Pseudolite Augmentation for OTF Ambiguity Resolution in Shipborne Mode

Abstract

The use of a shore-based pseudolite to augment DGPS for on-the-fly (OTF) carrier phase ambiguity resolution in the marine environment is investigated. OTF ambiguity resolution results in cm-level tridimensional accuracies. This high level of accuracy is required for applications such as depth determination in shallow navigation channels, dredging, and harbor maneuvers. Pseudolite augmentation results in improved satellite geometry and more effective and reliable ambiguity resolution. The use of a pseudolite can be crucial to resolving the ambiguities in areas where signal masking occurs due to topographic or man-made features. A shipborne test was conducted on Lake Okanagan, British Columbia, in November 1996, to validate these claims and assess pseudolite performance under field test conditions. A Stanford Telecom L1 pseudolite (Model 7201) was used, together with two NovAtel OEM-2 (RT-20) L1 12-channel Narrow Correlator receivers, which can receive the STel pseudolite signals without any modifications. The distance from ship to shore, where the reference receiver and pseudolite were located, reached 3,250 m. Between five and eight satellites were available during two tests. When using the full GPS constellation and with a masking angle of 10°, it was found that pseudolite augmentation reduced the average time to ambiguity resolution by 35 to 40, while improving the percentage of time when the ambiguities were solved correctly in the initial attempt from 95 to practically 100. Weak geometry was simulated by removing a satellite from the data processing. Under such a condition, pseudolite augmentation reduced the average time to ambiguity resolution by more than 50 and resulted in a significant improvement in ambiguity resolution reliability.