406MHz EPIRBs - how good are they?

[Fairwinds]

eg GME MT400 406Mhz EPIRB (CAT II)

Available for as little as £270

Says 'initial position location accuracy to within 5km'

How does it do this? I thought you had to have a GPS connected for this to work . . . does it just do it by triangulation on three or more satellites? ANd why only 'initial' position - doesn't this update the whole time it is broadcasting?

(Just reading Steve Callaghan's book 'Adrift' and going off the Hasler dictum of 'drowning like a gentleman')


- NB