A local area augmentation navigation system for determining the authenticity of the GPS signal. The system is provided with at least one reference station at a known location, a master station, a monitoring receiver, and a LAAS receiver. Each reference station receives a GPS signal from a GPS constellation and collects and outputs pseudo-range data from the GPS signal. The master station receives the pseudo-range data from the reference stations. The master station forms a correction message from the pseudo-range data and the known locations of the reference station. The master station broadcasts the correction message. The monitoring receiver receives the GPS signal from the GPS constellation and the correction message to determine the accuracy of at least one of the broadcasted correction message and the GPS signal. The LAAS receiver receives the correction message broadcast by the master station as well as a GPS signal from the GPS constellation and calculates the location of the LAAS receiver with the correction message and the GPS signal.