Exploiting ARPA

An officer of the watch (OOW) when on duty on the Bridge of a ship, his primary duty (responsibility) is to ensure Navigation safety of the ship. In the present era, the OOW has a number of modern navigation aids, which reduce his work load as well as provide him with the means to safely navigate his ship. One such Nav-Aid is ARPA.

Automatic Radar Plotting Aid (ARPA), is a device which is most essential on the Bridge. The primary purpose of ARPA is to provide information about the presence of other ships or objects in the ship’s vicinity and further out depending on the range of the radar and the range scale in use. Thus, helping in collision avoidance.

The availability of low-cost microprocessors and the development of advanced computer technology during the 1970s and 1980s made it possible to apply computer techniques to improve commercial marine radar systems. Radar manufacturers used this technology to create the Automatic Radar Plotting Aid (ARPA).

The modern day OOW needs to be well conversant with all the Radar-ARPA features so as to exploit it for Collision Avoidance. The OOW should have the requisite knowledge to tune the radar display using features such as Auto Gain control, Anti-Sea Clutter and Anti-Rain Clutter controls for optimal performance.

Utilizing the features of the Guard Zone, is most beneficial as it gives the OOW the flexibility to set the relative sector and range limits of the Guard Zone, depending upon the course of the ship. Once the Guard Zone is setup, any vessel or object entering inside the Guard Zone would be automatically acquired by the ARPA and tracked.

In addition to the auto-tracking of targets, ARPA also gives the OOW the facility for manual target acquisition for tracking. The target parameters for the chosen target are available on the display. The target parameters which need to be closely monitored such as how close will the target pass from own ship? This is known as the Closest Point of Approach (CPA), and in how much time will the target ship reach this CPA? This is known as Time for Closest Point of Approach (TCPA).

It would be a good practice for the OOW to ensure that all vessels crossing own ship in the open sea, to have a CPA of at least 2 n.miles and in navigable / pilotage waters to try and maintain a CPA not less than 5 cables if practicable or as much as is possible, such as when crossing in a narrow channel, in the prevailing circumstances.

The Relative Motion Mode of ARPA helps the OOW to understand the movement of the other vessel or object with respect to own ship. In this mode, if the CPA is Zero, the relative vector of the target vessel will be pointing towards own ship’s position which would be in the centre of the display. Although the target vessel could be doing a completely different course and speed, she is relatively heading for a collision towards own ship. In such a situation, the OOW would need to react in accordance with ROR rules and take the necessary collision avoiding action. The OOW would do well to alter own ship’s course to increase the CPA and if that is not sufficient, should increase or decrease own ship’s speed as well to avoid a collision with the target vessel.

In True Motion Mode the true course and speed of the target vessels vector can be seen on the ARPA display.

At sea the ARPA display should be always be set up in the ‘North up’ mode. This helps the OOW to correlate the land features as viewed on the ARPA display, with that as on the chart. But in the case of a gyro failure the ‘Head up’ mode would be considered useful to identify the visible land features as seen in the ahead sector with that on the radar and chart. Knowing the above fundamental features would be good for any mariner.

In conclusion, the primary function of ARPAs can be summarised in the statement found under the IMO Performance Standards. It states a requirement of ARPAs is: “to improve the standard of collision avoidance at sea: Reduce the workload of observers (OOW) by enabling them to automatically obtain information so that they can perform as well with multiple targets as they can by manually plotting a single target and provide continuous situation evaluation”.

As we can see from the above statement the principal advantages of ARPA are a reduction in the workload of bridge personnel and fuller and quicker information on selected targets. Hence, it is to the advantage of the mariner on the bridge to be fully conversant in utilizing ARPA features.?Regular simulator training would pose as an advantage to the mariner in knowing how to exploit ARPA effectively.