Resolving Carrier-Phase Ambiguities On The Fly, At More Than 100 km From Nearest Reference Site, With The Help Of Ionospheric Tomography

Abstract

In this paper we address the question of how to resolve GPS carrier phase ambiguities precisely and quickly, when the rover is more than 100 kilometers from the nearest reference site, in order to obtain sub-decimeter (r.m.s.) position in real-time (and, by implication, in postprocessing). To do this reliably, ionospheric refraction has to be corrected very accurately. For distances of up to a few hundred kilometers, dual-frequency GPS data from permanent control stations may be used to obtain the necessary ionospheric refraction information. The ionosphere over the area served by the stations has to be mapped using their carrier phase data, also in real-time, by computed ionospheric tomography. The resulting map is used to produce information that is transmitted to the user, along with range and time corrections. The user can then calculate very precise corrections for ionospheric refraction at the location of the roving receiver. After a successful preliminary test using 1997 data from the core control stations of the CATNET network in Catalunya, (Catalonia) Spain, a second test involving those, as well as a road vehicle, was conducted in March of 1999, at a distance of 116 km from the nearest reference station. The data were processed after the test, but simulating a realtime analysis. As reported here, repeated attempts at resolving all the L1 and L2 ambiguities proved successful over a total period of two hours.