Attaining Continuous Centimeter-Level Positioning on the Freeway in Combination with Error Detection

James W. Sinko and Joseph M. Strus

Abstract:	The development of reliable RTK GPS on most highways will enable many safety and convenience applications. An obvious application is vehicle 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. Real- time centimeter-level accuracy GPS for highway and motor racing applications requires specialized algorithms. Some of the difficulties encountered on the freeway are the frequent obscurations that cause the receiver to lose track of the signal, severe multipath caused by nearby large trucks, and phase delays caused by trees and other obstacles alongside the roadway (phase delay measurements from trees are included). This causes a loss of the L2 carrier phase for several seconds and even after track is reestablished the accuracy is degraded for several seconds. A second difficulty in obtaining centimeter-level positioning using GPS is that the RTK solution occasionally is based on the wrong integers and gives a solution that may be off by up to several meters. Our approach to the RTK GPS reliability problem has been to use two antennas and two GPS receivers on the vehicle along with an inertial measurement unit (IMU). A single epoch RTK solution (relative to the base station) is generated for each of the receivers. Then the solutions are checked to see that the distance between the two antennas is close to the measured distance. Otherwise, both solutions can be rejected, and the calculated inertial position used. Under typical conditions, incorrect solutions on the freeway were reduced from 2.1 percent to 0.07 percent using this method with a single epoch RTK algorithm. This is because RTK solution errors are caused by local multipath and phase delays that do not affect each antenna in the same manner. Vehicular applications can use altitude aiding to help significantly in resolving the ambiguities. Code differential GPS and/or inertial calculations give the location on the road with sufficient accuracy that an accurate height can be found from a detailed road database and used as a constraint in finding the solution. In addition to contributing to error detection, the IMU was used to maintain the continuity of the solution. To combine the RTK position and velocity with the inertial data, a 15 state extended Kalman filter is used. Since there are two antennas, azimuth initialization for the Kalman filter is from the two-dimensional GPS attitude system. Over a typical section of suburban freeway with overpasses and side obscurations, the combined system had a maximum horizontal error of 0.87 meter after a 29-second outage.
Published in:	Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002) September 24 - 27, 2002 Oregon Convention Center Portland, OR
Pages:	981 - 987
Cite this article:	Sinko, James W., Strus, Joseph M., "Attaining Continuous Centimeter-Level Positioning on the Freeway in Combination with Error Detection," Proceedings of the 15th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 2002), Portland, OR, September 2002, pp. 981-987.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In