ATPL General Navigation (061) key terms

The equipotential surface of the Earth's gravity field that coincides with mean sea level, extended under the continents; the reference surface for elevations and for defining the ellipsoid's position.

The mathematical solid formed by rotating an ellipse about its minor axis, used as the reference model for the Earth; its semi-major axis lies in the equatorial plane and its semi-minor axis is polar.

The ratio (a minus b) divided by a, where a and b are the equatorial and polar radii of the reference ellipsoid; it expresses how much the polar axis is shortened relative to the equatorial diameter.

The World Geodetic System 1984 datum, comprising an ellipsoid, an origin at the Earth's mass centre and an associated gravity model; it is the datum on which GNSS positions and most modern aeronautical coordinates are based.

The angle between the equatorial plane and the normal to the reference ellipsoid at the point concerned; it differs slightly from geocentric latitude except at the equator and poles.

The angle at the Earth's centre between the equatorial plane and the line joining the centre to the point on the surface; it is smaller than geodetic latitude in mid-latitudes.

The intersection of a sphere with a plane passing through its centre; it is the shortest surface path between two points and appears as a straight line on a gnomonic chart.

A curve on the Earth's surface that crosses every meridian at the same true angle; it plots as a straight line on a Mercator chart but is generally longer than the great circle between the same points.

The point of highest latitude reached on a great circle track; at the vertex the track direction is exactly 090 or 270 degrees true and the great circle is tangent to the parallel.

Half the difference between the initial and final great circle tracks between two points, equal to half the change of longitude multiplied by the sine of the mean latitude; it relates great circle and rhumb line bearings.

A conformal cylindrical projection on which meridians are equally spaced parallel lines and parallels are spaced by the secant of latitude, so that rhumb lines plot straight and scale expands with latitude.

A conformal conic projection using two standard parallels, on which a straight line closely approximates a great circle; widely used for aeronautical charts of mid-latitude areas.

A projection from the centre of the Earth onto a tangent plane; all great circles appear as straight lines, making it useful for planning great circle routes, but scale and shape are badly distorted away from the tangent point.

A parallel of latitude along which a conic projection's cone is tangent or secant to the Earth, so that scale is exactly correct there and departs from unity elsewhere on the chart.

The ratio of chart or grid distance to the corresponding true Earth distance at a given point; on conformal projections it varies with position and equals one on the standard parallels or central meridian.

The angle between the great circle direction and the chart grid or straight-line direction at a point, approximately equal to the change of longitude multiplied by the sine of latitude; it must be applied when converting between grid and true bearings.

The direction of the aircraft's fore-and-aft axis measured clockwise from the chart's grid north reference, rather than from true or magnetic north.

The angular difference at a place between true north and magnetic north, caused by the Earth's magnetic field; it is easterly or westerly and changes slowly with position and time.

The angular difference on a particular aircraft heading between magnetic north and the direction indicated by the compass, caused by the aircraft's own magnetic fields and metal structure.

A line on a chart joining points of equal magnetic variation; the agonic line is the special isogonal along which variation is zero.

The line joining places where magnetic variation is zero, so that true and magnetic north coincide along it.

The angle between the Earth's magnetic field lines and the horizontal at a place; it is zero at the magnetic equator and 90 degrees at the magnetic poles, and affects compass behaviour.

A magnetic compass whose card is read directly against a fixed lubber line, relying solely on the Earth's field and therefore subject to turning and acceleration errors.

A ground procedure in which the aircraft is aligned on successive headings and the residual deviations are measured and recorded on a deviation card.

The vector relationship in which true air velocity plus wind velocity equals ground velocity; it links heading and true airspeed with track and ground speed.

The direction in which the aircraft's longitudinal axis points, measured clockwise from true north, expressed in degrees from 000 to 359.

The actual path of the aircraft over the ground, measured as the direction from a known starting point to the present position, referenced to true north.

The angle between the aircraft's heading and the track along which it actually travels over the ground, caused by wind; it is named left or right of heading.

A vector giving the direction from which the wind blows (in degrees true) and its speed; in the triangle of velocities it is the vector closing heading and track.

The process of calculating present position by applying measured heading, true airspeed, elapsed time and estimated wind to the last known position.

A line on which the aircraft is known to lie, derived from a bearing, a range or a circle of equal distance; two or more intersecting position lines fix the position.

A confirmed aircraft position obtained by intersecting two or more position lines or by an independent navigation system, rather than by dead reckoning alone.

The magnetic bearing of a line extending outward from a VOR station, identified by the station's Morse code and selected with the OBS; it is the reciprocal of the bearing to the station.

The line-of-sight distance from the aircraft to a DME ground station, which exceeds the horizontal ground distance by an amount that becomes significant at high altitude and short range.

The time at a place based on the mean Sun's hour angle at that longitude; it differs from UTC by four minutes for every degree of longitude, being earlier to the west.

The primary time reference for aviation, based on atomic time and kept within one second of mean solar time at the prime meridian; it is used for all flight plan and position reporting times.

The civil time in force in a region, related to UTC by a whole number of hours (or half hours) and often altered seasonally by daylight saving adjustments.

Area navigation that allows the aircraft to fly a designated path between waypoints without overflying ground-based stations, using onboard computation of position and guidance.

A geographical point, defined by coordinates or by a named navigation aid, that the aircraft's route is designed to pass through and that RNAV equipment uses as a route reference.

A statement of the navigation performance, notably the lateral accuracy in nautical miles, that an aircraft must achieve for operation in a given airspace or on a given route.

A satellite-based navigation system in which a receiver determines position and time from the measured travel times of signals from several satellites whose positions are broadcast in the navigation message.

Receiver Autonomous Integrity Monitoring, a receiver function that detects and, with enough satellites in view, isolates faulty satellite signals so that a safe navigation solution is maintained.