The WGS84 broadcast TRF

The primary means of navigating in the WGS84 coordinate system is via the WGS84 positions of the GPS satellites, which are continuously broadcast by the satellites themselves. This satellite constellation is a TRF – that is, it is a general-purpose access tool making the WGS84 coordinate system available to users.

The WGS84 satellite positions are determined by the US Department of Defense using a network of tracking stations, the positions of which have been precisely computed. The tracking stations observe the satellites and hence determine the WGS84 coordinates of the satellites. The quality of the resulting satellite coordinates depends on the quality of the known tracking station coordinates. These were initially not very good (probably 10 metre accuracy) but have been refined several times. The tracking station coordinates are now very close agreement with the International Reference Meridian and International Reference Pole.

The network of GPS tracking stations can be considered the original WGS84 TRF. The satellite constellation, which is a derived TRF, can be seen as a tool to transfer this realisation ‘over the horizon’ to wherever positioning is needed in the world. The current coordinates of the tracking station antennae implicitly state the physical origin, orientation and scale of the system – they have been computed such that these elements are as close as possible to the theoretical requirements listed above. Of course, no TRF is perfect – this one is probably good to five centimetres or so.

Prior to May 2000, the full accuracy of the US tracking station TRF was not made available to non-military users. In the transfer of this TRF to satellite positions, positional accuracies were deliberately worsened by a feature known as selective availability (SA). This meant that a civilian user, with a single GPS receiver, could not determine WGS84 position to an accuracy better than about 100 metres. In May 2000, this intentional degradation of the GPS signals was officially switched off.

With a pair of GNSS receivers we can accurately measure their relative positions (that is, the three‑dimensional vector between the two receivers can be accurately determined). We must put one of these receivers on a known point and leave it there. This is known as relative GNSS positioning or differential GNSS. Fortunately, there are methods of accurately determining the real WGS84 position of the known point and hence recovering correct WGS84 positions, using the civil GNSS TRFs which are the subject of the following pages.