Satellite Orbit & Clock Correction Streams
To step forwards 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. In 2009 the IGS launched a pilot project to estimate and distribute such corrections in real-time, see IGS Real-Time Pilot Project (IGS-RT PP) for more details. Whereas IGS is currently focusing on satellite orbit and clock corrections for GPS which are related to ITRF2008 (or its IGS08 realization), EUREF is distributing corrections for GPS and GLONASS which are related to ETRS89.
The IGS-RT PP Analysis Centres (AC) are estimating GPS and partly also GPS & GLONASS satellite orbit and clock corrections. These corrections are uploaded to the IGS NTRIP broadcaster. The IGS-RT PP 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 ITRF2008.
For EUREF, a clearly specified product is made available as well. 14 parameter transformations between ITRF2008 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 ITRF2008 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.
It is planned to establish unambiguous mountpoints for the EUREF-related corrections streams. Currently, the following mountpoints are opened or will be established in near future for the ETRS89-related orbit & clock corrections:
Other mountpoints with individual solutions, e.g. mountpoint CLK41 as the individual GPS & GLONASS solution of BKG/CTU, are still available as well.
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 analyze GNSS real-time data is BNC, an NTRIP client for precise point positioning. 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).