Yeah, a few meters away is typically a VRS service because it uses your first position in single mode as the base position.
It works more or less as you described. The real GNSS data is collected from surrounding stations. The GNSS errors such as ionospheric and tropospheric delays are derived at each station. Then an error model can be computed within this sub network of stations. By converting observations from the closest real station to your position, and adding the modelized errors at your location, it gives GNSS data that looks like it is recorded from a base just near you.
Not sure I’m clear lol, but you have a ton of scientific papers about this on Internet if you are interested.
haha thanks - but my main interest in engineering - so as long as I roughly understand I’m happy.
But after my many experiments - I would say NTRIP is good - but it is not within any certainty published over multiple days/weeks. It is not really bad - but wanders around 10cm from average centre. I asked the “RTK Premium” for description of their offsets - but so far no response. The IGES station I get for free is as “precise” over multiple days/weeks as the RTK Premium but slightly offset from RTK Premium.
But RTK2Go is useless (IMO) in its current form and description - yes it is fine for relative position (mostly) but not for absolute positioning. And over time things can change - a site owner could suddenly figure out that they should offset for continental drift - and then old data is “useless” as you don’t know.
I’m trying a few others and will try to get a “review” out explaining the differences and offsets used.