Real-time Satellite Orbit and Clock Corrections to
Broadcast Ephemeris from IGS and EUREF Resources

EUREF's Real-time Analysis project and the IGS Real-time Pilot Project provide access to precise GNSS satellite orbits and clocks via NTRIP for test and evaluation.

1. Broadcast Corrections in RTCM Version 3 Format

Precise orbits and clocks can be derived from corrections to Broadcast Ephemeris. RTCM's 'State Space Representation' (SSR) Working Group has developed appropriate v3 messages to disseminate such Broadcast Corrections in real-time. The following messages are defined:

Orbit corrections are provided in along-track, cross-track and radial components. These components are defined in the Earth-centered, Earth-fixed reference frame of the broadcast ephemerides. Clock corrections are not adjusted for the 2nd-order relativistic effect. After applying corrections, the satellite position and clock is referred to the 'ionospheric free' phase center of the antenna which is compatible with the broadcast orbit reference. The orbit and clock corrections do not include local effects (like Ocean Loading or Solid Earth Tides) or atmospheric effects (ionosphere and/or troposphere). There is currently no RTCM SSR message for ionospheric state parameters. The development of ionospheric messages will be the next step in the schedule of the RTCM State Space Representation Working Group.

2. Broadcast Correction IDs

Any RTCM Version 3 Broadcast Corrections stream contains three types of IDs to identify its contents.

• A globally unique SSR Provider ID (PID) released by RTCM to identify a SSR service, see SSRProvider
• A SSR Solution ID (SID) which indicates different SSR services of one SSR provider.
• A Issue Of Data (IOD) SSR used to indicate a change in the SSR generation, which may be relevant for rover operation.

The following are IDs used in IGS and EUREF context.

3. Broadcast Correction Streams

Based on the SSR messages defined by RTCM, the following streams are made available as a common effort of EUREF and IGS:

Note that CoM stands for Center of Mass and APC for Antenna Phase Center. The RTCM v3 streams listed above may be used i.e. to support real-time Precise Point Positioning (PPP) applications. Daily comparisons of clocks from different Analysis Centers are carried out off-line and made available for download from ftp://cddis.nasa.gov/gps/products/rtpp,ftp://igsdepot.ensg.eu./pub/igs/products and ftp://nfs.kasi.re.kr/gps/products/rtpp.

3.1 Coordinate Transformations

Some of the above listed streams refer to regional or national coordinate reference systems. The following table presents links to NTRIP Broadcasters providing access to such streams.

The following Helmert transformation parameters are used in this context for transformations from ITRF2014/IGS14.

References:

ETRF2000: Claude Boucher, Zuheir Altamimi 2008: 'Specifications for reference frame fixing in the analysis of EUREF GPS campaign', see http://etrs89.ensg.ign.fr/memo-V7.pdf.

NAD83: Chris Pearson, Richard Snay 2013: 'Introducing HTDP 3.1 to transform coordinates across time and spatial reference frames', GPS Solutions, January 2013, Volume 17, Issue 1, pp 1-15.

GDA94: John Dawson, Alex Woods 2010: 'ITRF to GDA94 coordinate transformations', Journal of Applied Geodesy, 4 (2010), 189-199, de Gruyter 2010. DOI 10.1515/JAG.2010.019

SIRGAS2000: Personal communication with CGED-Coordenacao de Geodesia, IBGE/DGC - Diretoria de Geociencias, Brazil.
see ibge_manual_ppp

SIRGAS95: Personal communication with Laboratorio de Geodesia Fisica y Satelital at Zulia University (LGFS-LUZ), transformation parameters based on values from Table 4.1 of 'Terrestrial Reference Frames (April 10, 2009), Chapter 4', see http://tai.bipm.org/iers/convupdt/convupdt_c4.html.

DREF91: 'Referenzkoordinaten f�r SAPOS, Empfehlungen der Projektgruppe SAPOS-Koordinatenmonitoring 2008', Personal communication with Peter Franke, BKG, Germany.

4. Tools for Decoding RTCM v3 Broadcast Corrections

We so far don't know about GNSS receiver firmware which supports the RTCM v3 messages disseminated by EUREF and IGS. However, they are implemented in the BKG NTRIP Client (BNC) which is therefore capable of decoding and fowarding the orbit and clock corrections to your SSR application or to use them for real-time Precise Point Positioning. You may like to check out BNC's documentation and source code for details on how correctors are provided.