The Cospas-Sarsat System calculates the location of distress beacons using Doppler processing techniques. Simply stated the Doppler effect is a term used to describe the phenomena that the frequency of a signal "as heard" by a receiving device is affected by the magnitude of the relative velocity between the transmitter and the receiver. If the distance between the transmitter and the receiver is reducing, the frequency as heard by the receiver is raised by the Doppler effect. If the distance is increasing, the Doppler effect reduces the frequency as heard by the receiver. If there is no relative velocity, the frequency heard by the receiver is exactly the transmitted frequency.

When comparing InReach to a PLB, it might be useful to know if the Iridium satellites have the capability of triangulating locations where there is no encoded GPS signal. I suspect that the answer to that is "no."