15cm GPS accuracy has been available commercially since about 2002. Trimble and Novatel both sold units. But you had to get a subscription to a service which had a network of ground stations measuring propagation errors through the atmosphere. The correction signals came in from a geosync satellite. You also had to be able to see about five satellites.
More specifically you can use differential GPS for stuff like surveying with a reference receiver at a known location. Depending on how far away you are from the reference station you can get even better than 15 cm accuracy and just have a few inches of error.
That's more or less how this works. When you can see both the C/A and L5 signals, you can do some magic to help compensate for anything going on in the ionosphere
