Four Carrier-Phase Ambiguity Resolution (FCAR) Algorithm. An Alternative for Galileo OSPF Pre-processing

Miguel Belló-Mora, Antonio Fernández, José Diez, Juan F. Martín-Albo and Mariano Sánchez

Abstract:	Within the frame of the projects and studies developed by DEIMOS Space S.L. for Galileo C0 phase and the preparation of proposals for C/D phase, added-value algorithms for Ground Mission Segment (GMS) elements have been developed which would improve the accuracy performance of the Orbit Synchronisation Processing Facility (OSPF). In particular, this paper presents the ambiguity resolution algorithm developed for OSPF preprocessing. Between the two different observables to be used for Orbit Determination and Time Synchronisation (OD&TS) purposes in Galileo, it is carrier-phase measurement the one with the lowest noise level (<1cm), which would suggest to use of this observable as baseline for OD&TS purposes. However, the ambiguity in the number of cycles to extract the distance measurement must be resolved. Actually, this ambiguity must be resolved for each pair of satellite & ground station and each pass. In order to tackle this problem a Four-Frequency Carrier-Phase Ambiguity Resolution (FCAR) algorithm has been especially developed for this specific case, which would provide a single observable using the complete set of available information coming from both observables, i.e. carrierphase (CP) & pseudorange (PR), and the four available frequencies for Galileo. Therefore, the combined use of all the available information would provide the most accurate observable possible through the use of a very robust algorithm. In this paper, a description of the algorithm is presented starting from the hypothesis definition and scope of applicability. The use of a Square Root Information Filter (SRIF) as the basis of the algorithm is explained in detail, as well as the criteria used. Especial attention is paid also to the way the algorithm performs, which is illustrated by an example case. This example is obtained using a Raw Data Generator (RDG) prototype also developed within the previous study phases, which provides both satellites and ground stations dynamics and detailed measurement modelling. Using the simulated case as example, the FCAR algorithm resolves the ambiguities and, finally, a single ionospheric-free & ambiguity-free carrier-phase observable is obtained. This algorithm would provide an effective and robust way of introducing the most accurate observable possible to the OD&TS process, instead of using the classical smoothed pseudorange observable, taking profit from the four different frequencies available for Galileo and, therefore, optimising the achieved accuracy of the OD&TS process as a whole. Otherwise, using the sub-optimal (and baseline up-to-now) preprocessing GPS-like strategy based only on combinations of pairs of frequencies, Galileo would not extract all the possible accuracy coming from the four frequencies availability but it would produce OD&TS results with reduced accuracy.
Published in:	Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005) September 13 - 16, 2005 Long Beach Convention Center Long Beach, CA
Pages:	2083 - 2091
Cite this article:	Belló-Mora, Miguel, Fernández, Antonio, Diez, José, Martín-Albo, Juan F., Sánchez, Mariano, "Four Carrier-Phase Ambiguity Resolution (FCAR) Algorithm. An Alternative for Galileo OSPF Pre-processing," Proceedings of the 18th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS 2005), Long Beach, CA, September 2005, pp. 2083-2091.
Full Paper:	ION Members/Non-Members: 1 Download Credit Sign In