Long-Distance Kinematic GPS

The subject of this chapter is very precise navigation using the Global Positioning System (GPS) of navigation satellites, over distances of up to several thousand kilometres from the nearest reference receiver, particularly when the GPS data is post-processed. As the distances among receivers increase, sources of error that have almost the same effects when the receivers are a short distance apart, and cancel out in differential and interferometric GPS positioning (e.g., using single, double, or triple-differenced data), become different enough not to cancel out any longer. These errors now must be estimated and corrected out, or filtered, to determine the vehicle trajectory accurately. Because it is a more complex problem, the reliability and accuracy of long-range navigation, at this early stage of development, are not as good as for similar applications of GPS navigation over distances of a few tens of kilometres. One source of error that cannot be differenced away over long baselines is ionospheric refraction, limiting the ability to deal with carrier phase ambiguities even when using expensive dual-frequency receivers. Such problems aside, the experience obtained so far shows that decimetre-level positioning can be a practical proposition with many important uses, for distances of more than 1000 km. (For 2–3 metre-level, real-time navigation with pseudo-range, the closely related idea of Wide Area Differential GPS Augmentation concept, is being tested on a continental scale for future use in civil and commercial aviation.)