Measurement-level integration of carrier-phase GPS and laser-scanner for outdoor ground vehicle navigation

This paper introduces a navigation system based on combined global positioning system (GPS) and laser-scanner measurements for outdoor ground vehicles. Using carrier-phase differential GPS, centimeter-level positioning is achievable when cycle ambiguities are resolved. However, GPS signals are easily attenuated or blocked, so their use is generally restricted to open-sky areas. In response, in this work we augment GPS with twodimensional laser-scanner measurements. The latter is available when GPS is not and further enables obstacle detection. The two sensors are integrated in the range domain for optimal navigation performance. Nonlinear laser observations and time-correlated code and carrier-phase GPS signals are processed in a unified and compact measurement-differencing extended Kalman filter. The resulting algorithm performs realtime carrier-phase cycle ambiguity estimation and provides absolute vehicle positioning throughout GPS outages, without a priori knowledge of the surrounding landmark locations. Covariance analysis, Monte Carlo simulations, and experimental testing in the streets of Chicago demonstrate that the performance of the range-domain integrated system far exceeds that of a simpler position-domain implementation, in that it not only achieves meter-level precision over extended GPS-obstructed areas, but also improves the robustness of laser-based simultaneous localization and mapping.