For two-way authentication, the reverse process would be performed. In the current implementation, location signatures are 20,000 bytes. For continuous authentication, an additional 20 bytes per second are transferred. Re-authorization can be performed every few seconds or longer. The location signature is virtually impossible to forge at the required accuracy.
Continuous authentication provides further protection against such attacks.
GPS receivers are not suitable for location authentication because they compute latitude, longitude, and height directly from the GPS signals. Thus, anyone can report an arbitrary set of coordinates, and there is no way of knowing if the coordinates were actually calculated by a GPS receiver at that location. A hacker could intercept the coordinates transmitted by a legitimate user, and then replay those coordinates in order to gain entry. Typical code correlating receivers, available to civilian users, are also limited to 100 meter accuracy. DGPS methods attenuate the satellite orbit errors and cancel SA dithering effects.