Real-time orbit improvement for GPS satellites

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Many geodetic GPS applications require orbits of better accuracy than the predicted ones broadcast by the satellites themselves. However, orbits of high quality are available to users. Their generation is based on GPS data collected by dedicated tracking networks. Nevertheless, these orbits are available only after an interval of several days following data collection. For real-time positioning applications, one currently depends on the broadcast orbits. An alternative, real-time approach for orbit improvement is described here. This approach is designed to yield, in real-time, the best representation of orbits based on all available observations from a network of fiducial stations. The algorithm design is based on a unit, called the update step, which defines the length of the orbital arc over which the improvement takes place. The initial conditions computed in one orbital arc are propagated into the following one. The algorithm was implemented based on the UNB DIPOP software package, which was further modified to allow network adjustment including correlations between simultaneously observed baselines. The principle of the method has been tested using data collected by a network of 8 stations in Canada and the U.S., which are part of the IGS network. The orbital arcs generated with the method have been compared among themselves, in a test of orbit repeatability to test the orbit internal consistency, and also with the IGS orbits, in a test of external consistency. A subset of the 8-station network has been processed constraining the orbits generated by the real-time algorithm to assess their effect in geodetic positioning. These tests aimed to assess the quality of the orbits generated with the proposed method. The results show that the real-time orbits are at or below the 1 metre level 3drms. Their use in geodetic positioning yield baselines with relative error varying from 0.05 to 0.02 ppm, over baselines of hundreds of kilometres. This represents an improvement of 1 order of magnitude over the broadcast orbits, the only ones presently available for real-time applications.