> For the complete documentation index, see [llms.txt](https://docs.os.uk/more-than-maps/llms.txt). Markdown versions of documentation pages are available by appending `.md` to page URLs; this page is available as [Markdown](https://docs.os.uk/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/modern-gnss-coordinate-systems/realising-wgs84-with-a-trf.md).

# 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 %}


---

# Agent Instructions
This documentation is published with GitBook. GitBook is the documentation platform designed so that both humans and AI agents can read, navigate, and reason over technical content effectively. Learn more at gitbook.com.

## Querying This Documentation
If you need additional information that is not directly available in this page, you can query the documentation by asking a question.

Perform an HTTP GET request on the following URL with the `ask` and `goal` query parameters:

```
GET https://docs.os.uk/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/modern-gnss-coordinate-systems/realising-wgs84-with-a-trf.md?ask=<question>&goal=<user_goal>
```

`ask` is the immediate question: it should be specific, self-contained, and written in natural language.
`goal` is what the user is ultimately trying to achieve, the reason they need the answer. Sharing it helps GitBook give you a better, more relevant answer. A goal is most helpful when it describes the outcome the user wants rather than restating the question. For example, with `ask=how do I create an API token`, a goal like `automate deployments from our CI pipeline` lets GitBook tailor the answer to that use case.

The response will contain a direct answer to the question and relevant excerpts and sources from the documentation.

Use this mechanism when the answer is not explicitly present in the current page, you need clarification or additional context, or you want to retrieve related documentation sections.