GPS World, January 2011

INNOVATION GNSS Design Test FIGURE2 Prototype ground station at NASA GSFC FIGURE 3 Beacon simulator transmit antenna gain pattern Room for Improvement SARSAT fi rst became operational in the mid 1980s The current system uses instruments placed on LEO and GEO weather satellites to detect and locate mariners aviators and recreational enthusiasts in distress almost anywhere in the world at anytime and in almost any condition Previously dedicated Russian LEO satellites were also implemented but the use of these satellites was discontinued in 2007 Although it has proven its effectiveness as evidenced by the number of persons rescued over the systems lifetime the current capability does have limitations LEO spacecraft orbit the Earth 14 times a day and use the Doppler effect with satellite orbital ephemeris data to calculate the position of a beacon However a satellite may not be in a position to pick up a distress signal the moment a user activates the beacon Time is critical in responding to an emergency situation Unfortunately delays of two hours or longer are possible especially near the equator LEO spacecraft carry two instruments a Search and Rescue Repeater SARR supplied by the Canadian Department of National Defence and a Search and Rescue Processor SARP provided by the French Centre National dEtudes Spatiales CNES The SARR is a pure repeater which relays the beacon signal to a local ground station where the data is analyzed to obtain a location The SARP processes the received beacon signal by measuring the Doppler shift as Beacon signal parameter Required range Transmit power 5W 2 dB 37 2 dBm Modulation index 11 01 radians Bit rate 400 1 bits per second Unmodulated carrier duration 160 1 milliseconds Modulation rise fall times 150 100 microseconds TABLE 1 COSPAS SARSAT beacon specifications a function of time and decoding the digital message included in the 406 MHz signal This information is stored until it can be transmitted to a ground station using the SARRs downlink transmitter Under most conditions beacon locations can be determined to within a radius of 5 kilometers Geostationary weather satellites on the other hand orbit above the Earth in a fi xed location over the equator Although they do provide continuous visibility of much of the Earth they cannot independently locate a beacon unless it contains a GNSS receiver that determines its position and includes it in the beacons digital message Currently not all beacons contain integral GNSS receivers Furthermore even if a beacon contains a GNSS receiver the navigation signal may be obstructed by terrain or thick foliage The next generation system DASS overcomes these limitations and will improve accuracy and response time to provide an even more capable life saving system Distress Alerting Satellite System A 1997 Canadian government study of possible alternative satellite systems for SARSAT including commercial sources determined that the ideal system is based on medium Earth orbit MEO satellites A MEO system will be able to provide superior global detection and location data with fewer ground stations than the existing COSPAS SARSAT system The GPS constellation was identifi ed as an ideal MEO platform The concept of the DASS system is straightforward Three or more antennas track different GPS satellites equipped with search and rescue repeaters that receive the distress signal and retransmit the signal to the ground Since each satellite is in a different orbit each received signal has a different Dopplershifted arrival frequency and time of arrival Knowing the position and orbit of each satellite it is possible to determine the position of the distress beacon Future improvement in location accuracy is made possible by one of the strengths of the DASS space segment That is the DASS location algorithm optimizes location accuracy utilizing time and frequency measurements of beacon signals that were not designed for that purpose The DASS space segment allows for the beacon signal to be modifi ed in the future enhancing the performance of this type of location process Other advantages of DASS over the existing system are fairly obvious Reception of the emergency signal is immediate Locations can be determined after receiving a single GPS World January 2011 www gpsworld com 74