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Navigation and Communication

Navigation and Communication

Welcome to the Nav & Com section. All questions and answers are unverified regarding correctness. Please report any errors in our Pilot Forum.

  • What layer does in the atmosphere do HF radio waves reflect off?
  • If your assigned HF frequency was 8000Khz during the day, at night what frequency would you use?
  • Why do you get better HF radio transmission range using SSB?
  • What are the uploads from an IRS?
  • What do you know about the Trans-Polar routes Cathay Pacific is pioneering?
  • What is TAAATS?
  • The future Air Navigation System
  • What is ACARS?
  • How does a Radio Altimeter work?
  • What is Transport Wander of an uncorrected gyro?
  • How does an INS find true north?
  • Reference an INS, what is Schuler’s Loop?
  • What is the difference between an INS and an IRS?
  • What is an INS?
  • Tell me about an ILS
  • What can you tell me about an NDB/ADF?
  • What can you tell me about a VOR?
  • What are the advantages of GPS?
  • What is DGPS?
  • What can you tell me about GPS?
  • What is EGPWS?
  • What are the inputs to GPWS?
What layer does in the atmosphere do HF radio waves reflect off?

The Ionosphere
It is a deep layer of ionised particles which extends from about 200 000′ to 1 250 000′. The varying intensity of the ionosphere affects the passage and reflection of electromagnetic transmissions, both natural and man made. It effects especially noticeable around dusk and dawn and during periods of intense sunspot activity, where enormous quantities of ionised particles bombarding earth.

If your assigned HF frequency was 8000Khz during the day, at night what frequency would you use?

Must be lower as the ionosphere height changes, and at night it is lower so that higher frequencies tend to pass straight through it.

Why do you get better HF radio transmission range using SSB?

More power is transmitted in a narrower band.

What are the uploads from an IRS?

Flight Management Computer
Flight Control Computer
Thrust Management Computer
Anti-skid/Auto brake
WX radar
Air Data Computer
Flight Data Recorder

What do you know about the Trans-Polar routes Cathay Pacific is pioneering?

Flights between Hong Kong and North America, usually track over the pacific. More recently air routes that are more pole ward than ever before have been pioneered by Cathay Pacific. The new polar routes were designated Polar 1, 2, 3, and 4. These new routes are open to both twin engine and 4 engine aircraft due to the availability of alternate aerodromes within the 180-207 min ETOPS rules.

The first commercial trans polar flight [from Toronto to Hong Kong] was operated on May 18th 2000 by a Cathay A340-300, with a flying time of 14hrs 59mins.

Cathay stands to benefit a great deal from the new routes as they dramatically reduce flight times [between 2 and 3 hrs], which make them more attractive to passengers [no stop over in Anchorage].

Some problems operating at high latitudes include:

  • Low fuel temperature [Jet A1 freezes at around -50ºc]
  • Communications are solely by HF [Satcom stops working around 85ºN]
  • Exposure to increased levels of cosmic radiation [magnetic field]
What is TAAATS?

The Australian Advanced Air Traffic System. Australia is responsible for the airspace covering 11% of the world’s surface, totalling 56 million square kilometres. Faced with the challenges of rapidly advancing aeronautical technology and increases in air traffic within the region, Air services Australia introduced one of the worlds most advanced integrated air traffic management systems. TAAATS unites computer, radar and communications technologies in a system that makes all flight information available to all air traffic controllers facilitating the best use of air space for pilots and radically enhancing safety.

TAAATS also provides facilities for all the ATC environments, – Control Tower, Terminal Area Control, En Route Control in Radar Airspace, non-Radar Continental Airspace and Oceanic Airspace. TAAATS is dependent on data received from many other systems including radar and communications networks. Radar inputs to TAAATS come from the Airservices radar network of 19 sensors, which are sited along the busier air corridors with coverage augmented by radar data provided from six military radar sites.

Data Streams from TAAATS to other systems are equally important, facilitating the billing process and flight path monitoring systems. In addition to traditional radio communications, TAAATS provides two communication paths for the delivery of airways clearances to aircraft operators, by text message to an airlines flight operations computer or direct to an aircraft’s flight management computer.

Pre-departure clearances are automatically formatted by TAAATS and sent over the data communications networks to airline flight operations computers or duty controllers in the flight operations centre. They are then automatically routed to the aircraft flight deck or to a departure gate printer for presentation to flight crew.

TAAATS also enables air – ground exchange of control information as free text messages providing Controller Pilot Data Link Communications.

Tracking aircraft within the vast Australian FIR [Flight Information Region] is another major function of TAAATS for which it uses three methods of tracking.

  • Radar Data Processing, from primary and secondary radar coverage within 50 to 250 nautical miles
  • Automatic Dependant Surveillance when an aircraft is outside radar coverage
  • Flight Data Processing when neither RDP OR ADP is available.
The future Air Navigation System

The current air traffic management system is experiencing growing difficulty as air traffic around the world continues to increase. With air traffic predicted to grow at the rate of five percent annually, the industry must find a new air traffic management system that provides greater capacity. One potential solution is a concept called Future Air Navigation System, or FANS.

FANS offers a space-based method for handling increased air traffic, allowing operators to obtain maximum revenue from their operations while ensuring safe conditions for their passengers. The air transport industry has developed a new concept for air traffic management that involves significant changes to airplanes, infrastructure, and ground systems. Known as Future Air Navigation System, this system is becoming increasingly attractive as an option for coping more efficiently with current traffic levels, as well as with the increased traffic levels anticipated in the future.

