My understanding is that a lot of the imprecision in consumer GPS units is due to random signal delays caused by density and thickness variations in the earth's atmosphere. Many of the more sophisticated GPS units (even my little old Garmin Summit) allow you to subscribe to a local radio service that broadcasts a steady stream of "correction factor" information based on the local atmospheric conditions in real time. More than five years ago, when I was living in the UK, I listened to a documentary on Radio 4 where they were managing to get imprecision of only a centimetre or two using this system. The correction information was being broadcast on a sub-carrier of Classic FM.

clop