Navigation Aids
Navigation Aids
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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.
Major Standardizing Agencies
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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
aircraft.
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.
Reduction of Errors caused by Multi-Path Propagation
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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 :
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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 :
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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 :
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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.
Major Radio Navigation Aids
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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.
https://www.loc.gov/maps/?fa=contributor%3Au.s.+coast+and+geodetic+survey
https://en.wikipedia.org/wiki/U.S._National_Geodetic_Survey
For US statistics , see "The ATS Fact Book", issued annually by the FAA.
https://en.wikipedia.org/wiki/Federal_Aviation_Administration
ADF (Airborne Direction Finder)
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AEROSAT
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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.
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)
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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.
Omega
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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.
https://en.wikipedia.org/wiki/Omega_(navigation_system)
Radar (Airborne)
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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.
TACAN (Tactical Air Navigation)
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VOR (VHF Omni-directional Range : ICAO Standard)
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Navigation Characteristics (Part 1)
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(A) Inertia Navigation :
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* 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.
https://en.wikipedia.org/wiki/Inertial_navigation_system
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(B) Doppler :
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* 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.
https://en.wikipedia.org/wiki/Doppler_effect