Explanation: The COSPAS-SARSAT system primarily determines the position of a distress beacon, such as an EPIRB (Emergency Position-Indicating Radio Beacon), by measuring the Doppler shift of its 406 MHz signal. As a satellite passes over the beacon, the frequency of the signal received by the satellite changes due to the relative motion between the satellite and the stationary beacon. By tracking this change in frequency (Doppler shift) over time at several points during the satellite's orbit, ground stations can calculate the beacon's precise location on Earth. Option A is correct because it accurately describes this fundamental radio theory employed by the COSPAS-SARSAT system for position determination. Option B is incorrect as it describes a secondary method. While modern EPIRBs often include a GPS receiver and encode their position directly into the 406 MHz signal for faster relay, the COSPAS-SARSAT system's core ability to *determine* a position without prior knowledge relies on the Doppler shift, especially for older beacons or if the beacon's GPS fails. Option C is incorrect because a single Low Earth Orbit (LEO) satellite, observing the Doppler shift over its pass, can indeed establish an accurate position. While multiple satellites can improve the speed of location or provide redundancy, they are not strictly required for the *method* of position determination itself.