GPS World, January 2011

lation of satellites available would improve performance even more Furthermore the increase in satellite performance expected in the operational system will also improve probabilities of detection and location Location Accuracy Recall that location accuracy is measured as the percentage of location solutions obtained within fi ve minutes after beacon activation that are within fi ve kilometers of the actual beacon location The requirement is to obtain 95 percent of the locations to within 5 kilometers of the actual location and 98 percent within 10 kilometers within fi ve minutes after beacon activation As mentioned earlier the requirements included in the performance specifi cation assume a constellation of 24 DASS equipped GPS satellites POC testing was done with a system that had only eight DASS equipped GPS satellites available However location errors can be scaled to refl ect what the DOP would be if the satellite constellation contained all 24 GPS satellites Therefore it is the scaled results that can be used to determine whether performance will meet the requirement TABLE 4 therefore presents the location accuracy results as measured and after being scaled by DOP Another important performance metric for DASS is location accuracy obtained after a single beacon burst is received Even though there is not currently a requirement for single burst location accuracy it is a very desirable feature of DASS since an emergency situation does not guarantee that more than a single burst will be received Single burst location accuracy was therefore measured with the results shown in TABLE 5 Once again the results are scaled by DOP values to remove the effect of non optimal satellite geometry More insight into this performance can be gained by examining the single burst location accuracy distribution as a function of distance error as shown in TABLE 6 It can be seen that for these beacons computed locations are within 9 kilometers of the actual location 95 percent of the time Again the expectation is that having a full constellation of satellites available would improve this performance For instance having more satellites to choose from might allow the system to select data from satellites with stronger or less noisy links Conclusion The promise of search and rescue instruments on each satellite in the large and well managed GPS constellation will provide a signifi cant advancement in the capabilities of the already highly successful COSPAS SARSAT system The new system will provide global coverage for satellite supported search and rescue and provide capabilities for rapid detection and location of distress beacons while requiring fewer ground stations The DASS POC system has validated by test the predictions made by analysis during the defi nition and development phase The DASS POC testing has demonstrated reliable detection and accurate location of beacons within fi ve minutes of activation Accurate locations are also produced after even a single burst of a newly activated beacon which is a desirable feature of DASS since an emergency situation does not guarantee that more than a single burst will be received The performance obtained using a reduced constellation of satellites equipped with a modifi ed existing instrument not only demonstrates the existing capability but also confi rms the improvements to come with the operational system In fact the success of DASS is being emulated by the European Union in the design of their future Galileo GNSS constellation and the Russians in an upgraded GLONASS GNSS constellation all of which will be interoperable by international agreement DASS will contribute to NASAs goal of taking the search out of search and rescue Achieving this goal will not only improve the chances of GNSS Design Test INNOVATION rescuing people in distress quickly which is critical to their survival it will also reduce the risk to rescuers who often put themselves in dangerous situations to affect a rescue That is why the motto of the Search and Rescue Offi ce is Saving more lives reducing risks to search personnel and saving resources DAVID W AFFENS is the manager of the NASA Search and Rescue SAR Mission Office at the Goddard Space Flight Center GSFC in Greenbelt Maryland where he began working in 1990 He holds a degree in electronic engineering Before joining NASA he worked in various aspects of submarine warfare and intelligence gathering for the U S Navy over a span of 21 years ROY DREIBELBIS is a consultant who has worked in rescue related jobs since 1957 including helicopter rescue missions in Vietnam As an officer in the U S Air Force he was the director of Inland SAR at rescue headquarters for the coterminous 48 states was commander of the 33rd Air Rescue Squadron and served as deputy chief of staff for rescue operations at rescue headquarters from 1979 until 1981 Upon retirement from the Air Force he was employed by the State of Louisiana as flight operations director and chief pilot In 1987 he accepted employment with contractors in the District of Columbia area that supported NASA and NOAA SARSAT activities JAMES E MENTALL is the NASA GSFC Search and Rescue Instrument Manager He has a Ph D in physics and has spent more than 42 years of his professional life at GSFC For 15 of those years he has been responsible for the integration and test of the Search and Rescue Repeater and the Search and Rescue Processor on the NOAA Polar orbiting Operational Weather Satellites He has also served as the deputy mission manager for the Search and Rescue Mission Office and played a significant role in the procurement of the DASS antenna system and ground station GEORGE THEODORAKOS is the chief staff engineer for MEI Technologies Inc He received his B S summa cum laude and M S degrees in electrical engineering from the University of Maryland College Park Maryland in 1978 and 1987 respectively Since 2002 in his role as chief staff engineer at MEI he has provided technical management support to the Search and Rescue Mission Office at GSFC In this capacity he led the development of the Distress Alerting Satellite System MORE ONLINE Further Reading For references related to this article go to gpsworld com and click on Richard Langleys Innovation under Inside GPS World in the left hand navigation bar www gpsworld com January 2011 GPS World 79