Navigation Aids


Navigation Aids


The speed of propagation of radio waves (300,000 kilometer per second)

permits measurement of distance as a function of time and of direction as

a function of differential distance to two or more known points. In free space,

radio navigation is capable of considerable accuracy. Along the surface of the

earth, however, accuracy is reduced by the effects of multiple propagation

paths between transmitter and receiver. Most navigation systems area,

accuracy, and convenience of use. In general, complexity is minimized on the

vehicle at the expense of greater complexity at the fixed station. Since aircraft

and ships may move over large areas, systems which involve cooperation

between a vehicle and a ground station have required a high degree of

international standardization . These standards, once established, change

slowly. Within each country, additional military systems are in use; some of

these are compatible with the international civil systems.

November 21, 2020 9:37 PM


Major Standardizing Agencies


ITU : International Telecommunication Union (ITU), Geneva, Switzerland .

    An agency of the United Nations. Allocates frequencies for best use

    of the radio spectrum.

ICAO : International Civil Aviation Organization (ICAO), Montreal, Canada.

      An agency of the United Nations. Formulates standards and

     recommended practices, including navigation aids, for all civil aviation.

IATA : International Air Transport Association (IATA), Montreal, Canada. An

     association of scheduled airlines.

FCC : Federal Communication Commission (FCC), Washington, D.C. Licenses

     transmitters in the United States and aboard US registered ships and


FAA : Federal Aviation Administration (FAA), Washington, D.C.

     Formerly Federal Aviation Agency.

Operates navigation aids and traffic control systems, for both military

and civil aircraft, in the US and its possessions. (Not to be confused with

Civil Aeronautics Board (CAB), which regulates routes and fares of

inter-state and foreign airlines operating in US).

USCG : United States Coast Guard (USCG) , Washington , D.C.

      Operates navigation aids for shipping.

RTCA : Radio Technical Commission for Aeronautics (RTCA), Washington, D.C.

      Supported by contributions from industry and from government agencies.

      Seeks participation by manufacturers, users, and others in the generation

      of recommended standards for aviation electronics. Some of these

      standards have been adopted, at least in part, by the ICAO and by the FAA.

RTCM : Radio Technical Commission for Marine (RTCM), Washington, D.C.

      Similar to RCTA, but for shipping.

AEEC : Airlines Electronic Engineering Committee (AEEC), Annapolis Science Center,

      Annapolis, Maryland 21401. A division of Aeronautical Radio, Inc. (ARINC) ,

      owned by the scheduled US airlines. Holds frequent meetings with

      manufacturers, issues newsletters, and publishes technical

      recommendations on avionics hardware purchased by the scheduled

      airlines. Has worldwide influence on airborne-equipment design.

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Reduction of Errors caused by Multi-Path Propagation


In most radio navigation systems, the desired path of the signal is the

shortest one between transmitter and receiver; errors result from the

admixture with signals which have traveled by longer, often variable ,

paths. To reduce such multi-path effects, the following techniques are

commonly used.

(A) Pulse Transmission :


With suitable pulse duration and repetition rate, plus means at the

receiver to recognize the leading edge of the pulses, the direct signal

may be separated from that which has traveled a longer path.

Effectively used in radar, DME Loran-A, Loran-C.

(B) Space Diversity :


The larger the aperture of an antenna system, the greater the statistical

probability that the desired signals will add linearly while the multi-path

signals add randomly . Effectively used in doppler VOR, doppler DF.

(Antenna directivity is very frequently used as an equivalent to space

diversity. By proper shaping of the antenna pattern, energy may be

increased along the direct path and reduced along undesired potentially

interfering paths. In directional systems, such as ILS, horizontal directivity

may be employed for this purpose; in non-directional or omni-directional

systems, such as Tacan and DME, vertical directivity is used).

(C) Frequency Diversity :


While the line-of-sight path remains the same at all radio frequencies,

indirect paths may vary with frequency. In such case, spectrum-spreading

techniques may achieve the same result as space diversity.

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Major Radio Navigation Aids


For the specific extent to which these aids are currently implemented

throughout the world, see the navigational facility maps issued at

frequent intervals by the US Coast and Geodetic Survey, Washington

Science Center, Rockville, Maryland 20852.

For US statistics , see "The ATS Fact Book", issued annually by the FAA.

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ADF (Airborne Direction Finder)


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The Aeronautical Satellite Program (AEROSAT) is an experimentation and

evaluation program proposed by ESRO, Canada, and the United States.

