In addition to radar detectors (CARD) and radar transponders (SeeMe is one brand, as I recall), you have a third option: AIS. Due to GMDSS, vessels larger than 300 tons are now required to carry AIS active transponders (very expensive), which means those of us with small boats can receive that signal via a VHF antenna and then either display it on a small screen of a device like NASA builds - see e.g. http://www.allgadgets.co.uk/ag/produ...1&pf_id=AG3928
- OR feed the same signal into a laptop, perhaps overlaid with an electronic chart, so you can see the targets on your chart OR have it appear on your radar/plotter screen.
AIS will do the computations for you (smarter than your radar unless you are MARPA equipped) since it has your GPS data and the data from the other vessel, and give you warnings on those vessels that will come inside your CPA 'veil'. Moreover, it gives vessel name and - if your VHF is DSC equipped - their MMSI # so you can even 'dial them up' via VHF.
Things sure have changed!