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


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


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


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)


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


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


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.


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.



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.


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


Fig 2.4


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)


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


Fig 2.8

t

tT yy

xxB

yyxxD

arctan

)()( 21

21


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


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


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


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


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


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


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


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


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


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 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


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

Documents

Aviation Communication, Navigation, and Surveillance (CNS)