Distress Radiobeacon - Location By Doppler (without GPS)

Location By Doppler (without GPS)

The Cospas-Sarsat system was made possible by Doppler processing. Local unit terminals (LUTs) detecting non-geostationary satellites interpret the Doppler frequency shift heard by LEOSAR and MEOSAR satellites as they pass over a beacon transmitting at a fixed frequency. The interpretation determines both bearing and range. The range and bearing are measured from the rate of change of the heard frequency, which varies both according to the path of the satellite in space and the rotation of the earth. This triangulates the position of the beacon. A faster change in the doppler indicates that the beacon is closer to the satellite's ground track. If the beacon is moving toward or away from the satellite track due to the Earth's rotation, it is on one side or other of the satellite's path.

If the beacon's frequency is more precise, it can be located more precisely, saving search time, so modern 406 MHz beacons are accurate to 2 parts per billion, giving a search area of only 2 square km, compared to the older beacons accurate to 50 parts per million that had 200 square kilometers of search area.

In order to increase the useful power, and handle multiple simultaneous beacons, modern 406 MHz beacons transmit in bursts, and remain silent for about 50 seconds.

Russia developed the original system, and its success drove the desire to develop the improved 406 MHz system. The original system was a brilliant adaptation to the low quality beacons, originally designed to aid air searches. It used just a simple, lightweight transponder on the satellite, with no digital recorders or other complexities. Ground stations listened to each satellite as long as it was above the horizon. Doppler shift was used to locate the beacon(s). Multiple beacons were separated when a computer program analysed the signals with a fast fourier transform. Also, two satellite passes per beacon were used. This eliminated false alarms by using two measurements to verify the beacon's location from two different bearings. This prevented false alarms from VHF channels that affected a single satellite. Regrettably, the second satellite pass almost doubled the average time before notification of the rescuing authority. However, the notification time was much less than a day.