Computers & Technology

  • Author Ibrahim Ahmad
  • Published May 12, 2022
  • Word count 8,750


Air navigation is defined as "the process of determining the geographic position and maintaining the desired direction of an aircraft relative to the surface of the earth.

Navigation may be defined as the movement of a vehicle from one place to another. It is an art of practiced by all who travel but its development is rooted firmly in the fundamental laws of science. In today’s context it can be formally defined as the determination as a strategy for estimating the position of a vehicle along the flight path, giving output from the specified sensors.

In the early days, when man-made vehicle where surfaced bound (either or land or on sea) and they seldom ventured per beyond easily recognizable land marks, the act of navigation could be carried out by human using their sense to determine direction distance, speed and position. As vehicle become more sophisticated navigation instrument become necessary. Instead of known landmarks this instruments used information learned from celestial bodies, certain distance objects on the surface of the earth, and many other source of information carry out the job of navigation.


Whenever a purposeful change in location has to take place for an aircraft the following questions must be asked and answered;

i. Where is the aircraft now?

ii. (More specifically) where is the aircraft now with respect to where it should have been?

These questions are answered by navigation system.

These are number of reasons why sophisticated navigation system becomes so important in modern days, some of them are;

  1. Time large between measurement and decision to be reduced

  2. Number of aircraft in a given airspace has increased manifold in the past few decades

  3. Safety requirement have become crucial

A navigation system may provide information in a variety of forms, appropriate to the needs of aircraft. If the information is primarily for the benefit of the crew, it involves some types of display. Other output, however may involve steering and signal sent to a central computer. However, in the modern context one would consider this system as navigation-cum-guidance-system.

The basic principles of air navigation are identical to general navigation, which includes the process of planning, recording, and controlling the movement of a craft from one place to another. Successful air navigation involves piloting an aircraft from place to place without getting lost, not breaking the laws applying to aircraft, or endangering the safety of those on board or on the ground. Air navigation differs from the navigation of surface craft in several ways; Aircraft travel at relatively high speeds, leaving less time to calculate their position en route. Aircraft normally cannot stop in mid-air to ascertain their position at leisure. Aircraft are safety-limited by the amount of fuel they can carry; a surface vehicle can usually get lost, run out of fuel, then simply await rescue. There is no in-flight rescue for most aircraft. Additionally, collisions with obstructions are usually fatal. Therefore, constant awareness of position is critical for aircraft pilots.

The techniques used for navigation in the air will depend on whether the aircraft is flying under visual flight rules (VFR) or instrument flight rules (IFR). In the latter case, the pilot will navigate exclusively using instruments and radio navigation aids such as beacons, or as directed under radar control by air traffic control. In the VFR case, a pilot will largely navigate using "dead reckoning" combined with visual observations (known as pilotage), with reference to appropriate maps. This may be supplemented using radio navigation aids or satellite based positioning systems.

Route planning

The first step in navigation is deciding where one wishes to go. A private pilot planning a flight under VFR will usually use an aeronautical chart of the area which is published specifically for the use of pilots. This map will depict controlled airspace, radio navigation aids and airfields prominently, as well as hazards to flying such as mountains, tall radio masts, etc. It also includes sufficient ground detail – towns, roads, wooded areas – to aid visual navigation. In the UK, the CAA publishes a series of maps covering the whole of the UK at various scales, updated annually. The information is also updated in the notices to airmen, or NOTAMs.

The pilot will choose a route, taking care to avoid controlled airspace that is not permitted for the flight, restricted areas, and danger areas and so on. The chosen route is plotted on the map, and the lines drawn are called the track. The aim of all subsequent navigation is to follow the chosen track as accurately as possible. Occasionally, the pilot may elect on one leg to follow a clearly visible feature on the ground such as a railway track, river, highway, or coast.

The first step in navigation is deciding where one wishes to go. A private pilot planning a flight under VFR will usually use an aeronautical chart of the area which is published specifically for the use of pilots. This map will depict controlled airspace, radio navigation aids and airfields prominently, as well as hazards to flying such as mountains, tall radio masts, etc. It also includes sufficient ground detail – towns, roads, wooded areas – to aid visual navigation. In the UK, the CAA publishes a series of maps covering the whole of the UK at various scales, updated annually. The information is also updated in the notices to airmen, or NOTAMs.

The pilot will choose a route, taking care to avoid controlled airspace that is not permitted for the flight, restricted areas, and danger areas and so on. The chosen route is plotted on the map, and the lines drawn are called the track. The aim of all subsequent navigation is to follow the chosen track as accurately as possible. Occasionally, the pilot may elect on one leg to follow a clearly visible feature on the ground such as a railway track, river, highway, or coast.

The aircraft in the picture is flying towards B to compensate for the wind from SW and reach point C.

When an aircraft is in flight, it is moving relative to the body of air through which it is flying; therefore, maintaining an accurate ground track is not as easy as it might appear, unless there is no wind at all—a very rare occurrence. The pilot must adjust heading to compensate for the wind, in order to follow the ground track. Initially the pilot will calculate headings to fly for each leg of the trip prior to departure, using the forecast wind directions and speeds supplied by the meteorological authorities for the purpose. These figures are generally accurate and updated several times per day, but the unpredictable nature of the weather means that the pilot must be prepared to make further adjustments in flight. A general aviation (GA) pilot will often make use of either a flight computer – a type of slide rule – or a purpose-designed electronic navigational computer to calculate initial headings.

