Interchangeable Integration of GPS and Galileo by Using a Common System Clock in PPP

T. Melgard, J. Tegedor, K. de Jong, D. Lapucha, G. Lachapelle

Abstract:	1. Introduction Simultaneous use of two or more GNSS systems may be categorized at three different levels: 1) Compatible, 2) Interoperable and 3) Interchangeable, as for instance described in Pullen (2007) and discussed by Parkinson (2007). Compatible is when two or more GNSS can be used independently or together without interfering with each other. Interoperable is when the combination of two or more GNSS systems enhances the performance for some user applications. GPS-GLONASS applications can for example generally be considered both compatible and interoperable. Interchangeable is the highest level of integration where data from different GNSS constellations are used together in one solution without loss of accuracy or redundancy. Using a common clock is a big step towards interchangeable integration. And therefore the Fugro real-time (Melgard et al 2009) GPS-GLONASS PPP (Precise Point Positioning) solution is very close to being an interchangeable integration (Melgard et al 2011). Full interchangeable integration between GPS and Galileo is easier to achieve since Galileo uses CDMA (Code Division Multiple Access) coding like GPS does, and shares two common frequencies with GPS on L1/E1 and L5/E5a. When introducing Galileo in PPP solutions, common clock mode is very attractive because of the limited number of Galileo satellites in the early stages. And, in addition, it will always improve performance in signal blocking situations. When only a few satellites are in view, typically a mix of GPS and Galileo satellites will be received. The use of common clock mode will in such situations save one satellite, so that only four satellites are needed to calculate a mixed position. Common clock mode will increase both availability and reliability of the position solution. In the case of GLONASS PPP, it was found that the common clock offset to GPS changes over time (Melgard et al 2011). Therefore a procedure to continuously estimate the offset is implemented in the user software. In the case of mixing Galileo and GPS measurements, there are strong indications that the common clock offset will remain constant if correctly handled. Odijk & Teunissen (2012) shows that GPS/Galileo ISB (Inter System Bias) values are very stable over time. While work of Odijk & Teunissen (2012) is oriented towards baseline processing, i.e. RTK, this paper focuses on PPP applications. 2. Background Some of the benefits from knowing the common clock offset between different GNSS are mentioned above. When a position is calculated using a mix of satellites from two GNSS a minimum of five satellites are needed if the common clock offset is unknown. If the common clock offset is fully known, then the two GNSS can be operated in interchangeable mode and only four satellites are needed. I.e. the situation is then comparable to dealing with all satellites from the same GNSS. The common clock mode effectively results in observations from an additional satellite, while the ISB values ensures consistent observations across different receiver brands (for all signals) when operating in common clock mode. A fundamental requirement for this to work is that the GNSS receiver implementation is such that all GNSS measurements are referenced to the same oscillator. 3. Objectives and methodology The effects of common clock offsets are somewhat different in the case of float carrier-phase PPP and fixed ambiguity PPP. In the case of a float solution, the effects on the code and carrier-phase are again different. For instance, in the case of fixing the integer carrier-phase ambiguity in PPP, the common clock offset between different GNSS will affect the carrier-phase measurements directly if not accounted for. All these scenarios will be discussed in detail. Assuming receiver biases between different GNSS are constant, it is possible to calibrate these biases using zero baseline double-difference observables and removing the integer ambiguity. Transferring these biases to ISB values for the precise orbit and clock calculations will be discussed. This includes defining standards that allows for mixing of different receiver brands both at the network side and the user end. A demonstration is planned including calibration of the biases for different receiver brands, calculating precise orbits and clocks for combined GNSS constellations, correctly handling the biases and showing position results for mixed constellations in interchangeable mode. 4. Results The bias calibration tests confirm that code and carrier-phase biases between different receiver brands exist when combining different GNSS. These biases are found to be very constant over time and consistent for receivers of the same brand. An explanation of the origin of these biases will also be attempted. It is further expected that interchangeable integration of GPS and Galileo will be successfully demonstrated, including both the generation of calibrated precise clocks and orbits, and the application in an end-user PPP solution. 5. Conclusions and significance of work It is expected that this paper will enhance the understanding of biases between different receiver brands when combining different GNSS: why they exist and how to handle them. And, in particular the impact of these biases on PPP will be addressed; showing how interchangeable positioning with GPS and Galileo is possible. To the knowledge of the authors this is the first interchangeable implementation of GPS and Galileo at the decimetre level PPP. References Melgard, T., E. Vigen, K. de Jong, D. Lapucha, H. Visser, and O. Oerpen (2009) “G2 - The First Real-Time GPS and GLONASS Precise Orbit and Clock Service,” in Proceedings of ION GNSS 2009, 22-25 September, Savannah GA, U.S. Institute of Navigation, Fairfax VA Melgard, T., K. de Jong, G. Lachapelle, and D. Lapucha (2011) “Interchangeable Integration of GPS and GLONASS by Using a Common System Clock in PPP,” in Proceedings of ION GNSS 2011, 19-23 September, Portland OR, U.S. Institute of Navigation, Fairfax VA Odijk, D. and P. Teunissen (2012) “Characterization of between-receiver GPS-Galileo inter-system biases and their effect on mixed ambiguity resolution,” GPS Solutions, Springer-Verlag, pp. 1-13 Parkinson, B. (2007) “The Future of Satellite Navigation,” in Minutes of the Position, Navigation, and Time (PNT) Challenges and Opportunities Symposium. 6-7 November, Stanford Linear Accelerator Center (SLAC), Menlo Park CA
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	1198 - 1206
Cite this article:	Melgard, T., Tegedor, J., de Jong, K., Lapucha, D., Lachapelle, G., "Interchangeable Integration of GPS and Galileo by Using a Common System Clock in PPP," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 1198-1206.
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