I don't believe you will need a compass. An on-board GPS is there to determine one position and a the other position is static by definition. You can therefore calculate altitude and azimuth for the antenna, and I guess the GPS can do that chore for you.
But the device does not know its own orientation just from the GPS location.
Mine do, with better response than my E-Trek handhelds.
With all respect, oh great one, oh, yes they do. You get a reasonablly accurate 'course direction', with respect to either magnetic, or true, north after warmup and reception of three or more satalites.