The primary instrument of navigation is the magnetic compass. The needle or card aligns itself to magnetic north, which does not coincide with true north, so the pilot must also allow for this, called the magnetic variation (or declination). The variation that applies locally is also shown on the flight map. Once the pilot has calculated the actual headings required, the next step is to calculate the flight times for each leg. This is necessary to perform accurate dead reckoning. The pilot also needs to take into account the slower initial airspeed during climb to calculate the time to top of climb. It is also helpful to calculate the top of descent, or the point at which the pilot would plan to commence the descent for landing.

The flight time will depend on both the desired cruising speed of the aircraft, and the wind – a tailwind will shorten flight times, a headwind will increase them. The flight computer has scales to help pilots compute these easily.

The point of no return, sometimes referred to as the PNR, is the point on a flight at which a plane has just enough fuel, plus any mandatory reserve, to return to the airfield from which it departed. Beyond this point that option is closed, and the plane must proceed to some other destination. Alternatively, with respect to a large region without airfields, e.g. an ocean, it can mean the point before which it is closer to turn around and after which it is closer to continue. Similarly, the Equal time point, referred to as the ETP (also critical point), is the point in the flight where it would take the same time to continue flying straight, or track back to the departure aerodrome. The ETP is not dependent on fuel, but wind, giving a change in ground speed out from, and back to the departure aerodrome. In Nil wind conditions, the ETP is located halfway between the two aerodromes, but in reality it is shifted depending on the wind speed and direction.

The aircraft that is flying across the Ocean for example would be required to calculate ETPs for one engine inoperative, depressurization, and a normal ETP; all of which could actually be different points along the route. For example, in one engine inoperative and depressurization situations the aircraft would be forced to lower operational altitudes, which would affect its fuel consumption, cruise speed and ground speed. Each situation therefore would have a different ETP.

Commercial aircraft are not allowed to operate along a route that is out of range of a suitable place to land if an emergency such as an engine failure occurs. The ETP calculations serve as a planning strategy, so flight crews always have an 'out' in an emergency event, allowing a safe diversion to their chosen alternate.

The final stage is to note which areas the route will pass through or over, and to make a note of all of the things to be done – which ATC units to contact, the appropriate frequencies, visual reporting points, and so on. It is also important to note which pressure setting regions will be entered, so that the pilot can ask for the QNH (air pressure) of those regions. Finally, the pilot should have in mind some alternative plans in case the route cannot be flown for some reason – unexpected weather conditions being the most common. At times the pilot may be required to file a flight plan for an alternate destination and to carry adequate fuel for this. The more work a pilot can do on the ground prior to departure, the easier it will be in the air.


Instrument flight rules (IFR) navigation is similar to visual flight rules (VFR) flight planning except that the task is generally made simpler by the use of special charts that show IFR routes from beacon to beacon with the lowest safe altitude (LSALT), bearings (in both directions), and distance marked for each route. IFR pilots may fly on other routes but they then must perform all such calculations themselves; the LSALT calculation is the most difficult. The pilot then needs to look at the weather and minimum specifications for landing at the destination airport and the alternate requirements. Pilots must also comply with all the rules including their legal ability to use a particular instrument approach depending on how recently they last performed one.

In recent years, strict beacon-to-beacon flight paths have started to be replaced by routes derived through performance-based navigation (PBN) techniques. When operators develop flight plans for their aircraft, the PBN approach encourages them to assess the overall accuracy, integrity, availability, continuity, and functionality of the aggregate navigation aids present within the applicable airspace. Once these determinations have been made, the operator develops a route that is the most time and fuel efficient while respecting all applicable safety concerns—thereby maximizing both the aircraft's and the airspace's overall performance capabilities.

Under the PBN approach, technologies evolve over time (e.g., ground beacons become satellite beacons) without requiring the underlying aircraft operation to be recalculated. Also, navigation specifications used to assess the sensors and equipment that are available in an airspace can be cataloged and shared to inform equipment upgrade decisions and the ongoing harmonization of the world's various air navigation systems.

IN Flight

Once in flight, the pilot must take pains to stick to plan, otherwise getting lost is all too easy. This is especially true if flying in the dark or over featureless terrain. This means that the pilot must stick to the calculated headings, heights and speeds as accurately as possible, unless flying under visual flight rules. The visual pilot must regularly compare the ground with the map, (pilotage) to ensure that the track is being followed although adjustments are generally calculated and planned. Usually, the pilot will fly for some time as planned to a point where features on the ground are easily recognized. If the wind is different from that expected, the pilot must adjust heading accordingly, but this is not done by guesswork, but by mental calculation – often using the 1 in 60 rule. For example, a two-degree error at the halfway stage can be corrected by adjusting heading by four degrees the other way to arrive in position at the end of the leg. This is also a point to reassess the estimated time for the leg. A good pilot will become adept at applying a variety of techniques to stay on track.

While the compass is the primary instrument used to determine one's heading, pilots will usually refer instead to the direction indicator (DI), a gyroscopically driven device which is much more stable than a compass. The compass reading will be used to correct for any drift (precession) of the DI periodically. The compass itself will only show a steady reading when the aircraft has been in straight and level flight long enough to allow it to settle.