The current air traffic management system is based on ground navigational aids, radar, and voice communications, and will eventually be unable to cope with predicted air traffic growth. In response, Boeing has been working with the industry since 1983 to create FANS, which relies on space-based navigation and communication.

Operator benefits offered by FANS include reduced fuel burn and flight time, through direct routing, and increased payload capability for takeoff-weight-limited flights. If FANS were implemented, operators would be able to take advantage of several needed improvements:

Reduced Separation Between Airplanes
In non-FANS procedural airplane separation, errors in navigation and potential errors in voice communication between the flight crew and air traffic controller are considered when determining the necessary airspace separation between airplanes. The uncertainties of traditional voice position reporting and the delay associated with high-frequency relayed voice communications [20 to 45 min to make a high-frequency voice position report] require the air traffic controller to allow a tremendous amount of airspace between each airplane, typically 100 nm laterally and 120 nm longitudinally. This computes to 48,000 mi2 of airspace to protect one airplane, and means that airplanes often operate at less-than-optimal altitudes and speeds.

However, through a satellite data link, airplanes equipped with FANS can transmit automatic dependent surveillance reports with actual position and intent information at least every five minutes. The position is based on the highly accurate Global Positioning System [GPS].

Digital data communication between the flight crew and the air traffic controller drastically reduces the possibility of error, and allows greatly reduced airplane separations. The combination of improvements in communication, navigation, and surveillance allows authorities to reduce required separation distances between airplanes, which in turn allows airplanes to fly at their optimum altitude and burn less fuel.

More Efficient Route Changes
Oceanic operations currently are based on weather data that are 12 to 18 hr old. By using the satellite data link that is part of FANS, however, the latest weather data can be transmitted to an airplane while it is en route. Flight crews can then use these data to develop optimised flight plans, or those plans can be generated on the ground and transmitted to the airplane. Such dynamic re-routing may allow airlines to consider reducing discretionary fuel, which further reduces fuel burn or allows an increase in payload.

Satellite Communication
Satellite communication can reduce to a few minutes the response time for an airplane requesting a step climb to a new, optimum altitude to reduce fuel burn. Response time is currently 20 to 60 min.

No Altitude Loss When Crossing Tracks
To avoid potential conflict, an airplane that is approaching crossing tracks must be separated by altitude from any traffic on another track. As a result, one of the two airplanes can be forced to operate as much as 4,000 ft below optimum altitude. But if the air traffic controller has timely surveillance data, including projected intent, and the airplane is able to control its speed so that it reaches the crossing point at a given time, altitude separation would be required less frequently.

More Direct Routings In many cases, current air traffic routings are compromised to take advantage of existing navigation aids and radar coverage, resulting in less-than-optimum routings. Taking advantage of space-based navigation and communication would allow more direct (shorter) routes.

With FANS in place, operators could benefit from reduced fuel burn and flight time as well as increased payload capacity for takeoff weight-limited flights. As a result, costs associated with crew and engine maintenance could be reduced, allowing operators to apply the money saved toward implementing and operating new routes. Airplanes must be equipped for several functions to support implementation of FANS:

Airline Operational Control Data Link

The AOC link gives airline data systems the ability to transmit new routes, position reports, and updated winds through the data link network.

Automatic Dependent Surveillance
The ADS function reports the current flight position via satellite or VHF data link to the air traffic controller or to the airline. This improves the surveillance of en route airplanes.

Air Traffic Control (ATC) Data Link
This function replaces the tactical communication between the flight crew and air traffic controller, allowing the flight crew to request deviations to, or replacements of, the filed flight plan. The air traffic controller also has the ability to directly request tactical changes to the airplane flight plan.

Global Positioning System [GPS]
This improvement provides a more accurate position for en route operations and some approach operations. The navigation system must demonstrate that it can meet the required navigational performance criteria.

Required Navigational Performance [RNP]
RNP criteria address accuracy, integrity and availability as set forth in FANS. The actual navigation performance is constantly monitored; if it exceeds the required navigational performance, the flight crew is alerted so that they can compensate for a situation in which they have less accurate information than the route requires.

Required time of arrival [RTA]
This gives the flight crew the ability to assign a time constraint to a waypoint, allowing the airplane to cross a latitude or longitude at a specified time. The cruise speed is automatically adjusted to achieve that time, plus or minus 30 seconds. If the RTA is not possible, the flight crew is notified with a visual alert.

FANS represents a potential solution to the growing need for an air navigation system with greater capability. If all elements of the system were implemented, operators could expect such benefits as reduced fuel burn and flight time as well as increased payload and cargo. Possible flight operations improvements resulting from FANS include reduced space between airplanes, more efficient route changes based on updated wind models, satellite communication, no altitude loss when crossing tracks, and more direct routings.

What is ACARS?

Aircraft Communication Addressing and Reporting System ACARS is a digital data link system transmitted via VHF radio, which allows airline flight operations departments to communicate with the various aircraft in their fleet.

This VHF digital transmission system, used by many civilian aircraft and business jets, can be likened to “email for airplanes,” as the registration of each aircraft is it’s unique address in the system developed by aeronautical radio giant ARINC [Aeronautical Radio, Inc.]. Traffic is routed via ARINC computers to the proper company, relieving some of the necessity for routine voice communication with the company. With ACARS, such routine items as departure reports, arrival reports, passenger loads, fuel data, engine performance data, and much more, can be requested by the company and retrieved from the aircraft at automatic intervals. Before the advent of ACARS, flight crews had to use VHF to relay this data to their operations on the ground.

