Results of Galileo AltBOC for precise positioning

Abstract

The deployment of Galileo and the modernization GPS will provide additional signals with increasingly complex modulations and multiplexing schemes, enabling performance enhancements in terms of availability, accuracy, and robustness. With the Galileo Initial Operational Capability (IOC) scheduled around mid-decade with fourteen satellites already ordered in January 2010, 4 additional ordered in 2012 and with GPS L5 capability reaching 24 satellites in 2018, we will most probably witness in the forthcoming years the rise of new opportunities and applications, just as in the past decades when GPS was initially deployed. Galileo's E5 signal, with its Alternative Binary Offset Carrier (AltBOC) modulation, is one of the most advanced and promising signals of the Galileo system. Receivers capable of tracking this signal will benefit from unequalled performance in terms of measurement accuracy, precision, and multipath suppression. Pseudorange observables stemming from Galileo E5 AltBOC modulation have considerably higher accuracy — centimetre-level — when compared to GPS L1 and even modernised L5 pseudoranges, which although not at millimetre-level as in the case of carrier phase observables, can already benefit a wide range of users which may be interested in avoiding the hassles of carrier phase based positioning such as loss-of-lock and reinitialisations, amongst others. The high precision of the Galileo E5 AltBOC range measurements suggests that their modelling can benefit from available research results in precise point positioning (PPP). In contrast to carrier phase observables, pseudoranges are not ambiguous, hence it is expected that the convergence challenges of PPP will disappear or largely be mitigated when using cm-level precise pseudoranges. This would allow performing absolute positioning, both in real-time (using Galileo ultra-rapid orbits hopefully available in the future from IGS) or in postprocessing (similarly, using IGS final precise Galileo orbits), which can be of great use for regions with sparse reference stations, or none at all, and potentially simplify training and procedures in the case of professional surveying. In the frame of ENCORE (Enhanced Code Galileo Receiver) project, co-funded by the 7th Framework Programme under European GNSS Supervisory Authority (GSA) [RD-1] [RD-2], an AltBOC capable Galileo receiver prototype was implemented on a dedicated FPGA board including a dual frequency Galileo E5/E1 front-end as well as the application software and positioning algorithms required to operate the receiver. The prototype is currently being developed and upgraded for operation in urban environments in the frame of RXURB project co-funded by national Portuguese R&D program QREN and also under the project ATENEA+ co-funded by the Spanish R&D program INNPACTO. Positioning results are also presented using generated E5 and E1 signals with and without multipath, with and without a complete Galileo constellation, with and without GPS L1/L5 measurements, static and kinematic mode and broadcast ephemeris. These results are expected to contribute to a better understanding and consolidation of the use of AltBOC for precise positioning which could have an impact in future applications and the next generation of GNSS receivers.