Should the pilot be unable to complete a leg – for example bad weather arises, or the visibility falls below the minima permitted by the pilot's license, the pilot must divert to another route. Since this is an unplanned leg, the pilot must be able to mentally calculate suitable headings to give the desired new track. Using the flight computer in flight is usually impractical, so mental techniques to give rough and ready results are used. The wind is usually allowed for by assuming that sine A = A, for angles less than 60° (when expressed in terms of a fraction of 60° – e.g. 30° is 1/2 of 60°, and sine 30° = 0.5), which is adequately accurate. A method for computing this mentally is the clock code. However, the pilot must be extra vigilant when flying diversions to maintain awareness of position.

Some diversions can be temporary – for example to skirt around a local storm cloud. In such cases, the pilot can turn 60 degrees away his desired heading for a given period of time. Once clear of the storm, he can then turn back in the opposite direction 120 degrees, and fly this heading for the same length of time. This is a 'wind-star' maneuver and, with no winds aloft, will place him back on his original track with his trip time increased by the length of one diversion leg.

Another reason for not relying on the magnetic compass during flight, apart from calibrating the Heading indicator from time to time, is because magnetic compasses are subject to errors caused by flight conditions and other internal and external interferences on the magnet system.

Prior to the advent of GNSS, Celestial Navigation was also used by trained navigators on military bombers and transport aircraft in the event of all electronic navigational aids being turned off in time of war. Originally navigators used an astrodome and regular sextant but the more streamlined periscopic sextant was used from the 1940s to the 1990s. From the 1970s airliners used inertial navigation systems, especially on inter-continental routes, until the shooting down of Korean Air Lines Flight 007 in 1983 prompted the US government to make GPS available for civilian use.

Finally, an aircraft may be supervised from the ground using surveillance information from e.g. radar or multilateration. ATC can then feedback information to the pilot to help establish position, or can actually tell the pilot the position of the aircraft, depending on the level of ATC service the pilot is receiving.

The use of GNSS in aircraft is becoming increasingly common. GNSS provides very precise aircraft position, altitude, heading and ground speed information. GNSS makes navigation precision once reserved to large RNAV-equipped aircraft available to the GA pilot. Recently, many airports include GNSS instrument approaches. GNSS approaches consist of either overlays to existing precision and non-precision approaches or stand-alone GNSS approaches. Approaches having the lowest decision heights generally require that GNSS be augmented by a second system—e.g., the FAA's Wide Area Augmentation System (WAAS).

A flight vehicle regardless of whether it is a missile, an aircraft, or a launch vehicle needs the help of human intelligence in achieving it mission. This human intelligence manifest itself in various form like information about flight condition, gathering appropriate command to the flight vehicle and designing equipment to interpret these command and translate them into action onboard. Each flight vehicle has a mode or operation which might differ from another. Example, in missile or launch vehicle information is gathered by various sensors and conveyed to a computer which then appropriate decisions.

Irrespective of the kind of flight vehicle, the theory behind the design and analysis of all this take eventually emanate from a branch of applied mathematics called control theory. The application of control

The application of control theory aerospace may be divided into four (4) areas

  1. Flight planning: the determination of normal flight path and associated control histories for a giving flight vehicle to accomplish specific objectives with specified constraints.

  2. Navigation: the determination of a strategy for estimating the vehicle along the flight path, giving output from specified sensors.

  3. Guardians: the determination strategy for following the normal path in the presence of the normal coordination, wind disturbance and navigational uncertainties.

  4. Control: the determination of a strategy for maintaining angular orientation of the vehicle during the flight that is consistent with the guidance strategy, and the vehicle, crew and passenger’s constants

However, it should be kept in mind that these categories often overlap and the boundaries between them are not very sharp. Example, consider the aircraft velocity and its angular orientation. These are coupled and so the guidance control of aircraft must be considered together. To move or navigate from one place to another in the aircraft a pilot need to know the following;

• Starting point (point of departure)

• Ending point (final destination)

• Direction travel

• Aircraft speed

• Aircraft weight and balance information

With information flight planning can be done and the proper method of navigation can be put to be use

Direction is an angular distance from a reference. Direction, stated in whole numbers, is measured from 001° to a maximum of 360°. The reference for the angle can be either True North or Magnetic North. True North is the top of the earth whereas Magnetic North is the point from which all of the Earth's magnetic lines of force emanate.

Magnetic North is currently located near Hudson Bay in Canada. A magnetic compass system converts the energy from these lines of force to a cockpit indicator reading. Typical military aircraft have two compass systems: a primary and a secondary/back-up. The aircrew’s primary instrument for determining direction in the cockpit is the Remote Gyro Vertical Compass Card. This instrument is also referred to as a BDHI (Bearing Distance Heading Indicator) or EHSI (Electronic Horizontal Situation Indicator), but may vary by aircraft. In most modern aircraft, the inertial navigation system (INS) produces attitude and heading information for the aircrew through the use of a ring laser gyro (RLG) system and accelerometers.

Remote Gyro Vertical Compass Card

This data is used for pitch and roll displays as well as navigational computations. The ring laser gyro is a highly accurate way to measure changes in angular position (or angular rate) without the use of any moving parts or magnetic compass inputs. The laser gyros and accelerometers are positioned in the INS so that they are oriented along each of the three axes of the airplane. Strapping three ring laser gyros together with accelerometers on the X, Y, and Z axes of an aircraft, and then doing some math, allows for the continuous calculation of the attitude reference and changes in heading, pitch, and roll. The ring laser gyros are sensitive enough to detect the Earth’s rate of rotation and it uses that information to establish the heading of the airplane. The ASN-50 magnetic compass, also known as a flux valve, was once required to provide measurements of magnetic direction for older INS and mechanical gyro systems. The modern INS and RLG no longer require any external input from a magnetic compass to determine and maintain aircraft heading information (see Figure 1-3).

