Defensive radar systems for detecting and locating ballistic missiles must be designed to combat the degradation in system performance that can result from atmospheric and ionospheric refraction effects, clutter and scintillation. These phenomena become increasingly severe at low elevation angles and for high latitude stations there is a broad azimuthal sector where the effects are especially severe owing to the very irregular nature of the polar ionosphere and the presence of the aurora. A program of measurements and data analysis is in progress, primarily centered at the Millstone Hill radar facility, designed to evaluate tropospheric and ionospheric effects on radar measurements capability, with emphasis upon disturbed ionospheric conditions e.g., when radio aurorae exist. Emphasis in these measurements will be on attempting to separate and characterize the ionospheric and tropospheric components of refraction and scintillation. In order to do this, a UHF lobe-comparison satellite-beacon tracking capability is being added to the precise L-band tracking radar at Millstone. This will permit simultaneous dual-frequency measurements of angle- of-arrival, amplitude and phase fluctuations for paths traversing the polar ionosphere as well as in other directions less subject to propagation effects.