On the Extraction of Near-Surface Index of Refraction Using Radar Phase Measurements from Ground Targets

The speed at which electromagnetic waves travel between a radar and a target is dependent on the index of refraction of the atmosphere between the radar and the target. Modern radars can have sufficiently accurate time bases and digitizing equipment to observe small changes in the time it takes a radar signal to travel to a fixed target and back. These changes are related to small perturbations in the refractive index caused by changes in humidity, temperature, and pressure. Using the phase information from ground targets and its time evolution as a proxy for the changes in travel time of radar waves, a procedure for measuring the near-surface index of the refraction field around the radar is demonstrated and implemented on the McGill Doppler S-band radar. This paper describes the theory behind the measurement, and a technique used to extract refractive index data from ground targets. Early measurements of the index of refraction field are also presented, and some of the possibilities offered by this new radar-measured variable are identified.