| This thesis describes the theory and implementation of a high-performance navigation system which combines GPS with Low Earth Orbit Satellites (LEOS). Our objective is to rapidly acquire centimeter-level position, without placing constraints on the motion of the navigated platform. When tracking the carrier phase of satellite downlinks, the primary obstacle to accurate positioning is the resolution of cycle ambiguities, which arise since phase can only be measured modulus 2π. The rapid change in the line-of-sight vectors from the receiver to the LEO signal sources, due to the orbital motion of the LEOS, enables the robust resolution of these cycle ambiguities on the GPS signals as well as parameters related to the cycle ambiguities on the LEO signals. These parameters, once identified, enable real-time centimeter-level differential positioning of a user receiver several miles from a reference station. The technique requires no specialized navigation electronics, such as atomic oscillators, on-board the LEO satellites. As such, it can accommodate instabilities in the crystal oscillators on the satellites, multiple beam configurations of satellite links, bent-pipe communication architectures, and TDMA downlinks. A set of techniques have been developed for achieving centimeter-level navigation using pre-existent LEO transceiver hardware, designed for low cost data communication. The issues addressed include synchronization of the LEO and GPS hardware for errors below the centimeter level, precise time-tagging of carrier-phase measurements without the use of hardware accumulators and latches, an efficient data reduction algorithm for resolving cycle ambiguities while accommodating different frequency-dependent phase-lags in the receiver front ends, and the ability to scale the number of LEOS tracked, and the number of navigation antennas used, in software. By a combination of experimentation and simulation, we describe the system performance in terms of accuracy and integrity of navigation solutions. We also describe potential advantages provided by LEO satellites for meter-level navigation in high-loss environments, or multipath-prone environments. |
