Hmm ok. That makes it a fair bit worse. About 40-45% offline. About 6-7minutes at a time offline. Probably no certainty with the satellite coverage, so it could be up to 30-40min worse case. In air would be probably better, but for the challenge we don't need it in the air at all with the RFD900 available.
Antenna would be critical, as would RF noise floor and a good ground plane. Not sure what sensitivities they operate at, but it's probably similar to GPS. Stephen would know more.