ATPL Navigation key terms

The imaginary surface that coincides with mean sea level over the whole Earth, extended under the continents. It is an irregular shape used as the reference for height and for defining the Earth's gravitational field.

A mathematically regular surface produced by rotating an ellipse about its minor axis, used as a reference model of the Earth for coordinate and distance calculations. Different ellipsoids approximate the geoid with varying accuracy in different regions.

Angular distance measured north or south of the equator along a meridian, from 0 degrees at the equator to 90 degrees at the poles. It is expressed in degrees, minutes and seconds of arc.

Angular distance measured east or west of the prime meridian, from 0 to 180 degrees. Because meridians converge toward the poles, the surface distance represented by one degree of longitude shrinks with increasing latitude.

A great circle on the Earth's surface passing through both poles. All points on the same meridian share the same longitude, and the direction along a meridian is true north or south.

Any circle on the Earth's surface whose plane passes through the Earth's centre, dividing it into two equal halves. The shortest path between two points on a sphere always lies along a great circle arc.

A line on the Earth's surface that crosses every meridian at the same true track angle, appearing as a straight line on a Mercator chart. It is longer than the great circle between the same two points except on the equator or along a meridian.

The angular difference at a given place between true north and magnetic north, caused by the difference between the geographic and magnetic poles. It is shown on charts and changes slowly with time and position.

The angular error of a magnetic compass caused by magnetic fields generated within the aircraft itself. It varies with the aircraft's heading and electrical configuration and is corrected using a compass correction card.

A line drawn on a chart joining all points having the same magnetic variation. Isogonals allow a pilot to interpolate the local variation value between the printed lines.

The inclination of the Earth's magnetic field lines relative to the horizontal, being zero at the magnetic equator and 90 degrees at the magnetic poles. Large dip angles make magnetic compasses unreliable in high latitudes.

A magnetic compass whose indicating card is attached directly to the magnet assembly and read by the pilot in the cockpit. It is simple and independent of electrical power but suffers from turning and acceleration errors.

A compass system in which a detector unit senses the magnetic field and transmits the heading electrically to one or more cockpit displays. It reduces parallax and reading errors and can feed other navigation instruments.

A cylindrical chart projection on which meridians are parallel straight lines and the scale expands with latitude so that rhumb lines plot as straight lines. It is useful for plotting tracks but distorts areas at high latitude.

A chart projection using a cone that intersects the Earth, giving a straight-line representation of great circles and a nearly constant scale along a standard parallel. It is widely used for aeronautical charts covering mid-latitude regions.

A map produced specifically for air navigation, showing airspace, radio aids, obstacles, terrain and other information needed for flight planning and in-flight navigation. Its scale and projection are chosen to suit the phase of flight it supports.

A vector diagram combining true airspeed and heading with wind velocity to obtain ground speed and true track. It is the geometric basis for solving drift and wind correction problems in dead reckoning.

The angle between the aircraft's heading and the track actually made good over the ground, caused by wind. It is measured in degrees left or right of heading and must be corrected to hold a desired track.

The angle applied to the desired track to obtain the heading that will compensate for wind drift. It equals the drift angle but is applied in the opposite sense to keep the aircraft on the required track.

The speed of an aircraft relative to the surface of the Earth, obtained by combining true airspeed with the wind component along the track. It determines the time required to travel between two points.

The process of calculating an aircraft's present position by advancing a known position using heading, airspeed, elapsed time and estimated wind. It provides a continuous position estimate between fixes.

A determined aircraft position obtained from external references such as radio aids, visual landmarks or satellite systems. Fixes are used to update and correct the dead reckoning position.

A line on which the aircraft is known to lie, derived from a single navigation measurement such as a bearing or range. Two or more intersecting lines of position establish a fix.

The technique of comparing features visible on the ground with those depicted on a chart to establish position and orientation. It relies on identifying linear features, prominent landmarks and settlements.

A radio navigation receiver that indicates the bearing of a ground station relative to the aircraft's heading. The bearing is affected by compass errors and by radio propagation effects such as coastal refraction and night effect.

A very high frequency omnidirectional radio range that transmits signals allowing an aircraft to determine its magnetic bearing from the station. It provides a precise line of position largely free from static interference.

Distance measuring equipment that measures the slant range from the aircraft to a paired ground transponder by timing the round trip of pulses. The indicated distance is greater than the true horizontal distance, especially at high altitude.

The straight-line distance between an aircraft and a ground station, including the vertical component. It exceeds the horizontal ground distance, and the difference grows with altitude and with proximity to the station.

A precision approach aid transmitting a localiser beam for lateral guidance and a glide path beam for vertical guidance. Markers or distance information indicate the aircraft's progress along the approach.

The lateral guidance component of an instrument landing system, transmitting on a frequency in the 108 to 112 MHz band. Its beam defines the extended runway centreline that the aircraft follows during the approach.

The vertical guidance component of an instrument landing system, typically defining a descent angle of about three degrees to the runway. It is transmitted on a frequency paired with the localiser.

A satellite navigation system that provides position, velocity and time by ranging to multiple satellites. Accuracy depends on satellite geometry and signal quality and can be enhanced by augmentation systems.

Receiver autonomous integrity monitoring, a function that checks the consistency of satellite signals to detect a faulty satellite. It requires sufficient satellites in view to provide the redundancy needed for the check.

A method of navigation allowing an aircraft to fly any desired path within the coverage of navigation aids or self-contained systems, rather than routing directly between ground stations. It is the basis for modern airway and terminal route design.

A specification stating the navigation accuracy, integrity and continuity required for operation in a defined airspace, expressed as a distance value. Aircraft must be able to monitor their own performance against the stated value.

An integrated computer system that manages navigation, flight planning, performance and guidance. It uses a navigation database and sensor inputs to compute the aircraft's position and to command the autopilot along the programmed route.

A periodically updated digital store of waypoints, airways, procedures and radio aids used by the flight management system. Its validity period must be checked because outdated data can cause incorrect routing.

A navigation technique used in polar regions where meridians converge, employing a rectangular grid reference instead of true north. Headings are referenced to the grid direction and converted to true heading as needed.

A directional instrument using a gyroscope to provide a stable heading reference independent of magnetism. It drifts slowly and must be corrected for apparent wander caused by the Earth's rotation and aircraft movement.

The point along a route where the time required to reach either of two diversion airfields is equal. It is used in oceanic and remote operations to plan the most efficient diversion in an emergency.

The point on a route beyond which an aircraft can no longer return to its departure airfield with the fuel remaining. It depends on wind, fuel consumption and the distance already flown.