The total program will span almost ten years and plans cal for use of

satellite to provide improved communication and surveillance capability for

oceanic air traffic control. Information from this program will support ICAO

in defining an operational satellite system for the mid-1980's. A first

generation of L-Band avionics based upon the reference avionics interface

parameters of the AEROSAT system is being evaluated.

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ATCRBS (Air Traffic Control Radar Beacon System)


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DME (Distance Measuring Equipment : ICAO Standard)


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ILS (Instrument Low-Approach System : ICAO Standard)


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Loran-A, C ( Long Range Navigation )


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MLS (Microwave Landing System)


The MLS is an air-derived data system in which the airborne unit

obtains precision azimuth angle, elevation angle, and range data

referred to the runway. Angular position of the aircraft is measured

by reference to ground transmitters that generate angle-encoded

signals throughout the coverage sector in both azimuth and elevation.

The airborne unit extracts the modulated angle data that corresponds

to the line-of-sight angle from the ground antenna. Range

measurements are made via airborne interrogation of a ground

transponder. The replies from the ground beacon are tracked to

extract range data from the round-trip time delay.

The system is capable of transmitting auxiliary data such as runway

identity, equipment status, weather data, and siting constants to

airborne units. The airborne unit computes position data or flight

path deviation data suitable for inputs to the flight control system

and/or for display to the pilot.

The basic MLS system is comprised of the following functions or elements :

1) a basic C-band elevation and azimuth guidance element.

2) a DME operating in a separate portion of C band.

3) an elevation guidance element (elevation 2) for flare-out guidance to

  touchdown operating in Ku band.

4) a back course azimuth (and optional elevation) guidance.

A functional block diagram of the basic MLS equipment is presented in Fig. 8.

The azimuth and elevation angle transmitters, the DME ground beacon, and

the airborne unit, which extracts range and angle position data, are shown

in the figure (Fig. 8 ). This will be a Category III system. Limited production is

scheduled for 1977. The MLS objective is to replace ILS and become the IACO

standard before the year 2000.

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Omega is a world-wide VLF navigation system. Eight stations are

required to provide world-wide coverage. Each of the eight stations

sequentially transmits long, but precisely timed, pulses at three

different frequencies, 10.2, 11.33. and 13.6 kilohertz. The precise

timing permits automatic acquisition of the stations , and use of the

three frequencies reduces the residual "lane ambiguity" of the phase

measurement to 72 miles for hyperbolic operation and 144 miles for

direct ranging operation. The position error is of the order of 1 mile.

November 21, 2020 9:46 PM


Radar (Airborne)


Recent techniques permit the obtaining of semi-automatic position fix

by means of airborne radar. The geographic coordinates of the fix point,

or check point (CP), have previously been inserted into the navigation

computer and are stored there. The computer calculates the aircraft

position from its normal navigation sensor inputs. From these two data,

it generates bearing and distance from the aircraft to the CP. The latter

are converted into the equivalent radar coordinates, bearing ad slant

range. These are then displayed as cursors on the navigator's radar

indicator. When the navigator observes the predetermined CP target

on the radar image, he manually places the range and bearing cursors

on top of the CP target. This process cause any position error signals

which exist to be automatically fed back to th navigation computer,

thereby fix-correcting the position information of the navigation system.

November 21, 2020 9:47 PM


TACAN (Tactical Air Navigation)


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VOR (VHF Omni-directional Range : ICAO Standard)


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Navigation Characteristics (Part 1)


(A) Inertia Navigation :


 * Operates by double integration of measured acceleration.

 * Self-contained.

 * Unlimited coverage.

 * Passive operation.

 * Unlimited number of users.

 * Accuracy : 2 nmi/h or better.

 * Ambiguities : none.

 * Error characteristics : errors among users are uncorrelated,

                    aircraft separation is affected;

                    absolute error diverges with time.

 * Reliability assured through triple redundancy.


(B) Doppler :


 * Operates by integration of measured velocity.

 * Self-contained.

 * Unlimited coverage.

 * Radiate RF energy in the 10-GHz region.

 * Unlimited number of users.

 * Accuracy : position error less than 1.4 percent

            of distance traveled since calibrating.

 * Ambiguities : none.

 * Error characteristics : errors are uncorrelated among users;

                     aircraft separation is affected;

                     absolute error diverge with time.

November 21, 2020 9:50 PM