As a backup to the primary system, all aircraft have a Stand-by Compass (see Figure 1-4). This is a direct reading compass in which the measurement of direction is taken directly from a balanced/pivoted magnetic needle. The stand-by compass is sometimes called the "wet" compass because it is filled with a fluid to dampen needle movement. This compass is unstable during maneuvering, but it has the advantage of reliability and is independent of the aircraft’s electrical system.

Discussion of direction will continue in Lesson Topic 6.2 when charts and plotting techniques are introduced.

Time can be expressed in several ways: as the time of day (0815, 1400, etc.) or elapsed time. Elapsed time is written as hours and minutes or minutes and seconds. With elapsed time, the units are separated with a “+” sign (2+30, 3+15, etc.). It may also be expressed in a six digit format such as 09+15+20.

Estimated time of departure (ETD) and estimated time of arrival (ETA) can be expressed in four-digit time of day format, while elapsed time, such as estimated time en route (ETE), will be expressed in hours and minutes (or for short distances, minutes and seconds). All aircrew must be able to convert from local time to Greenwich Mean Time (Zulu time) and vice versa. This will be covered in greater detail in Lesson Topic 6.2.

Speed is the magnitude of the velocity of an aircraft. It is the distance traveled with respect to time and is stated in nautical miles per hour (knots). Lesson Topics 6.3 and 6.4 will cover speed in greater detail and explain how atmospheric conditions (altitude, temperature) affect it.

Speed = Distance Time


There are three method of navigation currently used.

• Navigation by pilotage (visual landmarks)

• Dead reckoning navigation

• Radio Navigation

• Celestials or astronomical navigation


Pilotage or Piloting is the most common method of air navigation. This method, the pilot keeps on course by following a series of landmarks on the ground. Usually before take-off, pilot will be making pre-flight planning; the pilot will draw a line on the aeronautical map to indicate the desired course. Pilot will knows various landmarks, such as highways, railroad tracks, rivers, bridges. As the pilot flies over each of landmark, pilot will check it off on the chart or map. If the plane does not pass directly over the landmark, the pilot will know that he has to correct the course.

Navigation fix aircraft position on map by observing non visible land marks. Using compass, the direction to next land mark or destination. Electronic pilotage is also use with the aid of air bone radar. This is called electronic pilotage. The micro search radar provided with a PPD (Plan Position Display) mapped to terrain is used for electronic pilotage.

Pilots have various navigation aids that help them takeoff, fly, and land safely. One of the most important aids is a series of air route traffic control, operated throughout the world. Most of the traffic control uses a radar screen to make sure all the planes in its vicinity are flying in their assigned airways. Airliners carry a special type of radar receiver and transmitter called a transponder. It receives a radar signal from control center and immediately bounces it back. When the signal got to the ground, it makes the plane show up on the radar screen.

Pilots have special methods for navigating across oceans. Three commonly used methods are:

  1. Inertial Guidance; this system has computer and other special devices that tell pilots where are the plane located.

  2. LORAN Long Range Navigation The plane has equipment for receiving special radio signals sent out continuous from transmitter stations. The signals will indicate the plane location

  3. GPS Global Positioning System. is the only system today able to show your exact position on the earth anytime, anywhere, and any weather? The system receiver on the aircraft will receives the signals from satellites around the globe


• Range is high (50 to 100 km)

• Accurate

If the ground is visible the navigator can see the principal features on the ground such as rivers, coastline, estuaries, rills etc. and fix his position. Even at night, the light beacons provide information about the position of the aircraft.


• Navigation by pilotage is possible only under of good visibility.

• Both the method of pilotage requires recognizable features in the terrain

• Cannot be used in the oceanographic regions (it will be useless over the stretch of sea if there are no island in the field of vision

• Pilotage depends upon the availability of accurate maps of the terrain


Dead Reckoning is defined as directing an aircraft and determining its position by the application of direction and speed data from a previous position. It is the basis for all types of air navigation. Navigation is both the history and prediction of an aircraft’s flight path. At the heart of DR are its four components: position, direction, time, and speed. Position is a set of coordinates that define the specific location of the aircraft above the earth’s surface. Direction is an angular measurement from a reference, which determines the actual flight path from a known starting point. Speed multiplied by time will produce the distance flown (or to be flown). The combination of these four components will allow the aircrew to determine the aircraft’s current position or to predict its future position. As with any mathematical relationship, if three of the four components are known, the fourth can be determined.

Dead Reckoning is the primary navigation method used in the early days of flying. It is the method on which Lindberg relied on his first trans-Atlantic flight. A pilot used this method when flying over large bodies of water, forest, deserts. It demands more skill and experience than pilotage does. It is based on time, distance, and direction only. The pilot must know the distance from one point to the next, the magnetic heading to be flown. Pilot works on the pre-flight plan chart, pilot plan a route in advance. Pilot calculates the time to know exactly to reach the destination while flying at constant speed. In the air, the pilot uses compass to keep the plane heading in the right direction. Dead reckoning is not always a successful method of navigation because of changing wind direction. It is the fundamental of VFR flight.

