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Performance Investigation on Elevation Cascaded Digital Beamforming for Multidimensional Waveform Encoding SAR Imaging
DOI: 10.12000/JR20107
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摘要: 在采用多维波形编码(MWE)技术的新体制合成孔径雷达(SAR)系统中，利用俯仰向的数字波束形成(DBF)来实现多个发射波形重叠回波的可靠分离是一个关键问题。该文详细研究了星上实时波束控制与地面后置零陷控制相结合的混合俯仰向DBF分离方法的成像性能问题。 作为一种包括星地两级DBF网络的级联结构，其中的星上部分通过在划分的多个天线子孔径上实现实时主瓣波束指向控制，确保在整个测绘上足够的信号接收增益; 而后置自适应DBF网络主要完成零陷抑制的任务，以消除其它发射波形带来的距离向干扰，可自适应于地形高度起伏变化带来的视角变化。根据对发射波形时频结构先验信息的利用情况，提出了两种星上实时波束形成器的实现方式。对混合DBF方法下的成像信噪比和模糊度性能进行了理论建模与仿真实验评估。实验结果表明，混合DBF方法可以为优化图像模糊度和信噪比性能提供额外的设计自由度，并降低星上数据通道数目。与纯地面DBF网络相比，采用混合DBF网络可以在显著减少星上输出数据量的同时获得满意的性能，在给出的实例中，实现相近性能的条件下，相应的星上数据通道数从10个减少到6个。Abstract: An important issue in a Synthetic Aperture Radar (SAR) system employing Multidimensional Waveform Encoding (MWE) is the fulfillments of Digital BeamForming (DBF) on receive in elevation for a reliable separation of the mutually overlapped echoes from multiple transmit waveforms. In this paper, the performance of a separation approach employing hybrid DBF in elevation by combining the onboard real-time beam-steering and a posteriori null-steering DBF on the ground is elaborately investigated. As a cascaded structure which comprises two subsequent DBF networks, the onboard part effectuates the steering of the mainlobes within multiple partitioned groups of antenna elements to ensure sufficient signal receive gain over the whole swath; the a posteriori adaptive DBF network on the ground mainly performs the task of placing nulls to cancel the range interference from other transmit waveforms, which enables adaptive beamforming to avoid the topographic height variation problem. Two type of onboard realtime beamformers are investigated, depending on the utilization of the transmit waveform structure information or not. The performance of the hybrid DBF approach is theoretically analyzed and evaluated in simulation experiment. It is shown that the hybrid DBF approach can provide additional dimensions of the trade-space to optimize the performance on range ambiguity suppression and signal-to-noise ratio improvement, as well as the onboard data volume reduction. In comparison with the a posteriori DBF on the ground, employing the hybrid DBF networks can get satisfactory performance while remarkably reducing the output data volume, in the presented example, the corresponding output channel number is decreased from 10 to 6.
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Figure 3. Cascaded DBF networks in elevation performing the SNR-preserving and interferences-free signal separation
Figure 4. Number of real multiplications per second of the proposed two types of onboard DBF processing and the reference beamforming processing given in Ref. [8] (N=5 and P= 8)
Figure 5. The array patterns of onboard Type-A beamforming at instantaneous time
$t = {\tau _{\rm{c}}}$ 
Figure 6. Amplitude of the distortion functions
$\mu _m^{(0)}(t)$ in the extent of each of 4 subpulses
Figure 7. Performance comparison on RASR between the cascaded hybrid Type-A DBF networks and ground DBF in Ref. [8]
Figure 8. Performance comparison on SNR between the cascaded hybrid Type-A DBF networks and ground DBF in Ref. [8]
Figure 9. The array patterns of onboard Type-B beamforming at instantaneous time
$t = {\tau _{\rm{c}}}$ 
Figure 10. Performance comparison on RASR between the cascaded hybrid Type-B DBF networks and ground DBF in Ref. [8]
Figure 11. Performance comparison on SNR between the cascaded hybrid Type-B DBF networks and ground DBF in Ref. [8]
Figure 12. Across comparison of performance on RASR and SNR between the onboard transmit signal-structure-dependent beamformers (Type-B) and the non-signal-dependent ones (Type-A)
Figure 13. Performance on RASR and SNR for Type-A and Type-B SNR-preferred beamformers with an increased element spacing
Figure 14. Performance on RASR and SNR of the cascaded DBF networks under the DOA mismatch condition in the presence of topographic height error, in comparison with the onboard real-time null-steering DBF in Ref. [7]
Table 1. Parameters used in the system simulation[8]
Parameter Value Parameter Value Wave length 0.031 m Number of subpulses 4 Swath width 100 km PRF 1310 Hz Off-nadir angle 18o～24o Azimuth subapertures 4 Azimuth resolution 1.5 m Antenna length 10.8 m Band width 250 MHz Antenna height 2.33 m Ground range resolution at center 1.5 m Processed Doppler bandwidth 4890 Hz Onboard elevation channel number 6 Azimuth ambiguity to signal ratio –30 dB Orbital altitude 800 km Subpulse duration/ interval 40 μs 
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