GPS World, January 2011

beacon burst since it does not rely on measuring the Doppler shift over time to determine position as in the current LEO system A full constellation of DASS equipped GPS satellites in orbit will ensure that four or more satellites are in view of the transmitting emergency beacon anywhere in the world while requiring fewer ground stations Another key strength of the DASS system is the promise of SARSAT transponders on each satellite in the large and wellmanaged GPS constellation There are at least 24 GPS active satellites in orbit at any given time currently 31 are active When the GPS constellation is fully populated by satellites with DASS transponders it will provide global coverage for satellite supported search and rescue and provide capabilities for rapid detection and location of distress beacons Efforts are ongoing to integrate a satellite beacon repeater instrument to be provided by the Canadian government onto the GPS Block III B and C satellites to provide the DASS space segment for operational use DASS Development DASS development will proceed in phases referred to as the defi nition and development proof of concept demonstration and evaluation initial operating capability and fi nal operating capability The proof of concept POC phase was completed in January 2009 The POC testing and results are summarized in this article At the time of this writing preparations are ongoing to initiate the demonstration and evaluation phase Definition and Development In 2000 as part of the defi nition and development phase the NASA GSFC SAR Mission Offi GNSS Design Test INNOVATION ce began discussions with the Department of Energys Sandia National Laboratories SNL to determine if it would be feasible to add a SAR repeater function to a Department of Energy DOE instrument on GPS satellites Sandia representatives thought it possible and NASA agreed to fund a study to determine if with minor modifi cation one could include a search and rescue repeater function to their instrument The SNL feasibility study concluded that the GPS DOE package could with minor modifi cations perform the SAR mission The study also determined that accurate locations could be calculated after a single beacon transmission and improved with each subsequent beacon transmission Based on this information NASA with the cooperation of the U S Air Force Space Command and SNL proceeded with the development of the new space based search and rescue system which was named the Distress Alerting Satellite System Proof of Concept In 2003 a memorandum of agreement MOA between NASA NOAA the Air Force the Coast Guard and the Department of Energy tasked NASA to perform a POC program for DASS The MOA included the development of a POC space segment and a prototype ground station to perform post launch checkout performance testing and implementation planning of an operational DASS sys Name Beacon ID Effective integrated radiated power dbm Modulation index radians Bit Rate bits per second Unmodulated carrier duration milliseconds Scenario 1 Maximum beacon parameter values ID1 370 11 400 1600 150 ID2 370 12 400 1600 150 ID3 370 11 404 1600 150 ID4 370 11 400 1616 150 ID5 370 11 400 1600 250 Scenario 2 Minimum beacon parameter values ID1 370 11 400 1600 150 ID2 370 10 400 1600 150 ID3 370 11 396 1600 150 ID4 370 11 400 1584 150 ID5 370 11 400 1600 50 Scenario 3 Variable power ID1 370 11 400 1600 150 ID2 400 11 400 1600 150 ID3 350 11 400 1600 150 TABLE 2 Beacon parameter values for each test scenario Modulation rise fall times microseconds www gpsworld com January 2011 GPS World 75