Land-based acft are not designed to touch
down routinely at the 6 to 16 ft/sec sink rate that exits along the glide path. Thus a flare maneuver must be executed to reduce the decent rate to less than 3 ft/sec at touchdown
During the approach, the angle of attack is maintained at a value that causes a lift force equal to the acft’s weight, & the speed is adjusted for a specified stall margin, typically 1.3 times the stall speed plus a margin based on reported wind speed & shear
The Mechanics of Landing (Decrab Maneuver)
1. The Decrab Maneuver & Touchdown In a crosswind Vcw, an acft will approach with
a cab angle b such that its ground-speed vector lies along the runway’s centerline. At an approach airspeed Va & a headwind Vhw,
b is usually less than 5 deg & is always less than 15 deg
After the decarb, the wind causes the acft to begin drifting across the runway.
)/(sin hwacw VVVb
The Decrab Maneuver & Touchdown
Table 13.2
The Mechanics of Landing (Rollout & Taxi) (1)
3. Rollout & Taxi Approximately 600ft after main-gera
touchdown, a large jet acft lowers its noise wheel & subsequently behaves like a ground vehicle
Some methods for guiding acft on taxiways1. Measuring runway stopping-distance by DME2. Guide the acft along a specific taxi route by
taxiway lights 3. Surface radars that aid in avoiding taxiway &
runway-incursion accidents
The Mechanics of Landing (Rollout & Taxi) (2)
4. Transponder-based systems 5. Radio broadcast of on-board derived
position & velocity6. Milliwatt marker-beacon transmitter
placed at all runway thresholds would give a visual & audible alarm on the flight deck of any acft that taxied onto an active runway
Automatic Landing Systems (1) Air carrier acft that are authorized for
precision-approach below category II must have automatic landing (auto-land) system.
1. Guidance & control requirements by FAA For category II: the coupled autopilot or crew
hold the acft within the vertical error of +or- 12 ft at the 100ft height on a 3deg glide path
For category III: the demonstrated touchdown dispersions should be limited to 1500ft longtudinally & -or+ 27ft laterally
Automatic Landing Systems (2)
2. Flare Guidance During the final approach the glide-slope
gain in the auto-land system is reduced in a programmed fashion. Supplementary sensors must supply the vertical guidance below 100ft
3. Lateral Guidance Tracking of the localizer is aided by heading
(or integral-of-roll), roll, or roll-rate signals supplied to the autopilot and by rate & acceleration data from on-board inertial system
Instrument Landing System(ILS) (1)
Is a collection of radio transmitting stations used to guide acft to a specific runway.
In 1996 nearly 100 airports worldwide had at least one runway certified to Category III with ILS
More than one ILS in high density airports About 1500 ILSs are in use at airports
throughout the US
Instrument Landing System(ILS) (2) ILS typically includes:
The localizer antenna is centered on the runway beyond the stop end to provide lateral guidance
The glide slope antenna, located beside the runway near the threshold to provide vertical guidance
Marker beacons located at discrete positions along the approach path; to alert pilots of their progress along the glide-path
Radiation monitors that, in case of ILS failure alarm the control tower, may shut-down a Category I or II ILS, or switch a Category III ILS to backup transmitters
ILS Guidance Signals (1)
The localizer, glide slope, and marker beacons radiate continues wave, horizontally polarized, radio frequency, energy
The frequency bands of operation are Localizer, 40 channels from 108-112 MHz Glide slop, 40 channels from 329-335 MHz Marker beacons, all on a signal frequency of
75 MHz
ILS Guidance Signals (2) The localizer establishes a radiation pattern in
space that provides a deviation signal in the acft when it is displaced laterally from the vertical plane containing the runway centerline
The deviation signal drives the left-right needle of the pilot’s cross-pointer display & may be wired to the autopilot/flight-control system for coupled approaches
The deviation signal is proportional to azimuth angle usually out to 5 deg or more either side of the center line
ILS Guidance Signals (3)
Fig13.4
Sum & difference radiation
patterns for the course (CRS) &
clearance (CLR) signals of
a directional localizer array
The Localizer (1) The typical localizer is an array usually located 600 to
1000 ft beyond the stop end antenna of the runway The array axis is perpendicular to the runway center line
Log-periodic dipole
antenna used in many localizer arrays
The Localizer (2)
Fig13.7
Category IIIB localizer
The Glide Slope (1) There are five different of glide-slope arrays in
common use; three are image systems & two are not
Image arrays depend on reflections from level ground in the direction of approaching acft to form the radiation pattern The three image systems are null-referenced system,
with two antennas supported on a vertical mast 14 & 28 ft above the ground plane
The sideband-reference system, with two antennas 7 and 22ft above the ground plane
