Orbits and Clocks for GLONASS Precise-Point-Positioning

In 2008 GMV introduced magicGNSS [Ref. 1], a web application providing a suite of tools for GNSS data processing featuring high-precision and integrity. The main application of magicGNSS is the calculation of GPS satellite orbits and clocks, and also of station/receiver coordinates, tropospheric delay and clock. magicGNSS current version (1.3) is available online for registered users at http://magicgnss.gmv.com. The Orbit Determination & Time Synchronization (ODTS) module was the first algorithm available in magicGNSS. ODTS generates orbits and clocks by processing dual-frequency code and phase measurements from a network of GPS stations distributed worldwide. Past and current data from a set of so-called core stations from the International GNSS Service (IGS, [Ref. 2]) is available on the magicGNSS server, and the possibility also exists to upload and process RINEX observation files from any user station. Since the beginning of 2009 the Russian constellation GLONASS has 19 operational satellites. The implementation of GLONASS data processing in addition to GPS in magicGNSS is then a natural step in order to take advantage of the extended satellite availability. The precise determination and prediction of GLONASS orbits and clocks poses a number of difficulties. One of them is the a priori unknown solar radiation model for the GLONASS satellites, actually the major limitation for sub-decimeter orbital dynamics accuracy. Also, when processing data from a network of GLONASS stations, a so-called inter-channel bias has to be estimated at station level in order to compensate for the different internal delays of the GLONASS signals and codes through the station hardware and software. Finally, the short-term stability of the Russian satellite clocks might pose a problem for clock estimation and interpolation. Precise-Point-Positioning (PPP) is a relatively new technique for centimeter-level error in positioning, and sub-nanosecond error in timing, using a stand-alone GNSS receiver and precise satellite orbit and clock products calculated beforehand (for example products from IGS). The user receiver can be fixed to the ground (static PPP) or be a roving receiver (dynamic or kinematic PPP). PPP is different from other precise-positioning approaches like Real Time Kinematics (RTK) in that no base stations or reference stations are needed. The only observation data that must be processed is the user receiver data itself, thus reducing the bandwidth and calculation power needed for the service. Another advantage of PPP is that since the input satellite orbit and clock products are by nature global, the PPP solutions are also global, i.e., the PPP approach works for a receiver located anywhere on or above the Earth surface, and the resulting PPP solution (coordinates) are referred to a well-known terrestrial reference frame (normally ITRF). A PPP software module is now available in magicGNSS. The PPP module processes GPS and GLONASS data. This paper describes the implementation of GLONASS data processing in the ODTS and PPP modules of the magicGNSS web application, from the point of view of both algorithms and usability, and presents the major results in terms of GLONASS orbit and clock estimation accuracy (ODTS), and in terms of the resulting positioning and timing accuracy at user receiver level (PPP). For PPP three scenarios are considered and reported: GPS-only, GLONASS-only, and GPS+GLONASS. Data intervals from 1 hour to 1 day are analyzed.