The ACARS system is comprised of the following elements:

The aircraft Airborne Sub-system, which consists of the:

  • Management Unit which receives ground-to-air messages via the VHF radio transceiver, and also controls the replies.
  • Control Unit with which the aircrew interface with the ACARS system, consisting of a display screen and printer.

The ARINC Ground System, which consists of the:

  • ARINC ACARS remote transmitting/receiving stations
  • ARINC computer and switching systems

The Air Carrier C2 [Command and Control] and Management Subsystem
, which is basically all the ground based airline operations such as operations control, maintenance, crew scheduling and the like, linked up with the ACARS system.

Messages can be categorized in two ways:

“Downlinks” which are those ACARS transmissions which originate in the aircraft
“Uplinks” are those messages sent from the ground station to the aircraft.

How does a Radio Altimeter work?

A Radio Altimeter operates in the super high frequency band [4200-4400 MHz] and works on the same principle as Radar, reflecting the signal off the surface directly underneath the aircraft.

What is Transport Wander of an uncorrected gyro?

An uncorrected gyro will remain aligned in space and not compensate for travel over the earth as a sphere. On landing at the destination the INS would still be aligned with the earth’s surface at the departure aerodrome.

How does an INS find true north?

An INS finds true north by using a process called gyro compassing. Inside the IRU, the 3 gyros sense angular rate of the aircraft. Since the aircraft is stationary during alignment, the angular rate is due to the earth’s rotation. The IRU computer uses this angular rate to determine the direction of true north. The magnitude of the earth’s rotation is also measured which allows the IRC to estimate the aircraft’s latitude. This latitude is then compared to the latitude entered into the computer by the pilot during initialisation.

Reference an INS, what is Schuler’s Loop?

Schuler tuning is the torqueing of the gyros in an INS to take into account the curved flight path of an aircraft as it follows the earth’s surface. The rate of angular correction is calculated by dividing the velocity of the aircraft with the earth’s radius, plus the height of the aircraft.

What is the difference between an INS and an IRS?

An INS is a collection of components that together provide a system that is able to provide navigation computations and solutions. An IRS is different in that it is a single box that contains an Inertial Reference Unit [IRU] and a computer to produce reference information. This information is then provided to numerous other aircraft systems and displays.

What is an INS?

INS are completely self-contained and independent of ground based navigation aids. After being supplied with initial position information, it is capable of updating with accurate displays of position, attitude, and heading. It can calculate the track and distance between two points, display cross error, provide ETAs, ground speed and wind information. It can also provide guidance and steering information for the pilot instruments.

The system consists of the inertial platforminterior accelerometers and a computer. The platform, which senses the movement of the aircraft over the ground, contains two gyroscopes. These maintain their orientation in space while the accelerometers sense all direction changes and rate of movement. The information from the accelerometers and gyroscopes is sent to the computer, which corrects the track to allow for such factors as the rotation of the earth, the drift of the aircraft, speed, and rate of turn. The aircraft’s attitude instruments may also be linked to the inertial platform.

The accuracy of the INS is dependent on the accuracy of the initial position information programmed into the system. Therefore, system alignment before flight is very important. Accuracy is very high initially following alignment, and decays with time at the rate of about 1-2 NM per hour. Position updates can be accomplished in flight using ground based references with manual input or by automatic update using multiple DME or VOR inputs.

To operate a typical system, power is applied and the INS is activated. As the gyro’s spin up and the platform is aligned with the aircraft’s attitude, a keyboard is used to advise the system of the aircraft’s present position, normally in terms of latitude and longitude, and magnetic variation. This information is integrated into a mathematical model within the computer and, by a procedure known as gyro compassing; the system reckons its north reference point.

As the system is aligning, the co-ordinates of each waypoint along a planned route are entered into the computer. Additional information such as ground tracks, ground speed, and desired ETAs may also be entered in some systems.

Once airborne, the required information is normally displayed on a control display unit [CDU] in either a CRT or digital format. The INS may also be interfaced with other equipment and instruments in the aircraft. For example, a HSI may receive and display the information or an autopilot may be connected to the INS so the navigation information may be used to manoeuvre the aircraft.

En route, the pilot recalls the desired waypoint from the computer. The computer provides and displays steering and distance information to the aircraft’s normal navigation instruments. Alterations or deviations from the pre-planned route may be carried out by entering the coordinates of the new waypoint into the computer.

Many factors contribute to errors in an inertial navigation system. In-flight errors arise from imperfections in gyros, accelerometers and computers. Initial misalignment may cause additional errors. Some errors and their effects are discussed in the following paragraphs.

Initial Levelling:
If the platform is not correctly levelled the resultant tilt angle will allow the accelerometer to “see” the effect of gravity and thus have outputs besides true vehicle accelerations. The result of this is distance errors.

The imperfect sensitivity and alignment of the accelerometers from which all information is drawn will lead to velocity and distance errors.

Integrator Errors:
Integration errors may be due to drift, faulty calibration, or non-linearity of errors in the initial conditions established [rounding off in the equation]. Depending on which stage of integration the errors occur, they may or may not increase with time and may be in any of the vc1ocity, distance or position solutions.

