Clark E. Cohen

Parity space methods for autonomous fault detection and exclusion using GPS carrier phase

Kinematic carrier phase positioning provides navigation integrity. The sub-centimeter precision of carrier phase measurements can be used to leverage receiver autonomous integrity monitoring (RAIM) in the sense that extremely tight fault detection thresholds can be set on the least-squares residual (ensuring navigation integrity) without incurring high false alarm rates. In addition, the high precision of carrier phase, when compared with code phase, lowers the integrity risk associated with the fault identification process. This is true because carrier phase provides a much cleaner observation of the effect of a given failure on the residual. Thus, for the same improvement in navigation continuity (obtained from fault isolation), misidentification will be less likely. This paper is focused on the use of parity space methods to investigate the limits of high-integrity and high-continuity GPS performance. In this regard, prototype algorithms for receiver autonomous fault detection and exclusion were developed with the goal of maximizing navigation continuity subject to the constraint of maintaining high integrity (by repressing mis-identifications). Fault detection and exclusion performance was demonstrated through analysis and extensive simulation. In addition, the prototype algorithms were implemented in a real-time airborne kinematic positioning architecture and tested by deliberately inducing failures in the post-processing of raw flight data.

A system using LEO telecommunication satellites for rapid acquisition of integer cycle ambiguities

This paper addresses the design of a cm-level carrier-phase navigation system which employs Low Earth Orbit (LEO) satellites for rapid resolution of integer-cycle ambiguities. In the short term, the system aims to combine the Navstar GPS fleet with satellites of the Globalstar Telecommunications constellation. We describe how cm-level carrier-phase positioning can be achieved using signals that were not designed for navigation. Our objective is to perform precision navigation without requiring alteration of the communication payload onboard the satellites. The technique accommodates the beam configuration of the satellite downlink, the bent-pipe architecture of the communication payload, the instabilities of the satellite oscillators and the frequency-dependent phase lags in the user and reference receiver front ends. The object of this paper is to characterize those components of the system necessary to achieve high-integrity high-precision performance objectives.

Application of GPS Attitude Determination to Gravity Gradient Stabilized Spacecraft

Recent advances in the Global Positioning System (GPS) technology have initiated a new era in aerospace navigation and control. GPS receivers have become increasingly compact and affordable, and new developments have made attitude determination using subcentimeter positioning among two or more antennas feasible for real-time applications. GPS-based attitude control systems will become highly portable packages which provide time, navigation, and attitude information of sufficient accuracy for many aerospace needs. A typical spacecraft application of GPS attitude determination is a gravity gradient stabilized satellite in low Earth orbit that employs a GPS receiver and four body mounted patch antennas. The coupled, linearized equations of motion enable complete position and attitude information to be extracted from only two antennas. A discussion of the various error sources for spaceborne GPS attitude measurement systems is included. Attitude determination of better than 0.3 degrees is possible for 1 meter antenna separation. Suggestions are provided to improve the accuracy of the attitude solution.

Autolanding a 737 using GPS and Integrity Beacons

Differential GPS and miniature, low-cost Integrity Beacon pseudolites were used to carry out 110 successful automatic landings of a United Boeing 737-300 aircraft. The goal was to demonstrate Required Navigation Performance including accuracy and integrity-for Category I11 Precision Landing using GPS. These autopilot-in-the-loop flighe tests using GPS Integrity Beacons (low-power, ground-based marker beacon pseudolites placed under the approach path) furnish evidence that GPS can provide the full performance necessary to meet the stringent specifications of Category 111. It has been demonstrated that Integrity Beacons can provide consistent accuracies on the order of a few centimeters. But perhaps even more important, this centimeter-level accuracy coupled with the built-in geometrical redundancy provided by Integrity Beacon ranging provides an exceptional level of intrinsic system integrity. This integrity is calculated to be easily better than the required one part in a billion probability of missed detection. Passenger safety is improved significantly because this level of integrity is achieved independently from ground-based monitors through Receiver Autonomous Integrity Monitoring (RAIM). For the flight tests, the GPS receiver and single-channel navigation computer calculated precise positions and calculated glide path deviations. An analog interface provided ILS localizer and glideslope signals to the autopilot. The 737 was equipped with a dual-channel flight control system which was previously certified for Category IIIA landings. The autolands were performed through touchdown without rollout guidance, The series of 110 automatic landings were carried out at NASAs Crows Landing facility in California over a four-day period during the week of October 10, 1994. A laser tracker was used as an independent means for characterizing flight performance. The feasibility demonstration was sponsored by the FAA.

Integration of wide area DGPS with local area kinematic DGPS

The Stanford University wide area DGPS network has provided a test bed for the development and evaluation of wide area augmentation system (WAAS) algorithms. Until recently, the accuracy performance of these algorithms was assessed only for static users and users on the ground. The only truth models available relied on the user being at a known location or on a surveyed runway. To remedy this situation, the WAAS system was integrated with the integrity beacon landing system (IBLS). IBLS is a local area kinematic DGPS system capable of providing centimeter-level positioning accuracy. The accurate trajectories provided by IBLS are used to assess WAAS performance in an airborne environment. The integration was achieved by porting both the IBLS user software and the WAAS user software to a real-time multi-processing operating system. Both systems now run simultaneously as separate processes on a single computer. The processes can communicate with each other for real-time comparison. They also store data to allow more detailed evaluation in post-processing. Results of flight tests of the Stanford WAAS network are presented.

Estimation of gyroscope polhode motion using trapped magnetic flux

A freely spinning body undergoes the well-known polhode motion of its spin axis. Applied to the cryogenic, electrostatically suspended spherical gyroscopes of the Gravity Probe B program, an unusual analysis technique is presented that allows reconstruction of the polhode motion, relying on the trapped magnetic flux in the superconducting rotor. Cooled to a temperature below 9 K, the niobium rotor coating becomes a superconductor, trapping the ambient magnetic field. Fixed with respect to the rotor body frame, this trapped magnetic flux is used as a reference against which to track the path of the rotor spin axis. The field is modeled as a series expansion in spherical harmonics. Consideration of any higher-order moment of the magnetic field along with the dipole moment is sufficient to completely define the spin axis coordinates in the body frame.