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Signal Processor For Scatterometer Radar Onboard Oceansat-2 Satellite

机译：散射仪雷达信号处理器船上海洋 - 2卫星

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Considering the enormous potential and the encouraging results obtained from ISRO's dedicated mission for oceanographic applications, Oceansat-1 launched in 1999, Space Applications Centre (SAC), ISRO has subsequently initiated the development of a new active microwave radar sensor viz. a Ku-band pencil beam scanning Scatterometer for its follow-on Oceansat-2 satellite mission. OCEANSAT-2 is configured around IRS bus and will be launched through ISRO's PSLV launcher, in 2006 A.D. This spaceborne Scatterometer will help in the estimation of radar backscattered power, sigma deg values for cell resolution grids of 50 Kms X 50 Kms over the complete ocean swath of 1400 Kms. and subsequent local and global wind vector retrieval. In a pencil beam scanning scatterometer, LFM transmit Modulation scheme is used to achieve better measurement accuracy and better range resolution. A Doppler shift within +-550 KHz range is imparted to echo return signal due to the relative motion of satellite and the earth and depending on the antenna scan position, earth rotation effects, satellite orbit parameters and spacecraft velocity. This Doppler shift can be compensated either in Transmitter by tuning the transmit carrier frequency, or in receiver by employing Doppler tracking filters. Considering the onboard storage and downlink data rate constraints of satellite, we have implemented an alternative configuration, which employs Real-Time onboard complex digital signal processing to carry out this Doppler compensation, with +-5 KHz accuracy and subsequent simultaneous estimation of signal+noise and noise-only energy for the various resolution cells over the total swath. The onboard signal processor performs real-time computation of Doppler shift frequency for the received echo signal and also carries out Doppler compensation by heterodyning and subsequent Deramping of the digitized data. The reference chirp signal for deramping is also generated in real time using Direct Digital Chirp Synthesis (DDCS) techniques. The subsequent signal processing is carried out either in time or frequency domains, and will reduce the maximum instrument data rate from 13 Mbps to about 256 Kbps. The frequency domain approach involves multiple Fast Fourier Transforms (FFT) of input data, while the time domain method involves multi-rate low pass filtering to extract signal around the Doppler shift frequency. The signal+noise energy is estimated for the processing bandwidth of 250 KHz for different resolution cells by performing binning over the 8 Km wide slices every PRF, while the noise-only energy is estimated separately by suitable digital filtering over 675 KHz bands, in lower and upper noise spectrum. These energy estimates are subsequently utilized in ground based processing for finding ocean surface wind speed and direction. Simulation studies have been carried out to evaluate various time and frequency domain processing algorithms for Range compression. Presently, the signal processor benchmarking is under process for various constituent modules for different approaches. The implementation approach will be finalized considering the application requirements like precision, memory storage, FFT frequency resolution etc. A prototype Data Acquisition and range Compression System (DACS), based on AD9054 digitiser and Xilinx Virtex XCV600 Field Programmable Gate Array (FPGA), has been designed, fabricated and tested. DACS has to cater to data acquisition, digital I/Q demodulation and the required range compression algorithm. A single channel I/Q demodulator involving IF sampling has been implemented with an 8-bit digitizer, Analog Devices, AD9054 with sampling frequency (62.5 MHz) of four times the IF frequency, as it offers better signal fidelity for low bandwidth signals and also reduces onboard analog and RF hardware. For onboard real time signal processing, FPGA based hardware solution was chosen over DSP or ASIC implementation in view of its higher performance, flex
机译：考虑到巨大的潜力和从ISRO专门的海洋应用任务获得的令人鼓舞的结果，Oceansat-1于1999年推出，空间应用中心（SAC），ISRO随后发起了一种新型活跃的微波雷达传感器Viz的开发。 KU带铅笔梁扫描散射仪，其后续海洋卫星卫星使命。 Oceansat-2围绕IRS总线配置，并将通过ISRO的PSLV发射器推出，2006年广告可帮助雷达反向散射电源，在整个海洋中为50公里x 50公里的细胞分辨率网格估计雷达反向散射电力，Sigma Deg值为50公里的x 50公里SWATH为1400公里。随后的本地和全球风向器检索。在铅笔束扫描散射仪中，使用LFM发射调制方案来实现更好的测量精度和更好的范围分辨率。由于卫星和地球的相对运动，并且根据天线扫描位置，地旋转效果，卫星轨道参数和航天器速度，因此赋予+ -550 kHz范围内的多普勒频移到回波返回信号。通过采用多普勒跟踪滤波器，可以通过调谐发射载波频率或接收器中的发射器来补偿该多普勒频移。考虑到卫星的板载存储和下行数据速率约束，我们已经实现了一种替代配置，它采用实时的车载复杂数字信号处理来执行该多普勒补偿，具有+ -5 kHz的精度和随后的信号+噪声估计。和噪声仅用于总条带上的各种分辨率细胞的能量。车载信号处理器对接收的回声信号执行多普勒换档频率的实时计算，并且还通过不同的存储和随后的数字化数据进行多普勒补偿。用于扩展的参考Chirp信号也使用直接数字啁啾合成（DDC）技术实时生成。随后的信号处理在时间或频域中执行，并且将从13 Mbps降低到大约256kbps的最大仪器数据速率。频域方法涉及输入数据的多个快速傅里叶变换（FFT），而时域方法涉及多速率低通滤波以提取多普勒换档频率周围的信号。通过每PRF在8km宽切片上执行Xinning，估计信号+噪声能量为250kHz的处理带宽，而仅通过675 kHz频段分开估计噪声能量，较低和上噪声谱。随后在基于地面的处理中用于寻找海洋表面风速和方向的这些能量估计。已经进行了仿真研究以评估范围压缩的各种时间和频域处理算法。目前，对于不同方法的各种组成模块，正在处理信号处理器基准测试。将最终确定实施方法，考虑到精度，内存存储，FFT频率分辨率等的应用要求等。一种原型数据采集和范围压缩系统（DACS），基于AD9054 DigIender和Xilinx Virtex XCV600现场可编程门阵列（FPGA），具有设计，制造和测试。 DAC必须迎合数据采集，数字I / Q解调和所需的范围压缩算法。涉及采样的单个通道I / Q解调器，如果采样已经用8位数字转换器，模拟设备，AD9054，采样频率（62.5 MHz）为IF频率的四倍，因为它为低带宽信号提供了更好的信号保真度，也是如此减少板载模拟和射频硬件。对于车载实时信号处理，考虑到其更高的性能，FLEX，FPGA基于FPGA的硬件解决方案选择了DSP或ASIC实现

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