Initial Azimuth Misalignment:
An error due to, misalignment in azimuth will give rise to velocity errors. Once integrated these velocity errors will lead to ever increasing distance and position errors.

Levelling Gyro Drift:
The random precession of gyros will tend to turn the platform away from the horizontal causing an oscillation action as the accelerometers try to correct. This oscillation, depending on its period, will cause velocity and subsequent distance errors.

Azimuth Gyro Drift:
Small position errors may occur due to azimuth gyro drift. However, gyro drift about the azimuth axis produces in-flight azimuth errors that are small compared to the initial misalignment errors in azimuth.

Computer Errors:
Errors in the computer are attributable to two basic causes; hardware limitations and approximations made in deriving equations. As modern digital computers eliminate most hardware/software problems only minor approximation errors remain.

Ring Laser Gyro in an INS
The ring laser gyro is a triangle shaped device with a silicone body and a gas filled cavity. A cathode and two anodes are used to excite the gas and produce two laser beams travelling in opposite directions. Reflectors in each corner are used to reflect the lasers around the unit’s body.

If the two laser beams travel the same distance, there will be no change in their frequency. However, if the unit is moved (accelerated), one light beam will travel a greater distance to the detector than the other beam. The beam travelling the greater distance will have a lower frequency than the beam travelling the shorter distance.

The detector analyses these frequency changes and sends the information to the computer, which then translates the data into movement in space.

By using three ring laser gyros and three accelerometers placed at right angles, it is possible to interpret all movement of the aircraft in space. This type of system has no moving parts and makes it ideal for “Strap-down” inertial systems.

Strap-down INS
A Strap-down INS is one that is “hard mounted” to the aircraft. There is no need for a stabilized platform such as that utilized in a conventional INS.

As the aircraft flies along, the ring laser gyros detect vertical acceleration, heading and velocity changes. Rather than continuously repositioning the sensor package, as in a conventional system, the computer recognizes the changes and mathematically processes them. By using computer software to maintain the inertial references, the stabilized platform with its motors, gimbals and angular measuring devices is eliminated.

Ring Laser Gyro advantages
The ring laser gyro INS combines high accuracy, low power requirements, small size and lightweight with an instant alignment capability. In addition, because there are no moving parts involved, the ring laser gyros INS generally has a very high serviceability rate.

Tell me about an ILS

The ILS provides the lateral and vertical guidance necessary to fly a precision approach, where glide slope information is provided. A precision approach is an approved descent procedure using a navigation facility aligned with a runway where glide slope information is given. When all components of the ILS system are available, including the approved approach procedure, the pilot may execute a precision approach.

ILS is classified by category in accordance with the capabilities of the ground equipment. Category I ILS provides guidance information down to a decision height [DH] of not less than 200 ft. Improved equipment [airborne and ground] provide for Category II ILS approaches. A DH of not less than 100 ft [radar altimeter] is authorized for Category II ILS approaches.

The ILS consists of:

  • Localizer transmitter
  • Glide path transmitter
  • Outer marker [can be replaced by an NDB or other fix]
  • Approach lighting system

Localiser transmitter
The primary component of the ILS is the localizer, which provides lateral guidance. The localizer is a VHF radio transmitter and antenna system using the same general range as VOR transmitters [between 108.10 MHz and 111.95 MHz]. Localizer frequencies, however, are only on odd-tenths, with 50 kHz spacing between each frequency. The transmitter and antenna are on the centreline at the opposite end of the runway from the approach threshold. The localizer back course is used on some, but not all ILS systems. Where the back course is approved for landing purposes, it is generally provided with a 75 MHz back marker facility or NDB located 3 to 5 NM from touchdown. The course is checked periodically to ensure that it is positioned within specified tolerances. The signal transmitted by the localizer consists of two vertical fan-shaped patterns that overlap, at the centre. They are aligned with the extended centreline of the runway. The right side of this pattern is modulated at 150 Hz and is called the blue area. The left side of the pattern is modulated at 90 Hz and is called the yellow area. The overlap between the two areas provides the on-track signal. The width of the navigational beam may be varied from approximately 3º to 6º, with 5º being normal. It is adjusted to provide a track signal approximately 700 ft wide at the runway threshold. The width of the beam increases so that at 10 NM from the transmitter, the beam is approximately one mile wide.

The localizer is identified by an audio signal superimposed on the navigational signal. The audio signal is a two-letter identification preceded by the letter “I”, e.g., ” I-OW “. The reception range of the localizer is at least 18 NM within 10º degrees of the on-track signal. In the area from l0º to 35º of the on-track signal, the reception range is at least 10 NM. This is because the primary strength of the signal is aligned with the runway centreline.

A localizer receiver receives the localizer signal in the aircraft. The localizer receiver is combined with the VOR receiver in a single unit. The two receivers share some electronic circuits and also the same frequency selector, volume control, and ON-OFF control. The localizer signal activates the vertical needle called the course bar.

In the overlap area, both signals apply a force to the needle, causing a partial deflection in the direction of the strongest signal. At the point where the 90 Hz and 150 Hz signals are of equal intensity, the course bar is centred, indicating that the aircraft is located precisely on the approach track. Full-scale needle deflection occurs at approximately 2.5º from the centre of the localizer beam.