The capture-effect system, with 3 antennas 14, 28, and 42 ft above the ground plane
The Glide Slope(2)
Fig 13.8
Category IIIB capture-effect glideslope &
Tasker transmissometer
The Glide Slope (3)
Fig 13.9
Glide-slope pattern near the runway. DDM counters are
symmetrical around the vertical, but signal strength
drops rapidly off
course
The Glide Slope (4) The cable radiators of the end-fire array are installed on
stands 40 in. high & are site alongside the runway near desired touchdown point
Fig 13.10 Fig 13.11
Standard end-fire glide-slope system layout
Front slotted-cable radiator of an end-fire
glide slope
ILS Marker Beacons (1)
Marker beacons provide pilot alerts along the approach path
Each beacon radiates a fan-shaped vertical beam that is approximately +or- 40deg wide along the glide path by +-85deg wide perpendicular to the path The outer marker(OM) is placed under the
approach course near the point of glide-path intercept & it is modulated with two 400 Hz Morse-code dashed per second
ILS Marker Beacons (2) The middle marker(MM) is placed near the point
where missed-approach decision would need to be made for Category I. MM is modulated with one 1300 Hz dash-dot pair second
The inner marker (IM) may be required at runway certified for Category II & III operations & is placed near the point where the glide path is 100ft above the runway. IM has six dots per second at 3000 Hz
Because of the real state problems the use of marker beacons is decreasing
The increase use of DME & ILS has diminished the pilot’s dependence on the markers
Receivers Filter the detector separate the 90
& 150 Hz tones which in the most basic circuit, are rectified & feed to a dc micrometer
ILS Limitations (1)
Major limitation is its sensitivity to the environment
At ILS frequencies, the very narrow beam widths, necessary to avoid significant illumination of the environment surrounding the approach course, require array structure which are too large to be practical
Accuracy degradations (beam bends) due to reflections from buildings, terrain, airborne acft, taxiing acft, and ground vehicles
ILS Limitations (2)
Fig 13.12
Formation of bends in the glide path
Microwave-Landing System (MLS) (1) Developed by U.S. military services to address the
ILS limitations Designs were sought that retained the desirable
features of the ILS while mitigating its weaknesses Same runway-residence of ILS because as the
landing acft approaches the runway, linear offset(due to the errors in the angular guidance) continually decreases, while the signal-to-noise ratio generally increases. Thus, in the most demanding phase of the flight close to
the ground, the positional accuracy is constantly improving & the noise content is generally decreasing
freq~ 5MHz
Microwave-Landing System (MLS) (2)
ILS sensitivity to environment is eliminated by narrow beam-width antennas that are physically small at microwave frequencies
The lack of available channels, which limits multiple ILS deployments in metro areas, would no longer be a problem
Microwave-Landing System (MLS) (3)
Never fully developed Being replaced by WAAS and GPS
Satellite Landing Systems (1)
Before GPS become operational efforts had been underway to use it for approach & landing
An operational concept called Special Category I Precision Approach Operations Using DGPS, based on the differential GPS (DGPS) technique, was developed, tested, and certified for specific airports
The test results have been very promising
Satellite Landing Systems (2) Augmentation Concepts The basic GPS, without differential correction,
cannot be used for precision approach & landing operations because;
1. Accuracy: The nominal error is +- 15m, compared with requirements (+-1.3m to +-8m for different Cats)
2. Integrity: The GPS design lacks a monitoring system which can provide timely warning of guidance-data faults within 10sec for Cat I, or less than 2sec for Cat III
3. Availability: The number of satellite in view in certain time periods may not be adequate
GPS has been improved but still not operable for landing systems
Future Trends (1)
Pilot aids Use several technologies to
reduce pilot work load during approach & landing
improve the pilot’s ability to monitor an automatic landing
Future Trends (2)
Satellite landing aids Solution to provide low-cost, non-
precision & near Cat I procedures at low-density airports
Airport surface navigation Spread the use of differential satellite-
based systems for guidance & surveillance of rollout, taxi & departure operations under low-visibility conditions
Accuracy Allocation Fig 13.1
Chapter Eleven
Data Links
Automatic Dependent Surveillance - Broadcast (ADS-B) A technology designed to address both airspace
and ground-based movement needs. Collaborative decision making is possible through
ADS-B surveillance information available to both ATC and aircrews.
ADS-B combined with predictable, repeatable flight paths allow for increased airspace efficiencies in high density terminal areas or when weather conditions preclude visual operations.
Additionally, ADS-B allow for enhanced ground movement management (aircraft and vehicles) and improved airside safety
ADS-B