# A Novel Spaceborne Sliding Spotlight Range Sweep Synthetic Aperture Radar: System and Imaging
## 1 Introduction
Due to the spatial variation of the CBSR, the SSS-RSSAR is characterized by non-uniform azimuth sampling, where the azimuth dependent PI array can be conveniently described by a fifth-order polynomial. Due to the perpendicular-to-BIS CBP and its resulted large squint geometry, the echo is to suffer from serious 2D coupling and therefore, the imaging algorithm applied for the SSS-RSSAR should be carefully chosen. The basic Back Projection Algorithm (BPA) is not preferred due to its cost-inefficient \(O\) (\(N^{3}\)) computational load [19]. The frequency domain algorithms that can be used for large-squint  
Figure 1: Data acquisition of: (**a**) conventional spaceborne Synthetic Aperture Radar (SAR); and (**b**) Spaceborne Sliding Spotlight Range Sweep Synthetic Aperture Radar (SSS-RSSAR). imaging, such as the Range Doppler Algorithm (RDA), Chirp Scaling Algorithm (CSA) and Range Migration Algorithm (RMA) [20; 21; 22], are all not ideal choices. This is because, if processed by these algorithms, the slant range that has been counteracted by the CVPI technique should be added back to the echo by using the zero padding, resulting in a much larger total data amount. The Polar Format Algorithm (PFA) seems to be the only frequency domain based algorithm that is capable of implementing a fast processing speed and a high data utilization efficiency [3; 23]. However, due to the non-uniform azimuth sampling caused by the CVPI technique, the PFA cannot be directly applied for the SSS-RSSAR imaging. In this paper, a Modified PFA (MPFA) is proposed to focus the SSS-RSSAR's echo received with the CVPI technique. The main modifications include an updated 2D dechirp filter and an interpolation-based data format correction based on a new wavenumber expression with respect to the CVPI technique. Based on the CVPI technique and the MPFA imaging method, this study will further discuss the parameter design methods for the SSS-RSSAR. Firstly, based on the analyses for the least required range sample number, an optimized pulse width can be achieved. Then, based on the discussion on the least required azimuth sample, the referential pulse interval used for initiating the non-uniform PI array generation can be calculated. Generally speaking, the carrier frequency and the chirp rate are both constant during the whole DAP. For the SSS-RSSAR, however, the constant carrier frequency and chirp rate will lead to serious spatial variation of the wavenumber and therefore, the range wavenumber bandwidth after the MPFA processing will be seriously compressed, leading to a much decreased range resolution. To overcome this shortage, a new Parameter-Adjusting (PA) technique is proposed to mitigate the range wavenumber variation by adjusting the carrier frequency and the chirp rate pulse by pulse based on the instant data acquisition geometry. The methods of designing the sliding spotlight factor and the DAP are also introduced. Finally, an integrate parameter design flow for the SSS-RSSAR is provided. The advantages of the SSS-RSSAR over the traditional Spaceborne Sliding Spotlight SAR (SSS-SAR) are also analyzed, mainly from two aspects: the total data amount and the total computational load. These analyses are made based on the assumption that both the SSS-RSSAR and the traditional SSS-SAR have perpendicular-to-BIS CBP. Based on the total data amount, the computational loads of the algorithms, including the MPFA, RDA, CSA and RMA, are compared. While the computational load of the MPFA is derived based on the total data amount collected from the tilted along-ROI BIS, the derivation of the computational loads of the RDA, CSA and RMA, on the other hand, are made based on the total data amount with respect to the parallel-to-orbit BIS. The reasons of choosing different data sets for the computational load comparison are: firstly, except for the MPFA, the echo of the SSS-RSSAR with the parameters specially designed based on the MPFA cannot be focused by the other algorithms.