Radar measurement of ionospheric scintillation in the polar region

Electromagnetic signals propagating in the ionosphere can exhibit rapid variation in amplitude and phase, referred to as scintillation. UHF radars that operate in the polar region can experience scintillation anytime of the day or night, particularly in the winters around solar maximum. Unless accounted for, scintillation is likely to disturb many radar functions, especially those that rely on the measurement of radar cross section. This paper discusses several estimators, based on radar track data, of the S₄ scintillation index that characterize the severity of amplitude scintillation. Estimates of S₄ are based on data (referred to as radar returns) obtained during track of targets which may have intrinsic RCS fluctuations. Key to this work is the consideration of thresholding, which is used in many radars to remove (from further processing) signals whose signal-to-noise ratio (SNR) is considered too low. For example, in the early warning radar (EWR) located at Thule, the instantaneous signal-to-noise ratio on every return is compared to a threshold before the return is applied in additional radar processing (track acquisition, track, etc.). Returns below threshold are discarded from further processing. We consider several estimators here. The direct estimator attempts to estimate the scintillation index through the direct calculation of the mean and standard deviation of the RCS from a number of radar returns. The maximum likelihood estimator uses multiple hypothesis testing to estimate the scintillation index that best fits the radar returns from some number of pulses. This estimator assumes that the statistics of the scintillation are well represented by the Nakagami-m distribution and uses a count of the number of pulses transmitted and the number of returns above threshold. Based on this work, we developed a modified maximum likelihood estimator with no pulse counter. We applied the modified estimator to several years of radar tracks in t- - he polar region to obtain the statistics of UHF scintillation as viewed from the EWR at Thule, Greenland. One-way S₄ was measured from five thousand horizon-to-horizon tracks of large calibration satellites during a two-year period after solar maximum in May 2000. The data is analyzed to quantify the exceedance, or the level of scintillation experienced at various probabilities.