> 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/the-international-gnss-service-igs.md).

# The International GNSS Service (IGS)

The IGS is essential for anybody requiring high accuracy GNSS derived positions. The IGS operates a global TRF of 505 GNSS stations (as of February 2018) and from these produces the following free products, distributed via the Internet:

* IGS tracking station dual-frequency GNSS data.
* Precise GNSS satellite orbits (ephemerides).
* GNSS satellite clock parameters.
* Earth orientation parameters.
* IGS tracking station coordinates and velocities; many of these stations are also listed in the ITRF.
* Zenith path delay estimates.

Of these, the first two are commonly used for general-purpose high-accuracy positioning, and the third is becoming increasingly important. The satellite ephemerides, clock parameters and Earth rotation parameters are available two days after the time of observation, and also in advance of the observation in a less accurate predicted version.

The IGS products give us access to a high-accuracy realisation based on the current ITRF. Used in conjunction with the ITRF coordinates of nearby IGS tracking stations and the dual‑frequency GNSS data from those stations, a user can position a single geodetic-quality GNSS receiver to within a few millimetres. Hence the IGS products are a vital part of the civil GNSS community’s access to the ITRF.

Because the subject of this booklet is coordinate systems, not GNSS positioning methods, no more will be said about IGS here. Please see the [Further information](/more-than-maps/a-guide-to-coordinate-systems-in-great-britain/further-information.md) page for more information on precise GNSS positioning and the IGS web page address.


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