Twenty-five minutes elapsed between the first radar contact and the collision between the liner Stockholm and the Andrea Doria. The Andrea Doria sank on July 25, 1956 with the loss of 54 passengers and crew.

So how could such a collision occur when both ships were equipped with radar? The answer is that while radar is an aid to collision avoidance, it is not a force field.

The Andrea Doria tragedy, and others that followed, led to the development of marine radar target plotting aids.

Back 25 or 30 years ago, a grease pencil was used to mark successive target positions on the face of the radar's CRT. The trail of marks displayed a vector prediction of the target's CPA. Contemporary radar sets are equipped with the electronic equivalent of a grease pencil, the electronic bearing line (EBL) and electronic range ring (VRM).

Relative target movement is determined by positioning the EBL and VRM so they intersect at the target's position on the screen. Successive observations show whether the target is maintaining its position on the EBL and tracking toward your vessel. Dual EBLs and VRMs, common on many radars, make it easy to track two targets simultaneously.

Although manual plotting for CPA, or collision threat, works well, the workload can become overwhelming when confronted with a large number of targets.

Salvation is found in ARPA, short for Automatic Radar Plotting. (Not: Some companies call this feature ATA for Automatic Tracking Aid). In its earliest incarnations ARPA required the radar operator to designate targets of interest by superimposing an on-screen cursor over the target return. After which, the system computer automatically tracked the target, creating an electronic grease pencil succession of position points. With know track positions and known time interval between points, the computer made its prediction.

The development of powerful microprocessors and mega memory capacity led to the fully automatic ARPA systems installed on commercial ships and megayachts. More recently the declining cost of computer hardware has allowed the introduction of an ARPA option on radar sets intended for boats as small as 20 feet. So what does this mean for the consumer?

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.

While radar can either be set for north-up, course-up or true motion display, the on-screen target data display is always true bearing, true course and speed over the ground or through water.

ARPA of this capability can add about $2,500 to the cost of the radar. Frankly, it's likely overkill for most recreational vessels. With this reality in mind radar manufacturers rather wisely offer less capable but still incredibly valuable mini-ARPA as an option on small vessel radar systems.

Typical mini ARPA systems automatically acquire and track up to 10 targets within a range of 0.1 to 32 nautical miles. Tracked target symbols are identical to the more capable systems. Vector times and prediction of future position typically range from 30 seconds to 30 minutes. Up to 10 past positions can be shown, at intervals from 30 seconds to six minutes. Data for any tracked target can be displayed on the screen when the target is selected with the plotting cursor.

With mini ARPA, somewhat less data is computed than with full capability ARPA. However the data field will show the prediction time selected for the future position vector, the course, speed, predicted closest approach distance and the time of closest approach. The cost of adding mini-ARPA is typically less than $600.

In order to function properly an ARPA-equipped radar must be provided with accurate heading information and in most cases with hull speed and speed over the ground data. Moreover, targets must return a consistent signal if they are to be successfully tracked by ARPA. The radar screen can be set to show targets in either true or relative motion mode.

When the relative motion display is selected (the one most recreational mariners are used to using) all tracked objects will display motion vectors, including buoys and similar stationary objects. In a true vector display, tracked stationary such as buoys will show no motion vector, while ships and other vessels will display their true motion vectors.

The difference has obvious use in identifying the nature of tracked targets. A good example is the legendary story of the stubborn skipper who demanded a radar target obey the rules of the road and give way. Whereupon said target identified itself as a lighthouse.