Automatic Radar Plotting Aid
An Automatic Radar Plotting Aid (ARPA) provides an interface between visually-oriented radar displays and or marine navigation computers. Where a radar screen displays a target as a "blip" of light relative to the radar antenna, such that the basic computation of bearing is manual, and determining course and speed requires multiple manual observations and measurement, the ARPA sends a digital message, for each target, to the "master" screen of the chartplotter (i.e., Electronic Chart Data and Information System (ECDIS)).
The ARPA device is, itself, a computer, usually a component inside the radar set rather than an external device; it supplements the traditional radar display. For each target, ARPA computes the actual position using GPS information. It stores the target positions between successive radar observations, so it can compute course and speed. The ARPA determines if it is on an "intercept course" (i.e., might collide if both the "own ship" and target maintain their current course and speed).
An ARPA message may include information either manually entered, or available from other sources such as the Automatic Identification System, the name of the target.
All bearing information is indicated as either True (i.e., in compass bearing) or Relative (i.e., angle with respect to own ship). Users should remember that the radar screen and the chartplotter screen do not necessarily use the same chart scale, so distances may seem different on both displays. Only GPS or equivalent coordinates are authoritative.
AIS, which relies on a GPS signal transmitted from the other target, is generally more accurate than ARPA; if the positions differ, a cautious seaman verifies both. Ideally, one's own chartplotter, ARPA, and AIS should either take information from the same GPS source, or separate GPS should periodically be checked.
Information sent by ARPA
- Message time
- Distance and bearing from own ship
- Target speed and course
- Distance to Closest Point of Approach (CPA) (i.e., potential collision); time to CPA (i.e., a negative value means the target is moving away)
- Units of speed and distance (knots/mph, kilometers/nautical miles/statute miles)
- Target name
- Tracking status (lost from tracking, currently reliably tracked, tracking data being acquired)
- Target acquisition method (manual or automatic designation)
- The commercial NMEA 0183 standard allows a maximum of 99