Electronic plotting devices
On larger ships, at least one of the radars carried is likely to have automatic radar plotting aid (ARPA) functions. Radars on smaller ships may be fitted with either automatic tracking aid (ATA) or electronic plotting aid (EPA) functions.
ATA uses ARPA hardware but with limited functionality: no trial maneuvre, target past position or guard zone features, and manual acquisition limited to 10 targets. EPA offers basic electronic plotting functions that are as good as a reflection plotter.
In comparison with standard radar, ARPA and ATA offer a number of automated collision avoidance features. However, watchkeepers should be aware of the dangers of being over-reliant on these devices and:
· understand the types of errors that are possible and recognise the operational warnings that appear on the display;
· understand the limitations of the devices;
· regularly test the devices using the built-in operational test facilities.
Heading and speed inputs
Correct and reliable speed and heading inputs into the ARPA or ATA are vital if targets are to be processed correctly.
Speed and heading inputs need to be sea stabilised (water tracked) to provide the ARPA or ATA with speed and course through the water. The use of these devices in a ground stabilised (bottom tracked) mode for assessing risk of collision could be particularly hazardous in sea areas that experience significant tidal streams and currents.
Automatic target acquisition
Features such as guard zones and target acquisition footprints are commonly used for the automatic acquisition of ARPA targets.
Such features should always be used with caution, especially in sea areas where radar inconspicuous targets can be expected.