Position is a geographic point defined by coordinates. There are several coordinate systems available to determine a specific location on the earth’s surface. The primary system used in aviation is the latitude/longitude system

At the simplest level, navigation is accomplished through ideas known as dead reckoning and pilotage. Pilotage is a term that refers to the sole use of visual ground references. The pilot identifies landmarks, such as rivers, towns, airports, and buildings and navigates among them. The trouble with pilotage is that, often, references aren't easily seen and can't be easily identified in low visibility conditions or if the pilot gets off track even slightly. Therefore, the idea of dead reckoning was introduced.

Navigation determines the aircraft position in relation to its last fix a known position. Starting at the fix (position) the navigation draws a line on the chart that represent the direction and the distance traveled (velocity and time are known variables). Most commonly used method same principles applied in later advanced methods like inertial navigation.

Dead reckoning involves the use of visual checkpoints along with time and distance calculations. The pilot chooses checkpoints that are easily seen from the air and also identified on the map and then calculates the time it will take to fly from one point to the next based on distance, airspeed, and wind calculations. A flight computer aids pilots in computing the time and distance calculations and the pilot typically uses a flight planning log to keep track of the calculations during flight.


The position of an aircraft at any instance of time is calculated from the previous determine position, the speed of its motion (i.e. its velocity vector) and the elapsed.


Navigation by dead reckoning over long distance is subject to errors unless intermediate check is possible


Radio Navigation is used by almost all pilots. Pilots can find out from an aeronautical chart what radio station they should tune to in a particular area. They can then tune their radio navigation equipment to a signal from this station. A needle on the navigation equipment tells the pilot where they are flying to or from station, on course or not.

With aircraft equipped with radio navigation aids (NAVAIDS), pilots can navigate more accurately than with dead reckoning alone. Radio NAVAIDS come in handy in low visibility conditions and act as a suitable backup method for general aviation pilots that prefer dead reckoning. They are also more precise. Instead of flying from checkpoint to checkpoint, pilots can fly a straight line to a "fix" or an airport. Specific radio NAVAIDS are also required for IFR operations. There are different types of radio NAVAIDS used in aviation

Radio/Electronic Navigation;

Using Electromagnetic (Radio) waves to find the position of the aircraft. Transmitters located on the ground keep transmitting the radio signals. Air bone receivers received the radio signals from one or more ground station and determine the position, bearing/direction and distance etc.


• Requires transmitter/receivers not a self-contained (dependent on external aids)

• Aircraft is dependent on external systems

• It requires line sight of communication


Automatic direction finder (ADF) is a marine or aircraft radio-navigation instrument that automatically and continuously displays the relative bearing from the ship or aircraft to a suitable radio station. ADF receivers are normally tuned to aviation or marine NDBs (Non-Directional Beacon) operating in the LW band between 190 – 535 kHz. Like RDF (Radio Direction Finder) units, most ADF receivers can also receive medium wave (AM) broadcast stations, though as mentioned, these are less reliable for navigational purposes.

ADF systems provide following navigation information;

• Relative bearing to/from the ground station

• Station identification

The ground transmitter, non-directional beacon transmits radio waves. A directional antenna points to the beacon to receive the signal and enabler the pilot knows the direction of the ground station. Relative bearing of aircraft to ground station is indicated on the radio magnetic compass.

Frequency: medium frequency range (200 kHz to 3000 kHz).


• Reliable operation possible using ground waves up to few 100 miles

The operator tunes the ADF receiver to the correct frequency and verifies the identity of the beacon by listening to the Morse code signal transmitted by the NDB. On marine ADF receivers, the motorized ferrite-bar antenna atop the unit (or remotely mounted on the masthead) would rotate and lock when reaching the null of the desired station. A centerline on the antenna unit moving atop a compass rose indicated in degrees the bearing of the station. On aviation ADFs, the unit automatically moves a compass-like pointer (RMI) to show the direction of the beacon. The pilot may use this pointer to home directly towards the beacon, or may also use the magnetic compass and calculate the direction from the beacon (the radial) at which their aircraft is located.

Unlike the RDF, the ADF operates without direct intervention, and continuously displays the direction of the tuned beacon. Initially, all ADF receivers, both marine and aircraft versions, contained a rotating loop or ferrite loop stick aerial driven by a motor which was controlled by the receiver. Like the RDF, a sense antenna verified the correct direction from its 180-degree opposite.

More modern aviation ADFs contain a small array of fixed aerials and use electronic sensors to deduce the direction using the strength and phase of the signals from each aerial. The electronic sensors listen for the trough that occurs when the antenna is at right angles to the signal, and provide the heading to the station using a direction indicator. In flight, the ADF's RMI or direction indicator will always point to the broadcast station regardless of aircraft heading. Dip error is introduced, however, when the aircraft is in a banked attitude, as the needle dips down in the direction of the turn. This is the result of the loop itself banking with the aircraft and therefore being at a different angle to the beacon. For ease of visualization, it can be useful to consider a 90° banked turn, with the wings vertical. The bearing of the beacon as seen from the ADF aerial will now be unrelated to the direction of the aircraft to the beacon.

Dip error is sometimes wrongly confused with quadrantile error, which is the result of radio waves being bounced and reradiated by the airframe. Quadrantile error does not affect signals from straight ahead or behind, nor on the wingtips. The further from these cardinal points and the closer to the quadrantile points (i.e. 45°, 135°, 225° and 315° from the nose) the greater the effect, but quadrantile error is normally much less than dip error, which is always present when the aircraft is banked.

