"Where you have one or more fixed local base stations that know their position accurately and continuously broadcast the difference between their known position and the satellite predicted position. GPS devices connected to their network can then receive the differences in real-time and apply the same correction factor to their own satellite predicted positions. This typically increases accuracy to less than a metre (with some of the better systems it can be accurate to just a few centimetres). This type of system would work well for a fixed open-air site like a battlefield."
There are some pictures of the reference stations here https://en.wikipedia.org/wiki/Differential_GPS. We don't know if this is what they are doing at Culloden but it would certainly be brilliant for a fixed setting like this. Thanks, Pete!
