Real-Time, Wide-Area, Precise Kinematic Positioning Using Data from Internet NTRIP Streams

A technique for real-time, wide-area differential kinematic positioning at the decimeter level has been tested with data from GPS stations in Europe and the USA, streamed in real time over the Internet by various organizations using the protocol known as NTRIP (Network Transport of RTCM via Internet Protocol). With the technique presented here, implemented in navigation software developed by the author in cooperation with personnel at NSWC, Dahlgren Division, it is possible, when used in differential navigation mode, to estimate corrections to the GPS broadcast ephemerides in real-time, as part of the navigation solution. Alternatively, more precise orbits can be used, such as the predicted Ultra-rapid IGS orbits (or IGU orbits), that are updated every few hours, and can be downloaded by anonymous FTP from IGS data repositories such as the CDDIS at NASA’s Goddard Space Flight Center. For this study, 1 Hz data from four stations, three in Spain and one in Portugal, have been used. These stations are separated by distances between 412 and 630 km, and their coordinates are precisely known in the EUREF frame. The data consists of the L1 and L2 carrier phase and pseudo-range observables from the station receivers, and the orbit information in the GPS Navigation Message. During the test, a station in Madrid, at the center of the network, was positioned kinematically relative to the other three, so the results could be compared to the precise fixed coordinates of the site, used as “truth”. Comparing results obtained with the precise predicted IGS orbits and the uncorrected broadcast orbits shows the errors using the latter to be much larger than with the IGS orbits. On the other hand, the results obtained with the IGS orbits, and those obtained with the broadcast orbits corrected in real time during the navigation solution, are quite similar, with the instantaneous kinematic coordinates differing from their “truth” values at the 10 cm level, after the initial period of convergence of the navigation Kalman filter.