DGCA CPL Air Navigation key terms

A circle on the Earth's surface whose plane passes through the Earth's centre. It gives the shortest surface distance between two points and appears straight on a gnomonic chart.

A path crossing every meridian at the same angle. It plots as a straight line on a Mercator chart, but it is longer than the great circle except along the equator or a meridian.

The angle between true north and magnetic north at a place, caused by the Earth's magnetic field. It is named east or west and slowly changes with position and time.

The angle at which the Earth's magnetic field dips below the horizontal at a location. It approaches 90 degrees near the magnetic poles and causes turning and acceleration errors in a compass.

A line joining places where magnetic variation is zero, so that true north and magnetic north coincide. Compass roses on or near it need no variation correction.

The angle at which two meridians meet when measured at a given latitude, equal to the change of longitude between them multiplied by the sine of that latitude.

The angle at a point between the great circle and the rhumb line joining two places. It equals half the convergence between the meridians of those places.

The time at a place based on the mean Sun, equal to UTC plus or minus the longitude expressed in time, with east longitudes ahead of UTC.

The worldwide time reference used in navigation, equal to local mean time on the prime meridian and kept within one second of atomic time by leap seconds.

The east-west distance covered along a parallel of latitude. It equals the change of longitude in minutes multiplied by the cosine of the latitude.

The error of a magnetic compass caused by magnetic fields produced by the aircraft itself. It varies with heading and is recorded on a deviation card.

A cylindrical chart projection with parallel meridians on which the rhumb line is straight; chart scale increases with latitude, so great circles appear curved towards the pole.

A chart projection using a cone that touches or cuts the Earth at standard parallels. Angles are preserved and a straight line closely approximates a great circle.

The relationship between a measured distance on the chart and the real distance on the Earth, usually given as a representative fraction or a statement of equivalence.

The angle between the aircraft's heading and the track it actually makes over the ground, produced by wind and quoted as degrees left or right.

A vector diagram linking heading and true airspeed with wind direction and speed to give the resulting track and groundspeed for flight planning.

Deriving the aircraft's position from a known starting point by applying heading, airspeed, elapsed time and forecast wind, without using external position fixes.

A position determined from external sources such as radio aids, GNSS or visual ground features, used to confirm or update a dead-reckoning position.

The time at which an aircraft is expected to arrive at a waypoint or destination, calculated from the distance remaining and the expected groundspeed.

The point along a route from which the time needed to reach either of two airfields is the same, useful for deciding where to divert in an emergency.

A ground station that transmits a continuous low-frequency radio signal. An aircraft's automatic direction finder uses it to display a relative bearing to the station.

The angle measured at the aircraft from its nose clockwise to the radio station or other object, displayed directly on the ADF indicator.

The magnetic bearing from an aircraft to a radio station. It is also the magnetic heading that would take the aircraft to the station in still air.

A region directly above a ground radio beacon where the signal is weak or absent, making bearings unreliable; pilots note the time of overhead passage.

A magnetic bearing measured from a VOR station outwards. An aircraft tracking inbound on the 090 radial is flying a magnetic heading of about 270 degrees.

The needle on a VOR or ILS display that shows displacement from the selected course, normally in degrees or dots left and right.

The straight-line distance from the aircraft to a DME ground station. It is greater than the horizontal distance, especially when close in and at high altitude.

The lateral guidance part of an instrument landing system, transmitting on VHF a course aligned with the runway centreline and showing left or right deviation.

The vertical guidance part of an instrument landing system, normally set at about three degrees, giving deviation above or below the correct descent path.

The furthest distance at which a ground-based radio aid can be received, limited by line of sight and extended by aircraft altitude and atmospheric refraction.

Area navigation that allows an aircraft to follow any chosen path within the coverage of ground aids or GNSS, instead of flying to or from individual stations.

A navigation specification stating the accuracy required in a block of airspace plus the aircraft's ability to monitor its own performance and warn of failure.

A defined point in the sky, given by coordinates or a named fix, at which a route changes direction and which is entered in area navigation equipment.

Precise orbital data for a navigation satellite, transmitted to receivers so they can calculate the satellite's position at the time of signal transmission.

A receiver function that checks whether the satellites being used give a consistent position and alerts the crew when navigation accuracy cannot be guaranteed.

A number describing how the geometry of the visible satellites amplifies ranging errors into position errors; high values indicate poor geometry and less reliable fixes.

A satellite-based augmentation system that broadcasts corrections and integrity information from geostationary satellites, improving GNSS position accuracy and enabling precision approaches.

A ground-based augmentation system that sends locally derived GNSS corrections to aircraft near an airport, supporting precisely guided approaches to multiple runway ends.

An on-board computer that stores the route and performance data, computes guidance, and supplies information to the navigation displays and autopilot.

Equipment that senses aircraft acceleration and rotation, integrating the measurements to provide position, velocity, attitude and heading without external radio aids.