The International Terrestrial Reference Frame (ITRF)

The ITRF is an alternative realisation of WGS84 that is produced by the International Earth Rotation and Reference System Service (IERS) based in Paris, France. It includes many more stations than the broadcast WGS84 TRF – more than 500 stations at 290 sites all over the world. Four different space positioning methods contribute to the ITRF – Very Long Baseline Interferometry (VLBI), Satellite Laser Ranging (SLR), Global Navigation Satellite Systems (GNSS) and Doppler Ranging Integrated on Satellite (DORIS). Each has strengths and weaknesses – their combination produces a strong multi-purpose TRF. ITRF was created by the civil GNSS community, quite independently of the US military organisations that operate the broadcast TRF.

Each version of the ITRF is supplemented with a 4 digit year code to identify it – ITRFyyyy. Each ITRFyyyy is simply a list of coordinates (X Y and Z in metres) and velocities (dX, dY and dZ in metres per year) of each station in the TRF, together with the estimated level of error in those values. The coordinates usually relate to the time yyyy.0 (i.e. 00:00 on 1st Jan of year yyyy). To obtain the coordinates of a station at any other time, the station velocity is applied appropriately. This is to cope with the effects of tectonic plate motion. Each ITRFyyyy is available as a SINEX format text file from the IERS Internet website – see 'Further information' for the address.

The datum realised by the ITRF is actually called ITRS (International Terrestrial Reference System) rather than WGS84. There used to be a difference between the two, but they have been progressively brought together and are now so similar that they can be assumed identical for almost all purposes. Because the ITRF is of higher quality than the military WGS84 TRF, the WGS84 datum now effectively takes its definition from ITRS. Therefore, although in principle the broadcast TRF is the principal realisation of WGS84, in practice ITRF has become the more important TRF because it has proven to be the most accurate global TRF ever constructed. The defining conventions of ITRS are identical to those of WGS84 given above.

The ITRF is important to us for two reasons. Firstly, we can use ITRF stations equipped with permanent GNSS receivers as reference points of known coordinates to precisely coordinate our own GNSS stations, using GNSS data downloaded from the Internet. This procedure is known as ‘fiducial GNSS analysis’. Secondly, we can obtain precise satellite positions (known as ephemerides) in the current ITRF that are more accurate than the ephemerides transmitted by the GNSS satellites. Both these vital geodetic services are provided free, via the Internet, by the International GNSS Service.