What is the point of active GPS stations?

[TrigChecker]

I gather they are GPS transmitters that are fixed to one location. What is the purpose of these providing constant GPS location information from a fixed location? Are the OS worried that part of the UK might move, and they will need to quickly adjust their maps accordingly?

[cjo]

Simply The GPS signal varies a little and has small errors. By measuring this at fixed locations it is possible to determine this error. Surveyors who require sub m….or even mm accuracy can calculate the error from this station and apply appropriate correction to their equipment.