For those who seem to be struggling with the SATCOM/INMARSAT operation the following gives a very good overview:
TMF Associates MSS blog » Understanding ?satellite pings??
Hopefully this will put-to-bed the repeated questions regarding this!
So the Pings allow the radius of a circle centred on the Inmarsat Satellite to be calculated to approximately 100 miles. The Aircrafts location being somewhere along the circumference deduced as a function of its presumed initial location and its potential range? Puts the SAR task into perspective.