Glide slope transmitter

The glide slope provides vertical guidance to the pilot during the approach. A ground-based UHF radio transmitter and antenna system, operating at a range of 329.30 MHz to 335.00 MHz, with 50 kHz spacing between each channel, produce the ILS glide slope signal. The transmitter is located 750’ to 1,250’ down the runway from the threshold, offset 400 to 600 ft from the runway centreline. Monitored to a tolerance of ± 1/2 degree, th

e UHF glide path is paired with [and usually automatically tuned by selecting] a corresponding VHF localizer frequency.

Like the localizer, the glide slope signal consists of two overlapping beams modulated at 90 Hz and 150 Hz. Unlike the localizer, however, these signals are aligned above each other and are radiated primarily along the approach track. The thickness of the overlap area is 1.4º or 0.7º above and 0.7º below the optimum glide slope.

This glide slope signal may be adjusted between 2º and 4.5º above a horizontal plane. A typical adjustment is 2.5º to 3º, depending upon such factors as obstructions along the approach path and the runway slope. False signals may be generated along the glide slope in multiples of the glide path angle, the first being approximately 6º degrees above horizontal. This false signal will be a reciprocal signal [i.e. the fly up and fly down commands will be reversed]. The false signal at 9º will be oriented in the same manner as the true glide slope. There are no false signals below the actual slope. An aircraft flying according to the published approach procedure on a front course ILS should not encounter these false signals.

A UHF receiver in the aircraft receives the glide slope signal. In modern avionics installations, the controls for this radio are integrated with the VOR controls so that the proper glide slope frequency is tuned automatically when the localizer frequency is selected. The glide slope signal activates the glide slope needle, located in conjunction with the course bar. There is a separate OFF flag in the navigation indicator for the glide slope needle. This flag appears when the glide slope signal is too weak. As happens with the localizer, the glide slope needle shows full deflection until the aircraft reaches the point of signal overlap. At this time, the needle shows a partial deflection in the direction of the strongest signal. When both signals are equal, the needle centres horizontally, indicating that the aircraft is precisely on the glide path. The pilot may determine precise location with respect to the approach path by referring to a single instrument because the navigation indicator provides both vertical and lateral guidance. With 1.4º of beam overlap, the area is approximately 1,500 ft thick at 10nm, 150 ft at l NM, and less than one foot at touchdown.

Marker beacons
Instrument landing system marker beacons provide information on distance from the runway by identifying predetermined points along the approach track. These beacons are low-power transmitters; that operate at a frequency of 75 MHz with 3W or less rated power output.

They radiate an elliptical beam upward from the ground. At an altitude of 1,000 ft, the beam dimensions are 2,400 ft long and 4,200 ft wide. At higher altitudes, the dimensions increase significantly. The outer marker [OM] [if installed] is located 3 1/2 to 6 NM from the threshold within 250 ft of the extended runway centreline. It intersects the glide slope vertically at approximately 1,400 ft above runway elevation. It also marks the approximate point at which aircraft normally intercept the glide slope, and designates the beginning of the final approach segment. The signal is modulated at 400 Hz, which is an audible low tone with continuous Morse code dashes at a rate of two dashes per second. A 75 MHz marker beacon receiver receives the signal in the aircraft. The pilot bears a tone over the speaker or headset and sees a blue light that flashes in synchronization with the aural tone. Where geographic conditions prevent the positioning of an outer marker, a DME unit may be included as part of the ILS system to provide the pilot with the ability to make a positive position fix on the localizer. In most ILS installations, the OM is replaced by an NDB.

Approach lighting systems
Various runway environment lighting systems serve as integral parts of the ILS system to aid the pilot in landing. Any or all of the following lighting systems may be provided at a given facility: approach light system [ALS], sequenced flashing light [SFL], touchdown zone lights [TDZ] and centreline lights [CLL-required for Category II [Cat II] operations]

What can you tell me about an NDB/ADF?

In Australia they are the oldest established, and most common radio navigation aid. The beacons are usually located at or near an airfield although a very few are still sited to mark waypoints along air routes.

NDBs transmit an omni-directional carrier signal in the low frequency band between 190 and 535 kHz. Their effective range is primarily dependent on the operating power. The rated coverage of each NDB is shown in the ERSA entry for the airfield or waypoint.

Low power NDBs, known as ‘locators’ with a range of 30 nm or less, are sited around major airports and are associated with their Instrument Landing Systems [ILS]. There are also high power [2–3 kilowatt] NDBs sited near the coast to provide guidance for over water routes, the over water range being much greater than their inland range.
The carrier wave is transmitted on a specific frequency but a two or three-letter Morse code signal is continually superimposed on the carrier for NDB identification. The companion airborne system which makes use of the NDBs – known as Automatic Direction Finding [ADF] equipment – can also receive transmissions in the 520 to 1611 kHz AM broadcast band.

The ADF, or radio compass, equipment consists of an

  • Antenna system
  • Receiver/control box system [panel mounted]
  • Indicator instrument

The antenna system comprises a loop antenna and a sense antenna, which depending on the age of any particular unit, may be completely separate or combined into one unit. The loop antenna nowadays may be a fixed square ferrite core with two perpendicular windings and may be coupled with a goniometer – (a device for measuring angles, with a great number of scientific applications) – in the receiver.

