Glossary Term
ARPA
ARPA (automatic radar plotting aid) is a radar function that automatically tracks selected targets and calculates their course, speed, closest point of approach (CPA) and time to CPA (TCPA), helping the bridge team assess the risk of collision.
On radar equipment installed since 1 July 2008, IMO performance standards describe the same capability as target tracking (TT), so manuals and bridge teams often use “ARPA” and “TT” for the same set of functions. ARPA supports, but does not replace, a proper lookout and the COLREG rules.
How ARPA works
The radar detects echoes. ARPA acquires selected echoes, manually or automatically, and follows them over successive antenna sweeps. From the change in each target’s range and bearing it calculates the target’s motion and shows it as a vector. To show true course and speed, ARPA also needs own-ship heading, from a gyrocompass or transmitting heading device, and own-ship speed. Speed comes from a speed log for sea stabilisation, or from GPS-derived course and speed over ground for ground stabilisation.
The main outputs are:
- CPA and TCPA. How close a target will pass and when, if neither vessel changes course or speed.
- True and relative vectors. True vectors show how a target moves through the water or over the ground; relative vectors show its motion relative to own ship, which makes a closing situation easy to see.
- Alarms. A warning when a target is predicted to pass inside the set CPA and TCPA limits, or when a tracked target is lost.
- Trial manoeuvre. A simulation of an own-ship course or speed change before it is made. IMO standards require it on radar for ships of 10,000 GT and above, so equipment for smaller vessels may not offer it.
ARPA, ATA, EPA and target tracking
SOLAS uses three names for plotting functions: electronic plotting aid (EPA), automatic tracking aid (ATA) and automatic radar plotting aid (ARPA). IMO Resolution A.823(19), adopted in 1995, set performance standards for ARPA. Resolution MSC.192(79), adopted in 2004, applies to radar installed on or after 1 July 2008 and builds target tracking into the radar performance standard itself, with a minimum tracking capacity by ship size:
| Ship size (MSC.192(79), Table 1) | Minimum number of tracked radar targets |
|---|---|
| Below 500 GT | 20 |
| 500 GT to below 10,000 GT, and high-speed craft below 10,000 GT | 30 |
| 10,000 GT and above | 40 |
SOLAS carriage requirements
SOLAS regulation V/19 links plotting and tracking aids to tonnage:
- V/19.2.3.3: ships of 300 GT and above, and passenger ships irrespective of size, carry an electronic plotting aid or other means to plot the range and bearing of targets.
- V/19.2.5.5: ships of 500 GT and above carry an automatic tracking aid or other means to plot other targets automatically.
- V/19.2.7.2: ships of 3,000 GT and above carry a second automatic tracking aid, or other means, functionally independent of the first. Ships of 10,000 GT and above are excepted from this item and fall under V/19.2.8 instead.
- V/19.2.8.1: ships of 10,000 GT and above carry an automatic radar plotting aid or other means to plot at least 20 other targets automatically, connected to a device indicating speed and distance through the water and capable of simulating a trial manoeuvre.
These are the convention’s requirements, subject to its transitional provisions for existing ships and the scope of Chapter V. Whether and how they apply to a particular yacht is decided by her flag Administration, under SOLAS or the yacht code she is certified to.
ARPA and AIS
AIS shows the position, course and speed that another vessel reports from her own sensors. ARPA calculates a target’s motion from your own radar. IMO’s guidelines on the operational use of AIS, Resolution A.1106(29), warn that AIS information may be inaccurate and state that AIS supports radar target tracking rather than replacing it. Where the two disagree, the reason needs to be found before the data is relied on.
Limits
A new track takes time to settle after acquisition, and vectors are less reliable just after own ship or the target manoeuvres. Sea and rain clutter, weak echoes and targets close together can cause lost or swapped tracks. Errors in heading or speed input produce wrong true vectors, and CPA and TCPA always assume that both vessels keep their present course and speed.
ARPA in a refit
Replacing a radar, moving its antenna or changing the heading and speed sensors affects how target tracking performs. Interfaces, settings and post-installation checks follow the manufacturer’s documentation. Radar, ECDIS and AIS upgrades are often planned together.
FAQ
What does ARPA stand for?
Automatic radar plotting aid: a radar function that tracks targets automatically and calculates their motion, CPA and TCPA.
What does ARPA mean on a ship’s radar?
It means the radar can acquire and track other vessels and show their course, speed and closest point of approach, with alarms when a target is predicted to come too close. On newer radar the same function may be labelled target tracking (TT).
Is ARPA the same as AIS?
No. ARPA calculates a target’s motion from your own radar echoes; AIS displays data transmitted by the other vessel. They complement each other, and AIS does not replace radar target tracking.
Is ARPA required on yachts?
It depends on the yacht’s tonnage, use and flag. SOLAS links automatic tracking aids to ships of 500 GT and above and ARPA to ships of 10,000 GT and above; the flag Administration decides how these and its yacht code apply to a particular yacht.
What is the difference between ARPA and ATA?
In SOLAS terms, an automatic tracking aid (ATA) plots targets automatically, while ARPA is the requirement for ships of 10,000 GT and above: at least 20 targets a connection to a speed-through-the-water device and a trial manoeuvre function. Radar built to MSC.192(79) describes both as target tracking, with capacity and functions set by ship size.