The U.S. Air Force is investigating the cause and effects of signal distortions observed from the GPS IIR-20(M) spacecraft that was launched on March 24, 2009.
Routine early orbit checkout procedures determined that GPS IIR-20(M) signals were inconsistent with the performance of other GPS IIR-M satellites. The signal distortion was initially observed as an elevation-dependent bias in ranging measurements from GPS monitor stations. Worldwide GPS users are not affected as this satellite is still in early orbit checkout and has not been introduced into the operational constellation.
A dedicated response team of U.S.A.F. and contractor experts are wrapping up their investigation of the cause. An extensive series of on-orbit signal quality measurements and ground tests clearly indicate that GPS IIR-20(M)’s observed behavior is related to the interface for the L5 demonstration payload. The demonstration payload, which is powered on and operating exactly as expected, successfully brought into use the L5 International Telecommunication Union frequency filing, ensuring the frequency allocation will be available to support future L5 users. The Air Force team has identified several parameters in the GPS IIR-20 (M)’s navigation message that can be adjusted to correct the bias and bring the satellite into compliance with the GPS Performance Standards. Over the next several months, the team will be testing, analyzing, and evaluating any potential impacts the corrective actions may have on military and civil GPS user equipment. The test strategy implements engineering analysis, modeling and simulation, and testing of real-life GPS receiver equipment to the greatest extent possible to ensure that there will be no inadvertent impacts to GPS users.
The Air Force expects to complete testing by the October 2009 timeframe. GPS IIR-20(M) will be introduced as a healthy satellite into the operational constellation at that time if no major issues are observed during the receiver testing campaign. Users will be kept informed of the status of GPS IIR-20(M) via standard Notice Advisory to Navstar Users (NANU) messages. The GPS constellation remains healthy, stable, and robust with 30 operational satellites. The Air Force has high confidence there is no related concern with the remaining IIR vehicles or the upcoming GPS IIR-21(M) mission. The Air Force is executing a nominal processing flow for an August 2009 GPS IIR-21(M) launch opportunity.
