Single-Epoch Ambiguity Resolution for Highway and Racetrack Applications

James W. Sinko

Abstract:	The development of reliable real-time kinematic (RTK) GPS on most highways will enable many safety and convenience applications, such as lane departure warning. Eventually, GPS could play an important role in fully automated highways. On the racetrack, RTK GPS can show car handling characteristics and driver performance to high precision. Real-time centimeter-level accuracy GPS for highway and motor racing applications requires specialized algorithms and L1/L2 receivers. The greatest difficulty encountered with this system is the frequent obscurations that cause the receiver to lose track of the signal. This results in a loss of the L2 carrier phase for many seconds (test results for a few popular brands of receivers are included), and even after track is reestablished, the accuracy is degraded for a couple of seconds. The term single-epoch ambiguity resolution is somewhat of a misnomer in that GPS receivers do have smoothing filters that introduce a significant correlation between successive epochs when sampling rates are 1 Hz or higher, and multipath errors are highly correlated in time. In this paper, autocorrelation functions for the residuals after correct ambiguity resolution are shown. The singleepoch method eliminates the prolonged carrying of incorrect integer solutions that occasionally occurs with Kalman filter and other multiple-epoch algorithms. After encountering an obstacle, the single-epoch method provides a solution as soon as the receiver recovers an adequate number of signals, although the probability of selecting the correct integers will improve after the receiver has had a few seconds of settling time. Over short baselines (a few kilometers) and with six or more satellites available, ambiguities are resolved correctly over 99% of the time. In clear areas, such as the Buttonwillow, California, racetrack, correct single-epoch ambiguity resolution has been demonstrated over 99.8% of the time. Vehicular applications can use altitude aiding to help significantly in resolving the ambiguities. Code differential GPS gives the location on the road with sufficient accuracy so that an accurate height can be found from a detailed road database and used as a constraint in finding the solution. The algorithm has been tested in a series of rooftop static tests during which the antennas were covered for short periods of time. On a relatively clear stretch of freeway with only one overpass, correct ambiguity resolution was demonstrated 88% of the time without altitude aiding and 96% of the time with altitude aiding. Over a more obscured stretch of freeway with trees and hills, ambiguities were correctly resolved 62% of the time without altitude aiding and 91% of the time with altitude aiding.
Published in:	Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001) September 11 - 14, 2001 Salt Palace Convention Center Salt Lake City, UT
Pages:	1575 - 1583
Cite this article:	Sinko, James W., "Single-Epoch Ambiguity Resolution for Highway and Racetrack Applications," Proceedings of the 14th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2001), Salt Lake City, UT, September 2001, pp. 1575-1583.
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