# Realising WGS84 with a TRF

So much for the theoretical definition of WGS84 – how can we use it? At first sight a coordinate system centred on the centre of mass of the Earth, oceans and atmosphere might seem very difficult to realise. Actually, this definition is very convenient for satellite positioning, because the centre of mass of the Earth (often called the geocentre) is one of the foci of the elliptical orbits of all Earth satellites\*, assuming the mass of the satellite itself is negligible. Therefore, observing a satellite can tell us, more or less, where the centre of the Earth is.

There are no fewer than three Terrestrial Reference Frames realising WGS84 that are very important to us in Britain. They are: the United States military ‘broadcast’ realisation; the International Terrestrial Reference Frame (ITRF) precise scientific realisation; and the European Terrestrial Reference Frame (ETRF) Europe-fixed realisation. We will look at each in turn. We will see that each of these actually realises a slightly different datum, although all of them are loosely referred to as ‘WGS84 realisations’.

{% hint style="info" %}
\*Orbiting satellites naturally move in ellipses, which have two focal points. The centre of mass of the earth-satellite system lies at one focus of the ellipse. A circle is the special case of an ellipse where the two focal points coincide.
{% endhint %}


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