In navigation, a course is the intended path of a vehicle over the surface of the Earth. For air travel, it is the intended flight path of an airplane or the direction of a line drawn on a chart representing the intended airplane path, expressed as the angle measured from a specific reference datum clockwise from 0° through 360° to the line. The reference can be true north or magnetic north and called true course or magnetic course respectively. Course is customarily expressed in three digits, using preliminary zeros if needed.
In order to be used in a chart, this reference has to be true north.
Determining the true course of a vessel
- Heading (2) is the direction the vessel, aircraft or vehicle is truly "pointing towards" (the heading of the ship shown in the image is 058°).
- Any reading from a magnetic compass refers to compass north (4), which is supposed to contain a two-part compass error:
a) The earth's magnetic field's north direction, or magnetic north (3), almost always differs from true north by magnetic variation (6), the local amount of which is given in nautical charts, and
b) ship's own magnetic field may influence the compass by so-called magnetic deviation (5).
Deviation only depends on the ship's own magnetic field and the heading, and therefore can be checked out and given as a deviation table or, graphically, as a Napier's diagram.
- The compass heading or compass course (7) has to be corrected first for deviation (the "nearer" error), from which results the magnetic heading (8). Correcting this for variation yields true heading (2).
- In case of a crosswind (9), and/or tidal or other current (10), the heading will not meet the desired target, as the vessel will continuously drift sideways; it is necessary to point away from the intended course to counteract these effects.
A track, also course over ground, is the actual path followed by a moving body, e.g. the vessel's track from A to B in the above given scheme. Some ambiguity exists in the fact that the path a navigator intends to follow, after evaluating and counteracting possible effects of wind and current, is also called track.
The track is equivalent to the heading (a bearing "right ahead"), if no crosswind and cross current occur (2), or the vessel is stationary, but this would hardly ever happen in aviation.
When wind is present, and is not a headwind or tailwind, the wind deflects the aircraft (or vessel) from its heading.
To correct for the wind, the aircraft or vessel points more or less into the wind. The amount depends on the vehicle's speed, the wind's speed, and the angle of the wind in relation to the vehicle. This so-called wind correction angle is computed in advance and is frequently checked while "en route". In the above scheme, the track would be (9) for wind from port side.
GPX is an XML schema for storing track logs.
An aircraft's heading is the direction that the aircraft's nose is pointing.
It is referenced by using either the magnetic compass or heading indicator, two instruments that most aircraft have as standard. Using standard instrumentation, it is in reference to the local magnetic north direction. True heading is in relation to the lines of meridian (north-south lines). The units are degrees from north in a clockwise direction. East is 90, south is 180 and west is 270 degrees.
Note that, due to wind forces, the direction of movement of the aircraft, or track, is not the same as the heading. The nose of the aircraft may be pointing due west, for example, but a strong northerly wind will change its track south of west. The angle between heading and track is known as the drift angle or crab angle.
- The above scheme shows a magnetic declination of 6° East, as is commonly encountered in areas of the Pacific Ocean, for instance, and a more-than-somewhat exaggerated deviation (taken from a fictitious deviation table for educational purpose) of +12°, for a compass heading of 040°. By conventional degaussing, deviation could usually be kept beyond 10°, and fluxgate compasses can be degaussed to almost D=0°.
- The possible influences of wind and current are maximized by presupposing a very slow boat in heavy wind and current.
- To increase readability of the scheme, all possible influences were given as positive, i.e. variation=East, + 12° deviation, wind and current from port side. The principle is the same for the opposite of any of the components.Initially a square symbol identifies a target in the beginning of acquisition and tracking phase. A line (vector) representing the target's relative direction of movement is displayed after 20 scans (usually less than one minute). Typically the square designator changes to a circle when steady state tracking is established after 60 scans. The end of the vector, representing target motion, predicts the position of the target after a time period between 0.5 and 30 minutes as selected by the operator. ARPA also shows pas positions of the target with a choice of 5,10 or 20 pas position dots at intervals of 0.5, 1,2,3, or 6 minutes. ARPA (Automatic Radar Plotting Aid) and ATA (Automatic Tracking Aid) have the ability to track 60 and 40 targets respectively at relative speeds of up to 150 knots. Tracked target data is output to other shipborne systems such as electronic chart systems (ECS). Targets may be acquired manually or by using the annular and polygonal automatic acquisition zones. The two conventional annular zones are of variable depth and provide protection over any arc up to and including a full circle around own ship. The polygonal zones can be drawn to virtually any shape and are particularly useful for shore-based or other static site applications. When target tracking, the operator is able to display full target data on any chosen target or CPA/TCPA data on six selected targets. The six targets may be selected manually by the operator or automatically by CPA or range.