[Talk-GB] Finally a RTK / NTRIP Broadcaster in London

Adrian ar2988-os2 at yahoo.co.uk
Fri Oct 2 18:00:46 UTC 2020

@ Grant Slater
Thank you for spotting this and making it known.

One thing you need to know when using an RTK base station, is what is the reference (datum). Unfortunately there is no convention for how this information is given, and it is often not given at all. The stations listed by Grant are on the Eurasian tectonic plate so the reference will be either ETRS or ITRS ("WGS 84"). WGS 84 is the datum used in OSM.

I have previously connected to several of the stations on the list: DARE, HERS, HERT and SHOE. All of these are on ETRS although HERS has a small position error. I tried connecting to LICC and it is on ITRS. (I estimate there is a position error, relative to ITRS, around 12cm too far north, 8cm too far west and 31cm too high.) I think a mistake has been made in configuring the LICC station. Incidentally, LICC is at Imperial College in South Kensington. There is a site information page at http://epncb.oma.be/_networkdata/siteinfo4onestation.php?station=LICC00GBR If you click on Data Provided you can see any warnings that have been logged. It was the warning here about a position error that prompted me to check it out. The website I just referred to evidently expects the reference to be ETRS.

The stations on Grant's list are of a type known as Continuously Operating Reference Stations (CORS). Stations of that type would be expected to produce results consistent to the millimetre. The ITRS position of LICC shifts by a millimetre every two weeks, so I hope they have an automated system, or at least a simple system, for updating the position it is broadcasting.

If you have a consumer SatNav you probably can't tell the difference between ETRS and ITRS. But if you are using RTK you certainly can tell the difference. In south-east England, at the time of writing, ITRS gives a position 52.0cm further north and 53.5cm further east than ETRS. This gives a horizontal distance of 74.6cm. The horizontal distance increases by 2.4cm per year. The altitude difference is less than 0.2cm.

If you record a tracklog using RTK from seven of the eight stations in the list, the tracklog will be in ETRS. You will need to convert it to ITRS for use in OSM. If the tracklog covers a small area, you can just apply a fixed offset to the latitudes and longitudes. Unfortunately I don't know of any tool which makes this simple to do.

Someone in France has organised funding for an independent network of open RTK base stations. (The availability of free RTK base stations was even worse in France than in the UK.) See https://centipede.fr/ They have produced detailed instructions for setting up a base station, including a shopping list, how to assemble the equipment, preconfigured software, and how to obtain the position of the base station to within a couple of centimetres. It also covers setting up a receiver for RTK. They have set up a server to broadcast all the streams and there are already several dozen stations in operation. They will soon be prevented from setting up a VRS, only by the cost of the software for doing that. The documentation is all at https://jancelin.github.io/docs-centipedeRTK/ It is in French, but for those who don't speak French, I expect a well-known online service would produce an adequate translation. There is a subscription RTK stream service covering many countries, which professionals use. They no longer quote prices on their web site, but when they did, the entry-level subscription for real-time access was around £4000 per year. IIRC, that gave the subscriber access to a maximum of five simultaneous VRS streams and included access to a two-way satellite link, for areas where there was no mobile phone signal.

More information about the Talk-GB mailing list