ADF receivers can be used to determine current position, track inbound and outbound flight path, and intercept a desired bearing. These procedures are also used to execute holding patterns and non-precision instrument approaches


A radio-magnetic indicator (RMI) is an alternate ADF display providing more information than a standard ADF. While the ADF shows relative angle of the transmitter with respect to the aircraft, an RMI display incorporates a compass card, actuated by the aircraft's compass system, and permits the operator to read the magnetic bearing to or from the transmitting station, without resorting to arithmetic.

Most RMI's incorporate two direction needles. Often one needle (the thicker, double-barred needle) is connected to an ADF and the other (generally thin or single-barred) is connected to a VOR. Using multiple indicators, a navigator can accurately fix the position of their aircraft using triangulation, without requiring the aircraft to pass over the top of the station. Some models allow the operator to select which needle is connected to each navigation radio. There is great variation between models, and the operator must take care that their selection displays information from the appropriate ADF and VOR.

This instrument display can replace a magnetic compass display in the instrument panel, but not necessarily the gyroscopic Heading Indicator. The Heading Indicator can be combined with information from navigation radios (primarily VOR/ILS) in a similar way, to create the Horizontal Situation Indicator. The HSI, along with the VOR system, has largely replaced use of the RMI, however the HSI's much higher cost keeps the older combination of an RMI and an Omni Bearing Indicator attractive to cost-conscious pilots.


Very high frequency Omni-directional range (VOR) [1] is a type of short-range radio navigation system for aircraft, enabling aircraft with a receiving unit to determine its position and stay on course by receiving radio signals transmitted by a network of fixed ground radio beacons. It uses frequencies in the very high frequency (VHF) band from 108.00 to 117.95 MHz Developed in the United States beginning in 1937 and deployed by 1946, VOR became the standard air navigational system in the world, used by both commercial and general aviation, until supplanted by satellite navigation systems such as GPS in the early 21st century. As such, VOR stations are being gradually decommissioned. In the year 2000 there were about 3,000 VOR stations operating around the world, including 1,033 in the US, but by 2013 the number in the US had reduced to 967.[6] The United States is decommissioning approximately half of its VOR stations and other legacy navigation aids as part of a move to performance-based navigation, while still retaining a "Minimum Operational Network" of VOR stations as a backup to GPS. In 2015 the UK planned to reduce the number of stations from 44 to 19 by 2020.

Very high Omni-directional Range (VOR) provides the following information.

• Bearing information to/from ground station

• Course deviation

• Identification of the ground station

Frequency range: 108 MHz – 117.5 MHz

Accuracy: it is generally plus or minus 1°-degree line of sight; depends on the altitude of the aircraft. Typically, the range is about 212 NM (nautical miles) at altitude of 30,000 ft. and reduced to 39 MM at 1000 ft. altitude.

A VOR ground station uses a specialized antenna system to transmit both an amplitude modulated and a frequency modulated signal. Both modulations are done with a 30 Hz signal, but the phase is different. The phase of one of the modulation signals is dependent on the direction of transmission, while the phase of the other modulation signal is not, in order to serve as a reference. The receiver will demodulate both signals, and measure the phase difference. The phase difference is indicative of the bearing from the VOR station to the receiver relative to magnetic north. This line of position is called the VOR "radial".

The intersection of radials from two different VOR stations can be used to fix the position of the aircraft, as in earlier radio direction finding (RDF) systems.

VOR stations are fairly short range: the signals are line-of-sight between transmitter and receiver and are useful for up to 200 miles. Each station broadcasts a VHF radio composite signal including the mentioned navigation and reference signal, station's identifier and voice, if so equipped. The station's identifier is typically a three-letter string in Morse code. The voice signal, if used, is usually the station name, in-flight recorded advisories, or live flight service broadcasts.

A VORTAC is a radio-based navigational aid for aircraft pilots consisting of a co-located VHF omnidirectional range and a tactical air navigation system (TACAN) beacon. Both types of beacons provide pilots azimuth information, but the VOR system is generally used by civil aircraft and the TACAN system by military aircraft. However, the TACAN distance measuring equipment is also used for civil purposes because civil DME equipment is built to match the military DME specifications. Most VOR installations in the United States are VORTACs. The system was designed and developed by the Cardion Corporation. The Research, Development, Test, and Evaluation (RDT&E) contract was awarded 28 December 1981.


In aviation, distance measuring equipment (DME) is a radio navigation technology that measures the slant range (distance) between an aircraft and a ground station by timing the propagation delay of radio signals in the frequency band between 960 and 1215 megahertz (MHz). Line-of-visibility between the aircraft and ground station is required. An interrogator (airborne) initiates an exchange by transmitting a pulse pair, on an assigned 'channel', to the transponder ground station. The channel assignment specifies the carrier frequency and the spacing between the pulses. After a known delay, the transponder replies by transmitting a pulse pair on a frequency that is offset from the interrogation frequency by 63 MHz and having specified separation.

DME provide distance from the station in nautical miles with a very high degree of accuracy

Frequency: 962 MHz and 1213 MHz

Airborne transmitter interrogates: Ground station response

Interrogation: paired pulse at a specific spacing are sent out from the aircraft and are received at the ground station

Reply: ground station responds (transmits)

DME systems are used worldwide, using standards set by the International Civil Aviation Organization (ICAO), RTCA, the European Union Aviation Safety Agency (EASA) and other bodies. Some countries require that aircraft operating under instrument flight rules (IFR) be equipped with a DME interrogator; in others, a DME interrogator is only required for conducting certain operations.

