Satellite Orbit & Clock Correction Streams
To step forward from real-time single point positioning (SPP - code-only analysis, with position precision of the meter level and below) to precise point positioning (PPP - code+phase analysis, with position precision of the dm or even cm level) it is necessary to have real-time access to several correction streams allowing to take full advantage of the high precision phase observables. An example of such corrections are real-time corrections to the GPS and GLONASS broadcast ephemeris enabling availability to high precision satellite orbit and clock information in real-time. Since 2014 the IGS operating a Real-Time Service (IGS RTS) to estimate and distribute such corrections in real-time. Whereas IGS is currently focusing on satellite orbit and clock corrections for GPS and GLONASS which are related to ITRF2014 (or its IGb14 realization), EUREF is distributing corrections for GPS and GLONASS which are related to ETRS89.
The IGS_RTS Analysis Centres (ACs) are estimating GPS GLONASS satellite orbit and clock corrections. Some ACs already started with processing and distributing Galileo and BeiDou corrections. The individual corrections are uploaded to the IGS NTRIP broadcasters. The IGS_RTS Analysis Coordinator is then combining the individual contributions to make available a clearly specified product to the users. These corrections are referred to the IGS realization of ITRF2014.
For EUREF, a clearly specified product is made available as well. 14 parameter transformations between ITRF2014 and various regional datums are introduced in the program BNC to additionally derive the corrections related to these regional datums. For EUREF, the values for the transformation between ITRF2014 and ETRF2000, epoch 2000.0, are introduced using the values from http://etrs89.ensg.ign.fr/memo-V8.pdf and extrapolated to the current epoch.
Note that in the current version of BNC only the satellite orbits are transformed. With regard to the large number of possible transformations, only a few transformations are hard-coded in the software, whereas individual ("custom") parameters could be introduced by the user.
The EUREF real-time product streams can be accessed using so-called mountpoints from one of the three regional NTRIP broadcasters. To get free access to the broadcasters you simply have to register, use column Operator & Registration Link for the details.
Unambiguous mountpoints for the EUREF-related corrections streams have been established. Currently, the following mountpoints are opened or will be established in near future for the ETRS89-related orbit & clock corrections:
Note that the correction streams given here are related to the antenna phase centre (APC) reference point because of in order to provide the necessary consistency between orbits and clocks required by the PPP procedure they will be used for.
One of the open source programs to process GNSS real-time data is BNC, an NTRIP client for precise point positioning. Another one, widely used, is RTKLIB. For a complete list of various corrections you may go to Broadcast Ephemeris Corrections to see the details.
The accuracy of the IGS real-time clock corrections is well below 0.3 ns with respect to the IGS rapid clocks. Using the IGS real-time orbit and clock correctors in the PPP gives accuracies of a few decimeters after 20 to 30 minutes convergence time. Permanent positioning using the various individual and combined clock & orbit corrections can be found on the real-time PPP monitoring page. Tests showed that the coordinate differences between the two sets of coordinates using either the transformed orbits & clocks or using the original orbits & clocks and transforming the coordinates afterwards below one centimeter for the ETRS89 transformation (Söhne, 2010).