GPS World, January 2011

INNOVATION GNSS Design Test Scenarios ID1 Nominal ID2 Mod index ID3 Bit rate Scenario 1 max 9911 9911 9956 9956 9970 Scenario 2 min 9941 9911 9959 9985 9941 ID1 ID2 ID3 Scenario 3 power 37 dbm 9955 40 dbm 9985 35 dbm 9880 TABLE 3 Probability of detection test results Location accuracy as measured Scenario Beacon ID Count 5 km Count 10 km Total Count ID4 Carrier Percent 5 km ID5 Mod rise fall Percent 10 km All All beacons 4696 5920 6194 76 96 Location accuracy after DOP scaling Scenario Beacon ID Count 5 km Count 10 km Total Count Percent 5 km Percent 10 km All All beacons 5942 6132 6194 96 99 TABLE 4 Location accuracy for 5 minute periods tem It stressed the need for DASS gave authority to each participating agency to participate in the POC demonstration and defi ned the roles of each The Air Force Space Command approved the addition of modifi ed equipment on GPS satellites The DASS POC space segment operates as a subcomponent of GPS Block IIR and IIF satellites Nine GPS Block IIR satellites carry experimental DASS payloads and all 12 IIF satellites are scheduled to Therefore the fi nal POC space segment will consist of 21 DASS equipped GPS satellites Each payload receives 406 MHz SAR signals on an extant GPS UHF antenna and relays the signals at a GPS S band frequency on a second extant antenna It is important to note that the performance of the DASS POC space segment will be exceeded by the performance of the operational space segment being designed specifi cally for DASS and planned for launch on GPS Block III satellites A prototype DASS ground station Figure 2 was funded by NASA and installed at GSFC The DASS prototype ground system consists of four antennas four receivers and the workstations and servers necessary to process the received Scenario Beacon ID Count 5 km Count 10 km Total Count Percent 5 km Percent 10 km All All beacons 5131 6203 6194 83 97 TABLE 5 Single burst location accuracy data command and control the operation of the ground station and display and analyze the results The antennas are located on the corners of the roof of a building connected by fi ber optic cable to signal processing equipment located in another building two kilometers away Proof of Concept Testing The overall objectives of the POC tests were to demonstrate the effectiveness of the DASS concept and to defi ne its technical and operational characteristics The primary technical objective was to demonstrate the systems ability to detect and locate 406 MHz emergency beacons under various controlled conditions This is the most important measure of the systems ability to perform as expected The specifi c objectives of the DASS POC demonstration were to Confi rm the expected performance of the DASS concept Determine if new or enhanced requirements needed to be established Defi ne preliminary performance levels that will be used to establish the scope and content of the next phase of development referred to as the demonstration and evaluation phase Therefore during POC testing performance measurements were taken for the probability of detection probability of location and location accuracy defi ned as follows Probability of detection is the probability of detecting the transmission of a 406 MHz beacon and recovering a valid beacon message from any available satellite Probability of location is the probability of obtaining a location solution within a given time after beacon activation independently of any encoded position data in the 406 MHz beacon message Location accuracy is the distance from the location solution obtained within 5 minutes after beacon activation to the actual beacon location The required performance is specifi ed as the probability that a given solution is within a given distance of the actual location It is important to note that the predicted performance of DASS assumes a full constellation of DASS equipped GPS satellites In fact one of the key strengths of DASS is the promise of DASS transponders on each satellite in the GPS constellation When a full constellation is equipped with DASS transponders there will typically be between seven and 13 GPS satellites visible at the NASA ground station Thus it will be possible to schedule the ground station an GPS World January 2011 www gpsworld com 76