Near-Field Reflectivity and Antenna Boresight Gain Corrections for Millimeter-Wave Atmospheric Radars

Abstract

Millimeter-wavelength (MMW) cloud radars operating at Ka band (35 GHz) and W band (95 GHz) are popular atmospheric research tools because they are compact, have low prime power requirements, and are highly sensitive to small hydrometeors. In order to maximize sensitivity, ground-based systems use large diameter high-gain antennas. However, these antennas have substantial far-field distances as large as several kilometers. The far-field distance is defined as rf = 2D2/?, where D is the antenna diameter and ? is the radar wavelength. In the Fresnel region, where 0.62D3/? < r < rf, the antenna gain and pattern shape vary significantly with distance. Processing radar measurements obtained in the Fresnel region using the conventional radar equations gives erroneous results because these relationships assume far-field antenna characteristics. Correction factors are needed to account for the radar response to targets that lie in the near field. This paper provides correction factors for responses to targets at a distance, r, that lie in the Fresnel region of an aperture antenna, that is 0.62D3/? < r < 2D2/?. Detailed antenna pattern simulations are presented, which predict the Fresnel region response to volume-scattering targets and to point-scattering targets that lie along the antenna boresight. These results are used to develop universal near-field reflectivity and boresight gain corrections that depend only on r/rf. They are independent of other radar characteristics, including transmit pulse length, receiver bandwidth, and antenna feed pattern. Although this paper focuses on MMW systems, these corrections are applicable to most radars using lens or dish antennas.