Hardware Implementation Design for An Efficient On-board Imaging Processing System in Spaceborne SAR
On-board Synthetic Aperture Radar(SAR)imaging technology is a key tool for time-sensitive remote sensing applications such as disaster monitoring and military reconnaissance.The typical construction of an on-board SAR imag-ing system using a Field-Programmable Gate Array(FPGA)coupled with a Digital Signal Processor(DSP)as the pro-cessing core is well-regarded for its heterogeneous computing power,energy efficiency,and flexibility.However,cur-rent FPGA+DSP systems are still under-researched in terms of algorithm support,large-grain processing,on-chip paral-lel acceleration,and complex matrix transposition,with significant room for performance enhancement.This paper analyses the Nonlinear Chirp Scaling(NCS)algorithm,suitable for large squint,high-resolution imaging,and parti-tions the algorithm into main and auxiliary paths based on computational complexity and type.A heterogeneous mapping scheme for the NCS algorithm within the FPGA+DSP system is subsequently proposed.To address the transformation of data storage formats post multi-channel Fast Fourier Transform(FFT)processing,which complicates pipelined process-ing,this work introduces a multi-channel FFT collaborative method based on time-frequency extraction switching to en-sure efficient parallel FFT operations.Additionally,to handle the complexity of transpose requirements across various granularities and parallelisms,a universal cross transpose solution using X-Direct Memory Access(XDMA)and on-chip segmented transposition is developed.Employing two VX690T FPGAs and two FT 6678 DSPs as core processors,this paper presents the development of an on-board SAR imaging card that implements the proposed system design.Moreover,a validation environment using simulated sources plus ground testing is established,processing simulation array data for strip/scan/spotlight/sliding spotlight/TOPS modes and actual data for strip/sliding spotlight modes.The ar-ray data demonstrates a two-dimensional peak sidelobe ratio of about-13.2 dB and an integrated sidelobe ratio of around-10.1 dB,indicative of good imaging quality.For instance,in strip mode with an image size of 32K×16K,the average imaging time using the NCS algorithm is 7.81 seconds,markedly accelerating imaging speed compared to existing approaches.
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