Air Force Requirements: The high speed test track at the Arnold Engineering and Development Center (ADEC) in Tulahoma, TN is used to test models of space vehicles, ordnance and aircraft. At launch speeds of up to Mach 26, the spatial integrity of the the track is important for successful experiments. The track is an 800 foot long series of rails arranged at 3,6,9, and 12 o’clock with a spacing of 2.5 inches between opposite rail faces. The track is composed of a series of 20 foot sections of rails constrained inside steel tubes. A launched model can fail if it experiences a discontinuity at the tube-to-tube interface. Failure of a model leads to a time consuming process of finding the model fragments, and repairing the rail where the high kinetic energy of the destroyed model damaged the rails. The two main types of discontinuity are positional, where the ends do not match, or slope, where the ends match up but the tubes meet at a slight angle. Conventional transits did not have the accuracy nor range. At best, the tubes were lined up using conventional optical instruments in a "leap frog" mode, and errors increased with distance. Innovative Technology A large diameter laser beam was used to establish a straight datum down the launch tubes. Unlike other laser alignment systems, the projected beam was monitored for motion and vibration. An electronic optical target placed at the end of the track monitored beam motion and transmitted real-time XY coordinates of the laser beam back to a central processor. Intermediate optical targets are deployed at strategic positions where the alignment of the tube section is required. These targets also sense beam movement. Since any movement of the laser beam occurs simultaneously at all targets, the magnitude of the motion is measured at the end target and is used to cancel these effects at all the intermediate target locations. Thus, repeatable straightness measurements can be made over long distances independent of movement of the laser beam caused by thermal, tidal or local disturbances. Company Benefit This compensated laser alignment system was commercialized. It is called EUCLID, named in part after the Greek geometrician and in the similar triangle compensation that is used to make stable measurements in the presence of laser beam movement. It has found wide use on Boeing 777 fuselage final assembly, and has be used to measure tunnel wall movement, structural beam bending and in alignment of other aerospace structures.