FY-3B/C中分辨率光谱成像仪在轨初期传递函数的评价和比较

针对极轨气象卫星的特点,提出了利用极地冰雪目标进行风云(FY)卫星中分辨率光谱成像仪(MERSI)传递函数(MTF)测试的刃边方法。该方法沿着冰雪和海洋之间的边缘进行取样,然后将获得的点扩展函数经过傅里叶变换得到最后Nyquist频率点处的MTF值。该方法能够应用于绝大多数MERSI通道。MTF测试结果表明:在轨初期,FY-3B/C-MERSI图像在卫星飞行方向的MTF结果都在0.31左右,图像质量较高,能够满足设计指标。但两颗卫星的MERSI传感器图像空间频率特性沿扫描方向都不是很好,1 000m分辨率的FY-3C图像测试结果集中在0.12左右,FY-3B集中在0.10左右,且FY-3C星结果系统偏高于B星,B星250m分辨率的测试结果只有0.07。分析认为导致这种图像频率特征质量在扫描方向严重下降的主要原因是设计MERSI时为保证信噪比保留的像元重叠扫描,建议在下一代FY-3号气象卫星设计时考虑这种重叠扫描的处理。

Evaluation and comparison of modulation transfer function for FY-3B/C MERSI on early orbit

In consideration of the characteristics of polar-orbiting satellites, an edge Modulation Transfer Function(MTF) test method using ice and snow targets in polar region is proposed for the China polar-orbit meteorological satellite FengYun (FY) 3-MEdium Resolution Spectral Imager (MERSI, 20 bands, with 250 m and 1 000 m resolution). It takes samples from the edge between ice block and sea to get the Point Spread Function (PSF). Then, the Fast Fourier Transform (FFT) method is used to calculate the PSF and retrieve MTF values at the Nyquist frequency point. This method is available to the most of MERSI bands. The experiment results for MTF measurement indicate that the MTFs of all bands along FY-3B/C flight direction are about 0.31 (>design index 0.27/0.25) with high spatial frequency feature quality. However, the spatial frequency feature qualities along MERSI scan direction are bad for both FY-3B and 3C satellites. The MTFs at 1.0 km resolution of FY-3B/C images are respectively approximate 0.10 and 0.12, which also show that the systematic results of FY-3C are higher than that of the FY-3B. The MTFs of FY-3B/MERSI are even just around 0.07 at 250 m resolution. This finding may be primarily attributed to 27% overlapped scanning in the original design of FY-3/MERSI imaging system to enhance the signal-to-noise ratio (SNR) of images. It suggests that the overlapped scanning should be processed in design of the next generation FY-3 atmospheric satellite.