A Real-Time Architecture for Kinematic GPS Applied to the Integrity Beacon Landing System

David Lawrence, Boris Pervan, Clark Cohen, H. Stewart Cobb, J. David Powell, and Bradford Parkinson

Abstract:	Recent flight tests of the Integrity Beacon Landing System (IBLS) have demonstrated the feasibility of using GPS for Category III precision landing. To achieve these results, an airborne architecture that provided position solutions in real-time was developed. Centimeter-level positioning accuracy was achieved using a single- frequency receiver without using integer search techniques. This capability distinguishes the Integrity Beacon Landing System from other proposed kinematic GPS landing systems. At the heart of the real-time architecture is a cycle ambiguity estimator. This estimator makes use of all available information to arrive at floating estimates of the integer biases associated with the GPS carrier phase measurements. The uncertainty in these estimates is stored in a covariance matrix. The estimates are updated in several ways: 1) After each carrier phase measurement epoch, the satellite phase measurements are transformed to a reduced measurement set that is only a function of the integers. The position and clock error terms are eliminated from the measurement, thereby partitioning the estimation of the constant integers from the estimation of the changing position. 2) Code DGPS measurements are incorporated into the estimates, achieving an effect similar to carrier- smoothed-code. 3) Phase measurements from the Integrity Beacons (low- power pseudolite transmitters placed under the approach path) provide a high-accuracy, high integrity update to the estimator. New satellites are added to the estimate and lost satellites are removed from the estimate’ with ease. Given redundant satellites, the estimator will converge toward the cycle ambiguities using satellite motion. With 7 satellites, the integer estimates typically converge to the cycle level in 15 minutes. During the pseudolite overpass, the estimates converge to the centimeter level in a matter of seconds. Receiver Autonomous Integrity Monitoring (RAIM) is performed during the pseudolite overpass to verify the consistency of the satellite and pseudolite measurements. Additionally, in all phases of flight RAIM is performed before each integer update to verify that the update is consistent with the existing integer estimates. Despite the flexibility of this architecture, it is straightforward to implement. The details of this implementation are presented.
Published in:	Proceedings of the 51st Annual Meeting of The Institute of Navigation (1995) June 5 - 7, 1995 Antlers Doubletree Hotel Colorado Springs, CO
Pages:	271 - 280
Cite this article:	Lawrence, David, Pervan, Boris, Cohen, Clark, Cobb, H. Stewart, Powell, J. David, Parkinson, Bradford, "A Real-Time Architecture for Kinematic GPS Applied to the Integrity Beacon Landing System," Proceedings of the 51st Annual Meeting of The Institute of Navigation (1995), Colorado Springs, CO, June 1995, pp. 271-280.
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