While stand-alone DME transponders are permitted, DME transponders are usually paired with an azimuth guidance system to provide aircraft with a two-dimensional navigation capability. A common combination is a DME collocated with a VHF omnidirectional range (VOR) transmitter in a single ground station. When this occurs, the frequencies of the VOR and DME equipment are paired. Such a configuration enables an aircraft to determine its azimuth angle and distance from the station. A VORTAC (a VOR co-located with a TACAN) installation provides the same capabilities to civil aircraft but also provides 2-D navigation capabilities to military aircraft.

Low-power DME transponders are also associated with some instrument landing system (ILS), ILS localizer and microwave landing system (MLS) installations. In those situations, the DME transponder frequency/pulse spacing is also paired with the ILS, LOC or MLS frequency.

ICAO characterizes DME transmissions as ultra-high frequency (UHF). The term L-band is also used.

Developed in Australia, DME was invented by James "Gerry" Gerrand under the supervision of Edward George "Taffy" Bowen while employed as Chief of the Division of Radio physics of the Commonwealth Scientific and Industrial Research Organization (CSIRO). Another engineered version of the system was deployed by Amalgamated Wireless Australasia Limited in the early 1950s operating in the 200 MHz VHF band. This Australian domestic version was referred to by the Federal Department of Civil Aviation as DME(D) (or DME Domestic), and the later international version adopted by ICAO as DME(I).

DME is similar in principle to secondary radar ranging function, except the roles of the equipment in the aircraft and on the ground are reversed. DME was a post-war development based on the identification friend or foe (IFF) systems of World War II. To maintain compatibility, DME is functionally identical to the distance measuring component of TACAN


LORAN, short for long range navigation, was a hyperbolic radio navigation system developed in the United States during World War II. It was similar to the UK's Gee system but operated at lower frequencies in order to provide an improved range up to 1,500 miles (2,400 km) with an accuracy of tens of miles. It was first used for ship convoys crossing the Atlantic Ocean, and then by long-range patrol aircraft, but found its main use on the ships and aircraft operating in the Pacific theater during World War II. Long range navigation: the plane has the requirement for receiving special radio signal sent out continuous from transmitter station. The signal will indicate the plane location.

It is accurate system for long range navigation based on the measurement of difference in time arrival of EM waves from one or more land based transmitters to the receiver of the aircraft.

LORAN, in its original form, was an expensive system to implement, requiring a cathode ray tube (CRT) display. This limited use to the military and large commercial users. Automated receivers became available in the 1950s, but the same improved electronics also opened the possibility of new systems with higher accuracy. The U.S. Navy began development of Loran-B, which offered accuracy on the order of a few tens of feet, but ran into significant technical problems. The U.S. Air Force worked on a different concept, Cyclan, which the Navy took over as Loran-C, which offered longer range than LORAN and accuracy of hundreds of feet. The U.S. Coast Guard took over operations of both systems in 1958.

In spite of the dramatically improved performance of Loran-C, LORAN, now known as Loran-A (or "Standard LORAN"), would become much more popular during this period. This was due largely to the large numbers of surplus Loran-A units released from the Navy as ships and aircraft replaced their sets with Loran-C. The widespread introduction of inexpensive microelectronics during the 1960s caused Loran-C receivers to drop in price dramatically, and Loran-A use began to rapidly decline. Loran-A was dismantled starting in the 1970s; it remained active in North America until 1980 and the rest of the world until 1985. A Japanese chain remained on the air until 9 May 1997, and a Chinese chain was still listed as active as of 2000.

Loran-A used two frequency bands, at 1.85 and 1.95 MHz These same frequencies were used by radio amateurs, in the amateur radio 160-meter band, and amateur operators were under strict rules to operate at reduced power levels to avoid interference; depending on their location and distance to the shore, U.S. operators were limited to maximums of 200 to 500 watts during the day and 50 to 200 watts at night.


A tactical air navigation system, commonly referred to by the acronym TACAN, is a navigation system used by military aircraft. It provides the user with bearing and distance (slant-range or hypotenuse) to a ground or ship-borne station. It is a more accurate version of the VOR/DME system that provides bearing and range information for civil aviation. The DME portion of the TACAN system is available for civil use; at VORTAC facilities where a VOR is combined with a TACAN, civil aircraft can receive VOR/DME readings. Aircraft equipped with TACAN avionics can use this system for en route navigation as well as non-precision approaches to landing fields. The space shuttle is one such vehicle that was designed to use TACAN navigation but later upgraded with GPS as a replacement.

Tactical air navigation: provides navigation information both bearing and range ground station can be portable and setup at temporarily air fields.

The typical TACAN onboard user panel has control switches for setting the channel (corresponding to the desired surface station's assigned frequency), the operation mode for either transmit/receive (T/R, to get both bearing and range) or receive only (REC, to get bearing but not range). Capability was later upgraded to include an air-to-air mode (A/A) where two airborne users can get relative slant-range information. Depending on the installation, Air-to-Air mode may provide range, closure (relative velocity of the other unit), and bearing, though an air-to-air bearing is noticeably less precise than a ground-to-air bearing. A TACAN only equipped aircraft cannot receive bearing information from a VOR only station.

