GPS World, August 2009

EXPERT ADVICE The SVN49 Pseudorange Error By Richard Langley T he seventh modernized GPS Block IIR satellite was launched on March 24 2009 Called SVN49 its sequence number in the long line of GPS satellites or PRN01 after its pseudorandom noise code identifier this satellite is special In addition to the equipment required to transmit the legacy GPS C A code and P Y code signals and the new civil L2Ccode and military M code signals on the standard L1 157542 MHz and L2 Not expected was the effect of the L5 filter and its associated cable run on the L1 and L2 signals 12276 MHz frequencies SVN49 carries an L5 demonstration payload L5 is the new civil signal to be transmitted on 117645 MHz by Block IIF and succeeding generations of GPS satellites The demo payload was included to claim the frequency which was allocated by the International Telecommunication Union before the August 26 2009 deadline The deadline had been imposed seven years earlier when the GPS Joint Program Office the forerunner of the GPS Wing applied for the frequency The Block IIF program schedule had slipped a bit and as a safeguard and one which eventually saved the day the demo payload was developed and assigned to SVN49 Shortly after the L1 L2 system on SVN49 was activated on March 28 it became clear that the satellite had a small Å FIGURE 1 Ionospheric refraction corrected SVN49 pseudorange residuals from data collected at 2 SOPS monitor stations courtesy GPS Wing problem The pseudorange data obtained by U S Air Force Space Commands 2nd Space Operations Squadron 2 SOPS monitor stations had larger than normal errors Typically the errors have a random characteristic with a mean of zero and a peak to peak variation of two meters or so But the SVN49 ionospherecorrected errors reached a level of about four meters and when they were plotted against the elevation angle of the satellite as viewed at each monitor station a clear trend emerged see FIGURE 1 Although larger than normal the errors still fell within the accuracy tolerances specified for GPS signals Nevertheless the anomalous behavior of SVN49 s signals was a cause of concern and the GPS Wing and its contractors mounted efforts to find the cause Payload Source They traced the source of the problem to the manner in which the L5 demo payload was added to the satellite To understand the problem we need to examine how the L1 and L2 signals are transmitted by a GPS satellite A primary and defining characteristic of GPS signals is that the received signal power should be approximately the same at any location on the Earths surface within view of the satellite In other words we should receive about the same signal power when a GPS satellite is overhead and closer to us as when it is low on the horizon and further away Any major variation in signal level seen by a receiver is typically due to the gain pattern of the receivers antenna To achieve a uniform power density at the Earths surface a GPS satellite uses an array of 12 helical antenna elements with an inner ring of four elements and an outer ring of eight fed by an antenna coupler network see FIGURE 2 The L1 and L2 signals are fed into the coupler through one of its two input ports port J1 The inner ring of elements transmits most of Å FIGURE 2 L band antenna element locations courtesy GPS Wing GPS World August 2009 www gpsworld com 8