Geostationary satellites have typically been station kept in 40 mile boxes since 1972. To save Reaction Control System fuel Inmarsat allowed inclination of the orbit to grow. This resulted in ~700 miles of N-S motion of the satellite during 7 hours of observations. The Burst Frequency Offset value matched in value and sinusoidal rate of change consistent with a 24 hour orbit. The satellite motion was then removed from the observed doppler which generated a vector to resolve the N S ambiguity on the line of position generated by the range measurement. Further Doppler signature analysis bracketed the speed and heading of successive handshakes. Doppler analysis was used for decades by SARSAT albeit with much more orbital inclination but less stable frequency ELT's. Inmarsat deserves credit for figuring it out, and VP Mark Dickinson for explaining it on CNN.
