I think I recall we could get it from the radar too
IIRC the expert in the rear seat explained that if the radar was in doppler mode & search, it would look down every couple of minutes or so & calculate a wind which was input to the nav computer. Main problem with the kit was, I think, that its errors increased dramatically at high latitudes; it seemed to work quite well in the Mediterranean, but was less than helpful at 70° N.