Failure Detection for a Pseudolite-Based Reference System

The 746th Test Squadron uses a flight reference system referred to as the Central Inertial and GPS Test Facility (CIGTF) Reference System (CRS). Currently the CRS is the modern standard flight reference system for navigation testing, but high accuracy is dependent on the availability of GPS. A pseudolite system is currently being developed to augment the CRS and supply the capability to maintain high accuracy navigation under normal and GPS-denied conditions. Pseudolite carrier phase measurements typically contain cycle slips and other errors (such as multipath, tropospheric error, measurement noise) that can affect reliability. Past work relied on the receiver-reported signal-to-noise (SNR) value to determine whether or not a cycle slip occurred in the phase measurements. However it has been shown that even when the SNR is relatively high, a cycle clip can occur. To reduce the error in the pseudolite measurements, the pseudolite system was tightly integrated with an inertial navigation system (INS). The integrated system detects failures through the residuals using a likelihood function. Integrating the inertial sensor provides a means for a filter to maintain the reliability of the pseudolite data which, in turn, increases the integrity of the resulting navigation solution. The navigation system consisted of a pseudolite network and an inertial sensor integrated through an error state extended Kalman filter. The update measurements of the pseudolite system are in the form of single difference carrier phase observables. Two different inertial sensors, a Honeywell HG1700 and a Microbotics MIDG II, are used in the navigation filter to analyze the impact of inertial quality in determining failures. A field experiment was conducted at the Advanced Navigation Technology (ANT) Center, at the Air Force Institute of Technology, using six pseudolites and a ground vehicle equipped with a pseudolite receiver, and both a commercial-grade and tactical-grade inertial systems. The field experiment was combined with simulations where the inertial data was combined with both real and simulated carrier phase measurements to evaluate the performance of both failure detection algorithms. Results from the field test and simulated experiments have shown cycle slips in the carrier phase measurements were detected and corrected using both commercial-grade and tactical-grade INS, but that performance, in terms of probability of detection and time to detect, was improved with the higher quality inertial data. Also the results have shown the most effective way to detect and remove cycle slips and slow growing errors would be to use an adaptive model approach that uses the residual monitoring method to initially detect a failure and the moving window algorithm to confirm there was a failure.