A new method of sensing the vibrational motions of a exible structure by measuring the dierence in GPS (global positioning system) carrier phase at separate antennas on the structure is presented. Differential carrier phase measurements have been recently used to determine the attitude of rigid bodies such as aircraft and spacecraft, and for aircraft landing with ground based GPS pseudolites. This technology is still rapidly developing and has often proven to be a powerful, low cost alternative to traditional sensing methods. This paper presents the main issues that must be considered to extend GPS to exible systems. Sensor bandwidth, measurement accuracy, and receiver architecture issues are discussed as well as the techniques for real-time state estimation. This paper emphasizes the following: 1. Why is GPS valuable as a structural deformation sensor? 2. What are the challenges to realizing this sensor? 3. How are these challenges being addressed?