GPS receiver relative calibration campaign preparation for Galileo In-Orbit Validation

Abstract

Among other tasks in the frame of the Galileo In-Orbit Validation (IOV) phase, the French National Metrology Laboratory (NML) LNE-SYRTE is engaged in the relative calibration of Global Positioning System (GPS) receiver delays: the GPS Common-Views (CV) using ionosphere free P3 technique is the time transfer backup mean for Two-Way satellite Time and Frequency Transfer (TWSTFT). The relative delay characterization of GPS equipment is scheduled today to be achieved by transport of a traveling equipment between Observatoire de Paris (OP) and another European NML, one Galileo Precise Timing Facility (PTF) and the United States Naval Observatory (USNO) successively. The goal here is to perform separate calibration of the P-Code delays as collected on either LI (PI) or L2 (P2) GPS carriers, by applying an original processing developed by LNE-SYRTE and derived from the standard TAIP3 processing. We describe the process, the techniques involved, the traveling equipment, the data processing and the uncertainty budget computation, according to former results obtained in the frame of the Galileo Time Service Provider (GTSP) Prototype development. Two possible applications of such a GPS receiver relative calibration campaign might be first an initial synchronization of Galileo System Time (GST) to UTC via UTC(OP), and second an initial GPS CV connection between GST and UTC(USNO), the UTC prediction of United States Naval Observatory, as a first step for the future computation of Galileo GPS Time Offset (GGTO). Therefore, we also propose here to study some limits of two links: first over a European baseline link, namely between OP and the Physikalisch Technische Bundesanstalt (PTB) in Germany; and second between OP and USNO, a long baseline compared to European links. We use the GPS TAIP3 files without any additional products to build Common-Views (CV), and we assess the quality of the results with respect to TWSTFT, and with respect to the computations of UTC - UTC(k) released monthly by the Bureau International des Poids et Mesures (BIPM) in its Circular T. Provided the relative calibration of GPS receiver delays would be successful, we assess both potential initial synchronizations of GST to be possible within an expanded uncertainty below 10 ns (k = 2) over an averaging period of two days or more.