The increasingly widespread use of the Global Positioning System (GPS) in determining the location of vehicles raises the possibility of using the information provided by GPS for vehicle control purposes. The use of a multi-antenna GPS system provides the ability to measure not only position and velocity, but vehicle heading and sideslip as well. This paper presents a validation of a GPS based system with an automotive grade two-axis optical sensor. The results show excellent agreement between the two sensor systems, confirming the accuracy of the GPS based system even in highly dynamic situations. Although any GPS based system is subject to dropouts from driving under trees and bridges, cornering stiffness estimates obtained when GPS is available enable construction of a vehicle state observer for use in the absence of GPS.