GPS Tensor An Attitude and Orbit Determination System for Space

GPS Receiver requirements for spacecraft operation are significantly more severe than for terrestrial or aeronautical operation. It takes several hours for a GPS SV to pass over an Earth-fixed location. In that time a low altitude orbiting satellite traveling at about 7 km/second can orbit the Earth multiple times. The development of the GPS Tensor receiver has had to address acquisition and tracking under these orbiting conditions along with the transitions between satellites available in the GPS SV constellation. Of note are issues associated with navigation, acquisition and tracking. Cold Start (no external aiding) conditions requires significantly different logic and time than for Warm Start (full external aiding) conditions that use ephemeris aiding. The paper discusses a software simulation of the Tensor acquisition that has been effective for analysis of the system operation in space and has resulted in improvements for the acquisition and tracking of SV signals. Two navigation solutions are generated in the GPS Tensor software. The Standard Position Service (SPS) solution requires the presence of four acceptable SV signals and is subject to position calculation outages. The alternate navigation solution uses the navigation filter algoritinm. The navigation filter incorporates up to nine pseudorange measurements (all receiver channels) but can propagate the position and clock bias solutions even with short term absences of SV signals. The interaction of these solutions in conditioning the pseudorange input data is discussed with emphasis on the melding of the integer millisecond roil-over from the Code Delay Lock Loop (DLL) and the millisecond resets and Pulse Per Second (PPS) output generated from the clock bias part of the navigation solution. The PPS output is more robust with the filter solution rather than the SPS solution which is more susceptible to outages. Another issue encountered during the development of the Tensor is the need for accurate estimates of the electrical delay properties of the system referred to as line-biases. Previous works (References 1, 2, and 3) have presented the solution of these line-biases for stationary platform which is satisfactory if such a test is possible and the line-biases do not change over time. Several situations have occurred where this test was not possible or the line-biases were expected to change significantly over time due to changing thermal conditions. The concurrent generation of attitude using differential phase measurements from an antenna array while performing phase calibration is also discussed.