Demonstrating In-The-Field Real-Time Precise Positioning (2012)

Precise Point Positioning (PPP) is a relatively new positioning technique providing centimeter-level error. PPP processes dual-frequency pseudorange and carrier-phase measurements from a single user receiver, using detailed physical models and corrections, and precise GNSS orbit and clock products calculated beforehand. PPP is different from other precise-positioning approaches like RTK in that no reference stations are needed in the vicinity of the user receiver. The only observation data that must be processed are measurements from the user receiver. Another advantage of PPP is that since the GNSS 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 position is referred to a well-known terrestrial reference frame (normally ITRF). PPP can be applied at post-processing level and also in real-time applications, provided that real-time input orbits and clocks are available. One disadvantage of standard PPP however is its relatively slow convergence time, which is of the order of an hour for decimetric accuracy, as compared to nearly instantaneous convergence with centimetric accuracy in short-baseline RTK. After the latest launch of GLONASS satellites, the Russian constellation is nowadays fully operational, with 24 operational satellites. The ‘GNSS constellation’, including GPS and GLONASS, provides currently 54 usable satellites. For the current GNSS user this means that up to 20 GPS+GLONASS satellites can be simultaneously visible in open-sky areas. This represents an increase of around 60% in satellite availability with respect to the GPS-only scenario, and does not count the upcoming Galileo and COMPASS systems. This high number of satellites in view is very interesting for PPP users, as the convergence time is sensibly improved when more satellites are used in the PPP solution (decimetric horizontal accuracy can be achieved in less than 20 minutes in many cases). However, the timely provision of accurate GPS and GLONASS orbits and clocks, which requires the proper consideration of inter-system and inter-channel biases, is a challenge for real-time applications. Over the last two years, GMV has developed an infrastructure for the generation of precise GPS and GLONASS orbits and clocks in real time. This infrastructure acquires via NTRIP data streams from fifty to sixty tracking stations distributed worldwide, and produces orbit updates every fifteen minutes and clock updates every second from a combined GPS and GLONASS solution that can be then used consistently for PPP applications. More recently, a real-time multi-GNSS PPP client has been also developed and integrated in a portable hardware device supporting in-the-field real-time PPP. This device connects to a standard geodetic-class receiver through a serial interface to retrieve the observations, and features mobile communications with the PPP corrections server using mobile Internet or Iridium. The communications have been optimized in order to provide a good balance between the data provider costs (mainly when using Iridium) and the positioning performances. This approach allows the use of a real-time PPP service with many existing geodetic-class receiver without the need for upgrading or replacing them, thus extending their operational capabilities. In addition to the algorithmic work in the server and client sides, a significant effort has been devoted to the development of the portable device and the integration of the algorithm in it, as well as to providing robustness to the service against anomalous events such as station or satellite losses or communication dropouts. The portable device is conceived as a demonstrator, and features a processing board which hosts the OS and the algorithms, a communications board integrating the mobile Internet and the Iridium modems, and a touchscreen with a custom user interface for evaluating the solution in real time. On the server side, several instances of the orbit and clock calculation algorithm can be run in parallel for redundancy. A specific piece of software monitors their outputs; in case of problems with the master solution, it switches automatically to another one. The system is being evaluated under several field scenarios representing many situations that potential users could address in real operations. These include static, kinematic and combined use cases. In the tests, different visibility conditions are 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. In this paper we describe the server and client developments undertaken, and we present both the server (i.e. orbit and clock) performances achieved and the resulting positioning performances under the different test scenarios. We also discuss the major challenges faced in all the process, and some ways under research to overcome the limitations of the technique.