Navigational Equipment

The Merchant Shipping (Navigational Equipment) Regulations 1993 require certain ships to be provided with a magnetic compass installation and other specified ships to be fitted additionally with a direction finder, an echo sounder, a gyrocompass, radar and ARPA installations, a speed and distance measuring installation and a rate of turn indicator. Provision is also made in the Regulations in respect of siting and serviceability of the installations and, in the case of radar and ARPA installations, the qualifications of the radar observers. A number of recent accidents have been caused by over-reliance on a single navigational aid. Watch-keepers must always ensure that positional information is regularly crosschecked using other equipment, as well as visual aids to navigation.

Some radars are equipped with Auto Tracking Aids (ATA) which enable targets to be acquired and automatically plotted. Such systems do not provide all the functions of ARPA. Radars for smaller vessels may be provided with Electronic Plotting Aids (EPA) which require the operator to plot each target manually. EPA provides the target calculations for each manual plot. Operators should be aware of the functional limitations of ATA and EPA.

The Use of Radar and Plotting Aids

Collisions have been caused far too frequently by failure to make proper use of radar and ARPA in both restricted visibility and in clear weather. A common error has been altering course on insufficient information and by maintaining too high a speed, particularly when a close quarters situation is developing or is likely to develop. Information provided by radar and ARPA/ATA in clear weather conditions can assist the watch-keeper in maintaining a proper lookout in areas of high traffic density. It cannot be emphasized too strongly that navigation in restricted visibility is difficult and great care is needed even with all the information available from the radar and ARPA/ATA. Where continuous radar watch-keeping and plotting cannot be maintained even greater caution must be exercised. A “safe speed” should at all times reflect the prevailing circumstances.

It is essential for the observer to be aware of the current quality of performance of the radar (which can most easily be ascertained by the Performance Monitor) and to take account of the possibility that small vessels, small icebergs and other floating objects such as containers may not be detected. When video processing techniques are employed, caution should be exercised.

Echoes may be obscured by sea or rain clutter. Correct setting of clutter controls will help but will not completely remove this possibility. When plotting larger targets on a medium range scale, the display should be periodically switched to a shorter range, and the clutter controls adjusted, to check for less distinct targets.

The observer must be aware of the arcs of blind or shadow sectors on the display caused by masts and other on-board obstructions. They must be plotted on a diagram placed near the radar display, which must be updated following any changes, which affect the sectors.

To estimate the degree of risk of collisions with another vessel it is necessary to forecast the closest point of approach. Choice of appropriate avoiding action is facilitated by the knowledge of the other vessel’s track. This can be obtained by the manual plotting methods or using EPA, or automatically, using ATA or ARPA. The accuracy of the plot, however, obtained, depends upon accurate measurement of own’s ship track during the plotting interval. Observers should be aware that an inaccurate compass heading or speed input will greatly reduce the accuracy of true vectors when using ARPA or ATA, and should therefore treat the apparent precision of the digital display with caution. This is particularly important with targets on near-opposite courses where a slight error of own-ship’s data can make the difference between a target apparently crossing ahead or passing clear.

The radar display should be kept on at all times when weather conditions indicate that visibility deteriorate, and at night wherever fog banks, small craft or unlit obstructions such as icebergs are likely to be encountered. This is particularly important when there is a likelihood of occasional fog banks so that vessels can be detected before entering the fog. The life of components, and hence the reliability of the radar, will be far less affected by continuous running, than by frequent switching on and off.

In restricted visibility the radar display should be permanently on and observed. The frequency of observation will depend on the prevailing circumstances, such as own ship’s peed and the type of craft or other floating objects likely to be encountered.

Oral presentation 6