Such a system automatically ascertains the direction of the transmitter relative to the longitudinal axis of the aircraft. Hence the reason for the term “Automatic” DF because in earlier days the loop antenna was a physical loop (mounted on top of, or beneath, the fuselage and often enclosed in an egg shaped fairing) which, simply put, had to be manually rotated by the operator to find the direction of the transmitter, which was read off a scale. At that time, and later, the sense antenna was a wire from the top of the tail fin to a fuselage connection.

The output from the receiver is fed to the panel-mounted instrument, which is a needle indicating the direction to the NDB, or broadcast station, as an angle relative to the aircraft’s longitudinal axis. Behind the needle is a circular card marked off in 5 degree azimuth divisions from 0° to 355° with a mark at the top dead centre [TDC] indicating the aircraft’s nose. Depending on the age of the instrument that card may be fixed, in which case 0° is always at TDC, or, more commonly, manually rotatable by turning a heading knob on the instrument.

If the card is rotated so that the aircraft’s current magnetic heading is situated at TDC then the head of the needle indicates the magnetic track to the transmitter and the tail of the needle indicates the reciprocal bearing – the aircraft’s magnetic bearing from the station.

Heavier aircraft are usually fitted with a more complex, and very expensive, form of ADF called a Radio Magnetic Indicator [RMI], which incorporates, or is slaved to, a directional gyro. It may also have a two-needle display.

NDB/ADF system errors:
Electrical interference Radio waves emitted by the aircraft alternator in the frequency band of the ADF.

Electrical energy emitted in the NDB band and will deflect the ADF needle towards the storm.

Twilight/night effect
Radio waves arriving at a receiver come both directly from the transmitter – the ground wave – and indirectly as a wave reflected from the ionosphere – the sky wave. The sky wave is affected by the daily changes in the ionosphere. Twilight effect is minimal on transmissions at frequencies below 350 kHz.

Terrain and coastal effects
In mountainous areas NDB signals may be reflected by the terrain, which can cause the bearing indications to fluctuate. Some NDBs located in conditions where mountain effect is troublesome transmit at the higher frequency of 1655 kHz. Ground waves are refracted when passing across coast lines at low angles and this will affect the indicated bearing for an aircraft tracking to seaward and following the shore line.

Co-channel interference
If signals from other NDBs operating on the same or adjacent frequencies is received with sufficient strength error may result. Thus NDBs are spaced geographically and frequencies are allocated to minimise error.

What can you tell me about a VOR?

The VHF Omni-directional Radio Range is a VHF ‘line of sight’ non-precision radio navigation aid operating in Australia in the 112.1 – 117.9 range.

The VOR basic principle of operation is to radiate 2 signals, 1 is an omni-directional phase, [i.e. radiating in all directions] known as the Reference phase. The other signal, the Variable phase rotates at 1800 rpm and varies in phase with azimuth. Phase [wavelength] relationship is used to compare the two signals to establish what radial the user is on using magnetic north is used as a baseline.

000M = in phase
090M = 90º out of phase etc.

System errors 

Ground station error
[transmitter, aerial and earth system]< +/-2º

Site effect error
[topographical features]< +/-3º

Terrain effect
[terrain reflecting signals]< +/-2º

Vertical polarization
[banked aircraft receiving only horizontally Polarised]

Airborne Equipment error
[various components]< +/-2º Aggregate error [algebraic sum of all above errors typically Not more than +/-5º]

What are the advantages of GPS?

  • No ground based equipment necessary
  • Aircraft equipment is light and relatively inexpensive
  • High degree of accuracy
  • Most errors are known and can be corrected or compensated for
What is DGPS?

Differential GPS [DGPS] is the regular Global Positioning System [GPS] with an additional correction [differential] signal added. This correction signal improves the accuracy of the GPS and can be broadcast over any authorized communication channel.

The GPS determined position of a reference station is computed and compared to its surveyed geodetic position. The differential information [some systems use the error in fix position, while others use individual satellite range errors] is transmitted to user receivers by radio or other means.

  • DGPS accuracy and integrity are better than GPS.
  • Accuracy improvement [2drms]: Positions of 10 meters or better are achievable using DGPS vs. 100 meters or better for GPS [Standard Positioning Service]
  • Integrity improvement: Provides an independent check of each GPS satellite’s signal, and reports whether it’s good or bad.
What can you tell me about GPS?

GPS is a US DoD developed, worldwide, satellite-based radio-navigation system that will be the DoD’s primary radionavigation system well into the next century.

The GPS consists of three major segments:

The space segment consists of 24 operational satellites [Space Vehicles, (SV)] in six orbital planes [four satellites in each plane]. The satellites operate in circular 20,200 km [10,900 nm] orbits at an inclination angle of 55 degrees and with a 12-hour period. The position is therefore the same at the same sidereal time each day, i.e. the satellites appear 4 minutes earlier each day.

The Control segment consists of five Monitor Stations [Hawaii, Kwajalein, Ascension Island, Diego Garcia, Colorado Springs], three Ground Antennas, [Ascension Island, Diego Garcia, Kwajalein], and a Master Control Station [MCS] located at Schriever AFB in Colorado. The monitor stations passively track all satellites in view, accumulating ranging data. This information is processed at the MCS to determine satellite orbits and to update each satellite’s navigation message. Updated information is transmitted to each satellite via the Ground Antennas.

The user segment consists of antennas and receiver-processors that provide positioning, velocity, and precise timing to the user.

