Application of cylindrical near-field measurement technique to the calibration of spaceborne radar antennas: NASA scatterometer and SeaWinds

Modern spaceborne radar scatterometers, such as the NASA scatterometer (NSCAT) and SeaWinds radar instruments, require precise determination of the normalized backscattered radar cross section within a few tenth of a decibel. This is needed to achieve the desired wind velocity and direction measurement accuracy of 2 m/s and 20/spl deg/, respectively. This high level of precision demands a priori prelaunch accurate knowledge and determination of the radar antenna's absolute gain and relative radiation patterns characteristics over wide angular range. Such characterizations may be performed on a far-field range, compact range, or in an indoor near-field measurement facility. Among the unique advantage of the near-field measurement is that most of the information of the radar antenna radiation properties can be obtained anywhere outside the near-field measurement surface. Two recently designed radar scatterometers are considered in this paper, NASA scatterometer (NSCAT) and SeaWinds, to demonstrate the utility of a newly completed cylindrical near-field measurement range. As an example of an advanced calibration methodology, the data based on a recently measured JPL/NASA scatterometer (NSCAT) radar antenna are used to experimentally demonstrate the role of the probe pattern compensation, probe multiple reflection effects, probe mispositioning effects, scan area truncation effects, etc. A measurement test on a standard gain horn (SGH) has been performed to achieve and verify the absolute gain calibration accuracy. A comparison between direct far-field measured data and those obtained from cylindrical near-field measurements for the SeaWinds radar antenna was found in excellent agreement. It is demonstrated that the near-field measurement technique is a viable approach in accurately characterizing the performance of spaceborne radar antennas.