The slant path atmospheric refraction calibrator: an instrument to measure the microwave propagation delays induced by atmospheric water vapor

The Slant Path Atmospheric Refraction Calibrator (SPARC) has been developed to measure the water-vapor-induced propagation delay experienced by a radio signal traversing the atmosphere. SPARC measures the difference in the travel times between an optical and a microwave signal propagating along the same atmospheric path. Since the refractivity of dry air is similar for both the microwave and optical signals, the difference in arrival times is a sensitive measure of the water-vapor-induced microwave delay. The instrument is capable of measuring the difference in the optical and microwave travel times with an accuracy of 15 ps or better. SPARC was developed to provide an accurate, independent method for evaluating other techniques used to determine the atmospheric water-vapor-induced delay