[jwzimm]

They ruled out the northern area by performing a Doppler frequency shift analysis on the signal they received. The satellite in question is stationed in a geostationary equatorial orbit but shifts slightly north and south. This relative movement results in a frequency shift depending on if the satellite was moving toward or away from the transmission source at time of transmission/receipt. They corroborated this analysis with signals received at the same time from known transmitters (other airplanes talking to this satellite). The result indicated that the signal originated from the southern direction.