GPS Receiver Performance Using Inertial-Aided Carrier Tracking Loop

Radio frequency interference (RFI) has been a perplexing problem, affecting the navigation quality of the Global Positioning System (GPS). The presence of the RFI, or even hostile jamming, will reduce the effective received signal power, and thus degrade navigation accuracy, continuity, and integrity of the system. A proposed next generation aircraft navigation system for the U.S. military called the Joint Precision Approach and Landing System (JPALS) [1, 2] is an example of a system that requires high performance even in severe RFI environments. Previous research has proposed using inertialaided GPS carrier-tracking loops as a component of the anti-jam solutions [3, 6]. It is believed that an inertialaided GPS receiver is more robust to wide-band RFI. By eliminating the need to track platform dynamics, the inertially-aided GPS tracking loops can operate at narrow noise bandwidths, thus lowering the tracking thresholds to a lower carrier-to-noise (C/N0). As a result, the lower C/N0 thresholds increase robustness to the RFI. The purpose of this paper is to verify the performance of an inertial-aided GPS receiver experimentally, since the performance has not been fully validated. A metric to evaluate the inertial-aided GPS receiver is, in wide-band RFI environments, the tolerable degradation of carrier-to-noise ratio (C/N0) and its corresponding loop bandwidth. A unique and flexible inertial-aided GPS test-bed has been developed to conduct experiments to include various combinations of inertial measurement units (IMUs) with GPS receiver clocks. In addition to different choices of IMU, the GPS carrier-tracking loop can be implemented and modified using different orders of tracking-loop filters and various noise bandwidths. Moreover, the level of C/N0 is tunable by injecting various power levels of white Gaussian noise into the same collected GPS front end signals. Future work will consider details of clock dynamics in inertial-aiding techniques. As shown in this study, the receiver clock phase error induced by the vibrating platform significantly limits the performance of the integrated GPS/INS navigation system. An accurate measurement of the power spectral density (PSD) of the receiver clock vibration and a precise estimate of the acceleration sensitivity vector are two building blocks for advancing this inertial-aiding technique. A better improvement of C/N0 is expected if a higher quality clock, such as an atomic clock, were used to drive the GPS receiver. In the low noise bandwidth range, the clock dynamics dominate the GPS receiver tracking performance.