Look at post #11.
It is essentially a receiver of radar signals, so alerts when radar is painting you. It also gives a rough indication of direction IIRC. Of course it can't work at the same time you are using radar on your boat as it would be continuously alerting.
If you have the power and money
(not much these days) go for both radar and AIS
. CARD is better on both power and cost but it only "sees" objects with radar transmissions, and can't indicate distance or precise direction to target. A limited solution at a limited price
is announcing their version of solid state radar, which looks to be the instant technology leader. Furuno
has the only recreational systems that actually have true ARPA capability; other systems have much more limited capability. ARPA tracks radar targets, calculates their movements and predicts potential collisions automatically (closest approach distance and time). It does require a new chartplotter
as well so it will mostly appeal to outfitting of new boats.