This page gives a nice overview on actual u-boat RWR technology:
http://www.uboataces.com/radar-warning.shtml
You can see that most of them did not give directional capability, often didn't detect all actual bands of radar use, and frequently had to be dismantled and taken below decks upon diving and re-installed on surfacing.
None of this is modeled in the game.
RWR in SH3 is basically a superweapon.
Steve