Carrier phase GPS augmentation using laser scanners and using low earth orbiting satellites

Abstract

Carrier phase measurements from the Global Positioning System (GPS) can potentially provide centimeter-level ranging accuracy for high-performance navigation. Unfortunately, positioning with carrier phase is only robustly achievable in open sky areas, within limited distance of another GPS receiver, and after substantial initialization time to estimate unknown cycle ambiguity biases. In response, in this research, two ranging augmentation systems are investigated to improve the availability of carrier phase positioning. First, GPS is integrated with laser scanners for precision navigation through GPS-obstructed environments. Second, GPS is augmented with carrier phase measurements from low-earth-orbit (LEO) Iridium telecommunication satellites for global high-integrity positioning.

In the first part of this work, carrier phase GPS and laser scanner measurements are combined for ground vehicle navigation in environments, such as forests and urban canyons, where GPS satellite signals can be blocked. Laser observations of nearby trees and buildings are available when GPS signals are not, and these obstacles serve as landmarks for laser-based navigation. Non-linear laser observations are integrated with time-correlated GPS signals in a measurement-differencing extended Kalman filter. The new navigation algorithm performs cycle ambiguity estimation and provides absolute vehicle positioning throughout GPS outages, without prior knowledge of surrounding landmark locations. Covariance analysis, Monte Carlo simulation, and experimental testing in Chicago city streets demonstrate that the integrated system not only achieves sub-meter precision over extended GPS-obstructed areas, but also improves the robustness of laser-based Simultaneous Localization and Mapping (SLAM).

The second augmentation system, named iGPS, combines carrier phase measurements from GPS and LEO Iridium telecommunication satellites. The addition of fast-moving Iridium satellites guarantees both large satellite geometry variations and signal redundancy, which enables rapid cycle ambiguity estimation and fault-detection using Receiver Autonomous Integrity Monitoring (RAIM). In this work, parametric models are defined for iGPS measurement error sources, and a new fixed-interval estimation algorithm is developed. The underlying observability mechanisms are investigated, and fault-free navigation performance is quantified by covariance analysis. In addition, a carrier phase RAIM detection method is introduced and quantitatively evaluated against known fault modes and theoretical worst-case faults. Performance sensitivity analysis explores the potential of iGPS to satisfy aircraft navigation integrity requirements globally.