GPS World, January 2011
tennas to receive data from the best satellites in terms of geometry signal strength processing capability and other factors However at the time of the POC testing there were only eight GPS satellites equipped with DASS transponders A maximum of three DASSequipped GPS satellites were visible at the same time at the NASA ground station above a 15 degree elevation angle and there were times when only one DASS equipped GPS satellite was visible Thus it was impossible to optimize satellite selection since there was never an opportunity to select from an excess of satellites that a full constellation would provide In particular satellite geometry and its effect on performance is never as optimal as what would be obtained from a full constellation of GPS satellites To predict the results of a full constellation using the results from a severely reduced constellation a calculation based on dilution of precision was used Dilution of precision DOP or geometric dilution of precision to be specifi c is used to describe the geometric strength of satellite confi guration on GPS accuracy When visible satellites are close together in the sky the geometry is said to be weak and the DOP value is high when far apart the geometry is strong and the DOP value is low Thus a low DOP value gives rise to a better GPS positional accuracy due to the wider angular separation between the satellites used to calculate a beacons position Location accuracy results can be scaled to refl ect the true DOP that would be obtained by a satellite constellation of 24 GPS satellites The DOP error caused by uncertainty in time and frequency measurements is used for scaling GNSS Design Test INNOVATION Distance error All beacons 5 km 83 6 km 88 7 km 91 8 km 94 9 km 95 10 km 97 TABLE 6 Single burst location accuracy by distance error The DOP of the satellites actually used to calculate a location solution denoted by ftDOP ACT is always bigger than the DOP that would have been available from a constellation of 24 GPS satellites ftDOP 24 The raw location errors need to be multiplied by the ratio ftDOP 24 ftDOP ACT to refl ect the results that would have been obtained if all 24 satellites were present The raw average location error KHUH QDYLJDWLRQ PHHWV 7R UHFHLYH HDUO UDWH UHJLVWHU XQWLO DQ WKH ZRUOG LQIR PXQLFK VDWHOOLWH QDYLJDWLRQ VXPPLW RUJ 3KRWR 0LVV 4XHOOH 3KRWRFDVH ZZZ PXQLFK VDWHOOLWH QDYLJDWLRQ VXPPLW RUJ www gpsworld com January 2011 GPS World 77
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