A GPS and VHF-FM marine radio won't allow you to track vessel, but this needed for vessels using Global Maritime Distress & Safety System (GMDSS). I am providing a link to the ShipPlotter program that tracks the Automatic Identification System (AIS). AIS is required for commercial vessel in the USA and most areas of the world. You won't be able to track pleasure boats with this system, since most don't have AIS installed. Using Shipplotter with a good outside antenna you should be see vessels 20+ nm away, plus you can share data from this program via the internet. ShipPlotter used is passive system and radar requires the radar unit to transmit a signal. Hope this helps you and just follow the links below for more study on the above subjects.