Survey model
Worldwide Geodetic System 1984 [WGS84] Is a mathematical model of the earth’s surface [Oblate spheroid]

GPS provides two levels of service:

Standard Positioning Service [SPS]
is a positioning and timing service, which will be available to all GPS users on a continuous, worldwide basis with no direct charge. SPS will be provided on the GPS L1 frequency, which contains a coarse acquisition [C/A] code and a navigation data message. SPS provides a predictable positioning accuracy of 100 meters [95 percent] horizontally and 156 meters [95 percent] vertically and time transfer accuracy to UTC within 340 nanoseconds [95 percent].

Precise Positioning Service [PPS]

is a highly accurate military positioning, velocity and timing service which will be available on a continuous, worldwide basis to users authorized by the U.S. P [Y] code capable military user equipment provides a predictable positioning accuracy of at least 22 meters [95 percent] horizontally and 27.7 meters vertically and time transfer accuracy to UTC within 200 nanoseconds [95 percent]. PPS will be the data transmitted on the GPS L1 and L2 frequencies. PPS was designed primarily for U.S. military use. It will be denied to unauthorized users by the use of cryptography. PPS will be made available to U.S. and military and U.S. Federal Government users. Limited, non-Federal Government, civil use of PPS, both domestic and foreign, will be considered upon request and authorized on a case-by-case basis, provided: It is in the U.S. national interest to do so.

Signal characteristics
The satellites transmit on two L-band frequencies: L1 = 1575.42 MHz and L2 = 1227.6 MHz. Three pseudo-random noise [PRN] ranging codes are in use.

Coarse/acquisition [C/A]

code has a 1.023 MHz chip rate, a period of 1 millisecond (ms) and is used primarily to acquire the P-code.

Precision [P] code has a 10.23 MHz rate, a period of 7 days and is the principal navigation ranging code.

is used in place of the P-code whenever the anti-spoofing [A-S] mode of operation is activated.

The C/A code is available on the L1 frequency and the P-code is available on both L1 and L2.

The various satellites all transmit on the same frequencies, L1 and L2, but with individual code assignments.

Due to the spread spectrum characteristic of the signals, the system provides a large margin of resistance to interference. Each satellite transmits a navigation message containing its orbital elements, clock behaviour, system time and status messages. In addition, an almanac is also provided which gives the approximate data for each active satellite. This allows the user set to find all satellites once the first has been acquired.


Selective availability and Anti-spoofing
Selective Availability [SA], or denial of full accuracy, is accomplished by manipulating navigation message orbit data (epsilon) and/or satellite clock frequency (dither).

Anti-spoofing [A-S] guards against fake transmissions of satellite data by encrypting the P-code to form the Y-code.

GPS System time
Its Composite Clock [CC] gives GPS system time. The CC or “paper” clock consists of all operational Monitor Station and satellite frequency standards.
GPS system time, in turn, is referenced to the Master Clock [MC] at the USNO and steered to UTC from which system time will not deviate by more than one microsecond. The exact difference is contained in the navigation message in the form of two constants, A0 and A1, giving the time difference and rate of system time against UTC [USNO, MC]. UTC itself is kept very close to the international benchmark UTC [BIPM], and the exact difference, USNO vs. BIPM is available in near real time. The latest individual satellite measurements are updated daily.

GPS time transfer
GPS is at the present time the most competent system for time transfer, the distribution of Precise Time and Time Interval [PTTI]. The system uses time of arrival [TOA] measurements for the determination of user position. A precisely timed clock is not essential for the user because time is obtained in addition to position by the measurement of TOA of FOUR satellites simultaneously in view. If altitude is known [i.e. for a surface user], then THREE satellites are sufficient. If time is being kept by a stable clock [say, since the last complete coverage], then TWO satellites in view are sufficient for a fix at known altitude. If the user is, in addition, stationary or has a known speed then, in principle, the position can be obtained by the observation of a complete pass of a SINGLE satellite. This could be called the “transit” mode, because the old TRANSIT system uses this method. In the case of GPS, however, the apparent motion of the satellite is much slower, requiring much more stability of the user clock.

GPS errors
GPS errors are a combination of noise, bias, and blunders.

Noise errors are the combined effect of PRN code noise [around 1 meter] and noise within the receiver noise [around 1 meter].

Bias errors result from Selective Availability and other factors.

Selective Availability [SA] is the intentional degradation of the SPS signals by a time varying bias. SA is controlled by the DoD to limit accuracy for non-U. S. Military and government users. The potential accuracy of the C/A code of around 30 meters is reduced to 100 meters [two standard deviations]. The SA bias on each satellite signal is different, and so the resulting position solution is a function of the combined SA bias from each SV used in the navigation solution. Because SA is a changing bias with low frequency terms in excess of a few hours, position solutions or individual SV pseudo-ranges cannot be effectively averaged over periods shorter than a few hours. Differential corrections must be updated at a rate less than the correlation time of SA [and other bias errors].

Other Bias Error sources:
SV clock errors uncorrected by Control Segment: 1-metre errors.
Ephemeris data errors: 1 metre
Tropospheric delays: 1 metre.

The troposphere is the lower part [ground level to from 8 to 13 km] of the atmosphere that experiences the changes in temperature, pressure, and humidity associated with weather changes. Complex models of tropospheric delay require estimates or measurements of these parameters.

Un-modelled ionosphere delays
: 10 metres.

The ionosphere is the layer of the atmosphere from 50 to 500 km that consists of ionised air. The transmitted model can only remove about half of the possible 70 ns of delay leaving a ten meter un-modelled residual.

