magicGNSS’ Real-Time POD and PPP Multi-GNSS Service

Guillermo Tobías, J. David Calle, Pedro Navarro, Irma Rodríguez and Daniel Rodríguez

Abstract:	Back in 2008 GMV released the first version of its magicGNSS suite, an online set of GNSS tools which have been tailored throughout the years to cope with the needs of a wide variety of GNSS users (over 1000 registered users by 2013), providing added-value services including multi-GNSS Precise Orbit and Clock Determination, High Accuracy Positioning, Integrity and local augmentation developments among others. The precise orbit and clock products for the different satellite constellations are generated in an Orbit Determination and Time Synchronisation (ODTS) process. This process receives as input dual-frequency code and phase measurements from a network of reference stations and produces as output precise satellite orbits and clocks, together with additional estimated parameters such as station clock biases, tropospheric delays and phase ambiguities. All GNSS satellites are processed together to ensure a consistent solution is provided. Due to GLONASS satellites transmitting individual frequencies (FDMA as opposed to CDMA used by GPS and Galileo), an inter-frequency bias must be estimated when processing GLONASS data together with the inter-system bias for Galileo. The quality of the multi-GNSS orbit and clock products generated by magicGNSS have been assessed by means of their comparison with the products generated by different Analysis Centers (TUM, CODE, CNES and GFZ) in the frame of IGS’ Multi-GNSS Experiment (MGEX) project with centimetric differences. The Precise Point Positioning (PPP) capability of magicGNSS suite provides centimetric positioing error, and sub-nanosecond error in timing, using dual-frequency observations in RINEX format from a stand-alone user receiver and precise GNSS orbit and clock products calculated beforehand (products from the International GNSS Service (IGS), multi-GNSS products generated by GMV or orbits and clocks generated by the user). The user receiver can be fixed to the ground (static PPP) or can be a moving receiver (dynamic or kinematic PPP). One of the main advantages of PPP with respect to other precise-positioning approaches like Real Time Kinematics (RTK) in that no ‘base station’ or surrounding stations are needed. Another advantage is that since the GNSS orbit and clock products are by nature global, and therefore, the PPP solutions are also global. Together with the magicGNSS online suite, GMV has been developing over the last two years an infrastructure for the generation of precise GPS, GLONASS and Galileo orbits and clocks in real-time. This infrastructure acquires via NTRIP data streams from around eighty IGS tracking stations distributed worldwide, and produces orbit updates every fifteen minutes and clock updates every second from a combined multi-GNSS solution that can be then used consistently for PPP applications. These real-time satellite orbits and clocks are available as a real-time ephemeris correction stream in standard RTCM format and can be used for real-time PPP using GMV’s magicGNSS PPP demonstrator or any real-time PPP client which processes RTCM ephemeris corrections. This real-time High-Accuracy (HA) service is comparable to the IGS service; the clock RMS is around 0.3 nanoseconds compared to the IGS rapid product (75ps RMS) and the prediction orbit is around 6 cm of the IGS rapid orbit product (~2.5 cm). GMV’s magicGNSS real-time PPP demonstrator was developed in parallel to the real-time HA multi-GNSS products generation platform to evaluate real-time PPP performance in realistic scenarios (kinematic and static). It provides an end to end process including redundancy, communications, robustness and reliability and the ability to run on portable devices. The system has already been evaluated under several field scenarios representing many situations that potential users could address in real operations for many different applications. These include static, kinematic and combined use cases. In the carried out tests, different visibility conditions have been evaluated (open sky or different types of obstacles such as trees or walls), as well as the robustness of the solution against communication losses of different durations. The real-time PPP solutions are validated against RTK and/or post-processed PPP. As an example of the aforementioned tests, a receiver was mounted onto the roof of a car to test the real-time PPP solution in kinematic mode. This test showed that during the collection of data (a 15 km route around a base station) the solution degraded when a lower amount of satellites were in view due to obstacles such as trees and buildings. This degradation can result in a re-estimation of the ambiguities which can take up to 20 minutes to achieve. It was noticed that the inclusion of multiple GNSS observations during this testing was an advantage as more satellites were in view with respect to the GPS-only case, limiting the instances of a re-estimation of the ambiguities being needed. Currently intensive research is being carried out at GMV for developing strategies to reduce convergence times, which are normally around 30 minutes for decimetric accuracy. One such strategy that is used is the ‘quick start’ feature which involves starting at a point with calibrated (few centimetres) a-priori coordinates. Use of this feature can lead to convergence within a few seconds which is crucial for certain PPP users. Throughout this paper, magicGNSS` real-time orbits and clocks server and the real-time PPP demonstrator developments undertaken are described, together with both the server (i.e. orbit and clock) performances achieved with respect to IGS and the resulting positioning performances. We also discuss the major challenges faced in the development process, and some under research ways to improve the overall service.
Published in:	Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014) September 8 - 12, 2014 Tampa Convention Center Tampa, Florida
Pages:	1046 - 1055
Cite this article:	Tobías, Guillermo, Calle, J. David, Navarro, Pedro, Rodríguez, Irma, Rodríguez, Daniel, "magicGNSS’ Real-Time POD and PPP Multi-GNSS Service," Proceedings of the 27th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2014), Tampa, Florida, September 2014, pp. 1046-1055.
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