The Construction of a Three-Dimensional Antenna Gain Matrix and Its Impact on Retrieving Sea Surface Mean Square Slope Based on Aircraft Wave Spectrometer Data

Xiuzhong Li School of Marine Science, Nanjing University of Information Science and Technology, Nanjing, China

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Yijun He School of Marine Science, Nanjing University of Information Science and Technology, Nanjing, China

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Biao Zhang School of Marine Science, Nanjing University of Information Science and Technology, Nanjing, China

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Chenqing Fan First Institute of Oceanography, State Oceanic Administration, Qingdao, China

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Abstract

In this study, a rotating frequency-modulated continuous wave (FMCW) radar is installed on an aircraft to retrieve the sea wave spectra. Because the aircraft attitude angles produce the incorrect antenna gain used in the radar equation, the incorrect normalized radar cross section (NRCS) of the sea surface will be acquired. To eliminate the effect of the angles, a three-dimensional matrix of the radar antenna gain is constructed by means of coordinate transformation and interpolation, based on a large set of configurations of the aircraft attitude angles (roll, pitch, etc.). With the application of the matrix, the NRCS of the sea surface is corrected and the calculating time is reduced. Then the sea surface mean square slope (MSS) is obtained from the echoes of the airborne wave spectrometer. Considering a weak periodicity of MSS due to low sea state, four images are presented to show the variation of the MSS after aircraft attitude angle correction. The results indicate that the accurate incidence angle of the antenna beam center is critical for retrieving the sea surface MSS, and that the magnitude of the MSS from three cycles of radar echoes can be changed by as much as 40% within 5° of the attitude angles. Furthermore, the MSS becomes more periodic and regular after correction.

Corresponding author address: Dr. Yijun He, School of Marine Science, Nanjing University of Information Science and Technology, 219 Ningliu Road, Nanjing 210044, Jiangsu, China. E-mail: yjhe@nuist.edu.cn

Abstract

In this study, a rotating frequency-modulated continuous wave (FMCW) radar is installed on an aircraft to retrieve the sea wave spectra. Because the aircraft attitude angles produce the incorrect antenna gain used in the radar equation, the incorrect normalized radar cross section (NRCS) of the sea surface will be acquired. To eliminate the effect of the angles, a three-dimensional matrix of the radar antenna gain is constructed by means of coordinate transformation and interpolation, based on a large set of configurations of the aircraft attitude angles (roll, pitch, etc.). With the application of the matrix, the NRCS of the sea surface is corrected and the calculating time is reduced. Then the sea surface mean square slope (MSS) is obtained from the echoes of the airborne wave spectrometer. Considering a weak periodicity of MSS due to low sea state, four images are presented to show the variation of the MSS after aircraft attitude angle correction. The results indicate that the accurate incidence angle of the antenna beam center is critical for retrieving the sea surface MSS, and that the magnitude of the MSS from three cycles of radar echoes can be changed by as much as 40% within 5° of the attitude angles. Furthermore, the MSS becomes more periodic and regular after correction.

Corresponding author address: Dr. Yijun He, School of Marine Science, Nanjing University of Information Science and Technology, 219 Ningliu Road, Nanjing 210044, Jiangsu, China. E-mail: yjhe@nuist.edu.cn
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