Multi-path effects: 0.5 metres.

Multi-path is caused by reflected signals from surfaces near the receiver that can either interfere with or be mistaken for the signal that follows the straight-line path from the satellite. Multi-path is difficult to detect and sometime hard to avoid.

Blunders can result in errors of hundred of kilometres. Control segment mistakes due to computer or human error can cause errors from one meter to hundreds of kilometres. User mistakes, including incorrect geodetic datum selection, can cause errors from 1 to hundreds of meters. Receiver errors from software or hardware failures can cause blunder errors of any size. Noise and bias errors combine, resulting in typical ranging errors of around fifteen meters for each satellite used in the position solution.

Geometric Dilution of Precision [GDOP] and Visibility
GPS ranging errors are magnified by the range vector differences between the receiver and the SVs. The volume of the shape described by the unit-vectors from the receiver to the SVs used in a position fix is inversely proportional to GDOP.

Poor GDOP, a large value representing a small unit vector-volume, results when angles from receiver to the set of SVs used are similar.

Good GDOP, a small value representing a large unit-vector-volume, results when angles from receiver to SVs are different.

GDOP is computed from the geometric relationships between the receiver position and the positions of the satellites the receiver is using for navigation. For planning purposes GDOP is often computed from Almanacs and an estimated position. Estimated GDOP does not take into account obstacles that block the line-of-sight from the position to the satellites. Estimated GDOP may not be realizable in the field. Good Computed GDOP and Bad Visibility Equals Poor GDOP GDOP terms are usually computed using parameters from the navigation solution process.

In general, ranging errors from the SV signals are multiplied by the appropriate GDOP term to estimate the resulting position or time error. Various GDOP terms can be computed from the navigation covariance matrix. ECEF XYZ DOP terms can be rotated into a Northeast Down [NED] system to produce local horizontal and vertical DOP terms.

GDOP Components

PDOP = Position Dilution of Precision [3-D], sometimes the Spherical DOP.
HDOP = Horizontal Dilution of Precision [Latitude, Longitude].
VDOP = Vertical Dilution of Precision [Height].
TDOP = Time Dilution of Precision [Time].

While each of these GDOP terms can be individually computed, they are formed from covariances and so are not independent of each other. A high TDOP [time dilution of precision], for example, will cause receiver clock errors, which will eventually result in increased position errors.


Error source Potential error Typical error
Ionosphere 10.0 meters 0.4 meters
Troposphere 1.0 meters 0.2 meters
Ephemeris data 1.0 meters 0 meters
Satellite clock drift 1.0 meters 0 meters
Multi-path 0.5 meters 0.6 meters
Measurement noise 1.0 meters 0.3 meters
Total 14.5 meters 10 meters

Calculating a Position 
A GPS receiver calculates its position by a technique called satellite ranging, which involves measuring the distance between the GPS receiver and the GPS satellites it is tracking. The range [the range a receiver calculates is actually a pseudorange, or an estimate of range rather than a true range] or distance is measured as elapsed transit time. The position of each satellite is known, and the satellites transmit their positions as part of the “messages” they send via radio waves. The GPS receiver on the ground is the unknown point, and must compute its position based on the information it receives from the satellites.

Measuring Distance to Satellites
The first step in measuring the distance between the GPS receiver and a satellite requires measuring the time it takes for the signal to travel from the satellite to the receiver. Once the receiver knows how much time has elapsed, it multiplies the travel time of the signal times the speed of light [because the satellite signals travel at the speed of light, approximately 186,000 miles per second] to compute the distance. Distance measurements to four satellites are required to compute a 3-dimensional [latitude, longitude and altitude] position. In order to measure the travel time of the satellite signal, the receiver has to know when the signal left the satellite and when the signal reached the receiver. Knowing when the signal reaches the receiver is easy, the GPS receiver just “checks” its internal clock when the signal arrives to see what time it is. But how does it “know” when the signal left the satellite? All GPS receivers are synchronized with the satellites so they generate the same digital code at the same time. When the GPS receiver receives a code from a satellite, it can look back in its memory bank and “remember” when it emitted the same code. This little “trick” allows the GPS receiver to determine when the signal left the satellite.

Requirements for navigation
3 3D position fix
4 RAIM [with baro-aiding]
5 RAIM [without baro-aiding]
6 RAIM maintenance [i.e. redundant satellite]

What is EGPWS?

EGPWS integrates 3 alerting functional areas into a single line replaceable unit:

  • Ground proximity warning [including callouts]
  • Terrain [or obstacle] Awareness Display [TAD]
  • Terrain Clearance Floor [TCF]

EGPW Computer
Cockpit audio system
Alert Annunciators, mode selector switches and Terrain display units Envelope modulation

What are the inputs to GPWS?

Input Warning
Barometric pressure change Sink rate”
ILS/GS “Glide slope”
Radar Altimeter “Terrain”
Aircraft configuration “Too low, gear/flap”



Mode 1: Excessive decent rate
“Sink rate, Whoop Whoop pull up”

Mode 2: Excessive terrain closure rate
“Terrain, Whoop whoop, pull up”

Mode 3: Altitude loss after take-off or go-around
“Don’t sink”

Mode 4:Unsafe terrain clearance during high speed flight or while not in the landing configuration
“Too low Gear/flap, Too low terrain”

Mode 6: Excessive bank angle at low altitude
“Bank angle, Bank angle”