Most commercial vessels will use radar and very soon every one over 300 gross GRT will have AIS too (July 2007).
Why would AIS be a good adjunct to the use of ARPA?
Typically the ARPA will track the most reflective part of the vessel, on a VLCC this would be the superstructure. Before a change of heading of such a ship is detected on the ARPA, the VLCC will probably have turned 30-40 degrees, equivalent to 4 or 5 minutes. Another 1 to 2 minutes are needed to obtain a steady vector of the target, for smaller vessels this is somewhat less, however an average of five to six minutes between wheel-over and recognition of this fact on ARPA is common.
In contrast, the data of an AIS-ship, sailing at a speed greater than 14 knots and turning, will be updated every 2 seconds, showing the course alteration virtually from the moment that the wheel was put over.