Three or Four Carriers - How Many are Enough?

U. Vollath, K. Sauer, F. Amarillo, J. Pereira

Abstract:	The use of three carriers to perform high precision carrier phase based positioning has been widely accepted now for the modernized GPS as well as for the planned Galileo satellite navigation system. In principle, instantaneous (one-epoch) ambiguity resolution becomes feasible for a broad range of applications. A boost of system availability and reliability is recognized as well. For Galileo, in addition to the open access service (OS) a commercial service (PRS) is planned. Besides other improvements a fourth carrier frequency E6 will be available. It has to be proved that the corresponding fees are justified by the actual Quality of Service. Theoretical analyses (ADOP analyses) on the impact of a fourth frequency on the ambiguity resolution reliability and availability are presented. An increase of 1 to 15 orders of magnitude is predicted for the reliability of four-carrier single-epoch ambiguity resolution with respect to using three carriers only. Based on a contract with the European Space agency (ESA) European Space Research & Technology Center (ESTEC), an extensive hardware simulation of the planned three- and four-carrier options has been performed. The paper provides a detailed description of this experiment. The simulation set-up consists of a modified satellite signal simulator and a prototype receiver capable of tracking more than two carriers. Ambiguity resolution and positioning were computed using the latest state-of-the-art multi-carrier algorithms. The second subject of the experiment assesses the differences between modernized GPS and Galileo in terms of the signal structure. Galileo makes extensive use of Binary Offset Carrier (BOC) instead of Bi-Phase Shift Key (BPSK) modulation. Therefore different noise and multipath characteristics and corresponding performance indicators result. The paper presents performance comparisons for three-carrier GPS, three-carrier Galileo and four-carrier Galileo scenarios. To cover a broad range of applications, different ionospheric and multipath conditions and varying height separations of user and reference station are covered. The user segment comprises rural vehicle, aeronautical en-route and precision approach environments.
Published in:	Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003) September 9 - 12, 2003 Oregon Convention Center Portland, OR
Pages:	1470 - 1477
Cite this article:	Vollath, U., Sauer, K., Amarillo, F., Pereira, J., "Three or Four Carriers - How Many are Enough?," Proceedings of the 16th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS/GNSS 2003), Portland, OR, September 2003, pp. 1470-1477.
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