OMEGA was the first global-range radio navigation system, operated by the United States in cooperation with six partner nations. It was a hyperbolic navigation system, enabling ships and aircraft to determine their position by receiving very low frequency (VLF) radio signals in the range 10 to 14 kHz, transmitted by a global network of eight fixed terrestrial radio beacons, using a navigation receiver unit. It became operational around 1971 and was shut down in 1997 in favor of the Global Positioning System.

It was the first global radio navigation system for aircraft. It enables the aircraft to determine their position by receiving very low frequency (VLF) radio signals transmitted by a network of terrestrial radio beacons, using a receiver unit


Taking a "fix" in any navigation system requires the determination of two measurements. Typically, these are taken in relation to fixed objects like prominent landmarks or the known location of radio transmission towers. By measuring the angle to two such locations, the position of the navigator can be determined. Alternately, one can measure the angle and distance to a single object, or the distance to two objects.

The introduction of radio systems during the 20th century dramatically increased the distances over which measurements could be taken. Such a system also demanded much greater accuracies in the measurements – an error of one degree in angle might be acceptable when taking a fix on a lighthouse a few miles away, but would be of limited use when used on a radio station 300 miles (480 km) away. A variety of methods were developed to take fixes with relatively small angle inaccuracies, but even these were generally useful only for short-range systems.


Navigation determines the location of aircraft by observing certain celestial bodies – the sun, the moon, the stars, like north stars measures the elevation of celestial body by sextant, an instrument that measure the angular distance of the celestial body above the horizon and note down the precise time at which at which the measurement is made with chronometer. The two measurements are enough to the position of an aircraft on a circle on the face of the globe.


• Independent of external aids – self contained


• Visibility should be good to take elevation angles of heavenly bodies

• Horizon should be located


• Almanac: an annual publication containing Tide Tables astronomical ephemerals etc.

• Altitude: the angular distance of a celestial body above the viewer’s horizon.

• Anabatic Winds; caused by warm air rising up a slope to be replaced by cooler air, as opposed to kabatic, descending winds.

• Apparent wind; the wind as felt on board, this will be the actual wind

• Barometer: an instrument for measuring atmospheric pressure.

• Bearing; the compass reading taken of an object in relation to the observer, horizontal direction to the wind grouped stations or from the ground station.

• Course: course is the direction in which the pilot wants the aircraft to go (Line drawn on a chart) also known as desired trades.

• Cardinal points: the four main points of the compass, North, East, South and West.

• Nautical mile: distance aviation is measured in nautical miles

• Headings: actual direction toward which aircraft is headed (nose is pointed) considering the effect of cross wind.

• Air-speed: Airspeed is an important perimeter in aviation. It depends in the density of air and that depends on temp and pressure at any altitude greater than sea level, air will be less dense than sea level.

Air Navigation needs

Each model as reference

A coordinate system to identify position and to compute distance. Navigation aids for reducing the workload of navigator/pilot. Navigation requires determination position of aircraft in which the a/c has to reach the desired destination.

Basic Navigation Aids

Aeronautic charts

Specialized map that shows more than geographic features, chart provide airways which are highways in the air. Location of airports, landmark, mountain, rivers and lakes

Magnetic compass

A compass is a navigational instrument that measures direction (or point-news) along with intermediate directions. North correspondent to zero degree and the angle increase in clockwise so east is 90° degrees, south is 180° and west is 270°



An inertial navigation system (INS) is a navigation device that uses a computer, motion sensors (accelerometers) and rotation sensors (gyroscopes) to continuously calculate by dead reckoning the position, the orientation, and the velocity (direction and speed of movement) of a moving object without the need for external references. Often the inertial sensors are supplemented by a barometric altimeter and sometimes by magnetic sensors (magnetometers) and/or speed measuring devices. INSs are used on mobile robots and on vehicles such as ships, aircraft, submarines, guided missiles, and spacecraft. Other terms used to refer to inertial navigation systems or closely related devices include inertial guidance system, inertial instrument, inertial measurement unit (IMU) and many other variations. Older INS systems generally used an inertial platform as their mounting point to the vehicle and the terms are sometimes considered synonymous.

Geometry pig is composed of low-frequency communication system, computer, major probe, cup, odometer wheel. It can inspect pipeline with size of 6” ~ 48”. The probes with deformation ability can be used to detect the pipeline internal diameter change. It can detect dent, orality, accessories causing pipe internal diameter change, etc.

During it’s running, the ground marker is used to monitor the accurate position of geometry pig and to collect pipeline deformation. Special data analysis software carried by the pig replays the detection data collected on site so that pipe diameter geometric changes could be showed clearly in the form of data curves. Through data analysis, pipe deformation size can be quantized accurately and meanwhile the deformation points also can be located combined with welding point information of installation and construction records.

Geometry pig can identify dents, ovality, change of wall thickness, pipe accessories causing the change of pipe inner diameter, pipe length, valve, T-joint, girth weld, bend, etc. It also helps to find the exact location of deformation point and determine the size to reconstruct pipeline timely, eliminate hidden risks and ensure pipeline operates safely, efficiently and smoothly.

It is a self-contained navigation system comprises of gyros accelerometers and navigation computer which provide the aircraft position and navigation information in response to signals resulting from inertial effect on system components and does not require information from external references

  1. Bowditch, Nathaniel (1995). "Glossary". The American Practical Navigator (PDF). Vol. 9. Bethesda, Maryland: National Imagery and Mapping Agency. p. 815. ISBN 978-0-939837-54-0. Archived from the original (PDF) on 2011-05-20. Retrieved 2010-12-14.

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