GPS World, January 2011

GNSS Design Test INNOVATION licized event is one of thousands of successful rescues made possible by years of NASA research and development Background The beginnings of satellite aided search and rescue date back to 1970 when a plane carrying two U S congressmen crashed in a remote region of Alaska A massive search and rescue effort was mounted but to this day no trace of them or their aircraft has ever been found At the time search for missing aircraft was conducted by search aircraft fl ying over thousands of square kilometers hoping to sight the missing aircraft As a result of this tragedy Congress recognized this ineffi cient search method and passed an amendment to the Occupational Safety and Health Act of 1970 requiring most aircraft fl ying in the United States to carry emergency locator beacons ELTs to provide a local homing capability NASA then developed the technology to detect and locate an ELT from ground stations using the beacon signal relayed by satellites to provide more global coverage This concept evolved into a highly successful international search and rescue system called COSPAS SARSAT COSPAS is an acronym for the Russian words Cosmicheskaya Sistema Poiska Avariynyh Sudov which translates to Space System for the Search of Vessels in Distress SARSAT is an acronym for Search and Rescue Satellite Aided Tracking Established by Canada France the United States and the former Soviet Union in 1979 the system has 43 participating countries and has been instrumental in saving more than 28000 lives worldwide including 6400 in the U S all as a result of NASAs innovations Since this auspicious beginning NASA has continued to perform SAR research and development as a member of the National Search and Rescue Committee and supports the National Search and Rescue Plan through an interagency memorandum of understanding with the Coast Guard the Air Force and NOAA NOAA is responsible for operation of the U S portion of current COSPAS SARSAT system that relies on SAR payloads on weather satellites in low earth and geostationary orbits As shown in Figure 1 the satellites relay distress signals from emergency beacons to a network of ground stations and ultimately to the U S Mission Control Center USMCC operated by NOAA The USMCC distributes the alerts to the appropriate search and rescue authorities the U S Air Force or the Coast Guard The Air Force coordinates search and rescue for the mainland U S SAR region and operates the Air Force Rescue Coordination Center The Coast Guard performs maritime search and rescue and oversees the U S national SAR policy Beacons Three types of distress emergency locator beacons are in use that are compatible with the COSPAS SARSAT system EPIRBs emergency position indicating radio beacons FIGURE 1 Overall concept of search and rescue system designed for maritime use ELTs emergency locator transmitters for use on aircraft PLBs personal locator beacons for personal use These can be used by persons engaged in high risk activities such as mountain climbing and backcountry skiing Originally emergency locator beacons transmitted an analog signal on two frequencies 1215 MHz and 243 MHz in the civil and military aeronautical communications bands respectively so that they would be audible over aircraft radios Later a signal that was encoded with a digital message and transmitted at 406 MHz was added Since February 1 2009 only the 406 MHz encoded signals are relayed by satellites supporting the international COSPAS SARSAT system Therefore older beacons that only transmit the 1215 243 MHz signals are now only detectable by ground based receivers and aircraft overfl ying a crash site The 406 MHz beacons transmit an approximately halfsecond message or burst approximately every 50 seconds beginning 50 seconds after being activated The actual time of burst transmission is dithered in time so that no two beacons will have all of their bursts coincident A 406 MHz beacon may also have an integral global navigation satellite system GNSS receiver Such a beacon uses the GNSS receiver to attempt to determine its location for inclusion in the transmitted digital message In this way the beacon will be located once it is detected by a low Earth orbit LEO or geostationary orbit GEO satellite Distress messages contain information such as The beacons country of origin A unique 15 digit hexadecimal beacon ID Location when equipped with an integrated GNSS receiver Whether or not the beacon contains a 1215 MHz homing signal www gpsworld com January 2011 GPS World 73