Synthetic Aperture Radar (SAR) system operating in squint working mode has high flexibility and can be widely used in many fields. This paper mainly focuses on decoupling and imaging processing of squint stripmap SAR data. A modified Chirp Scaling Algorithm (CSA) based on the Linear Range Walk Correction (LRWC) method is proposed for this squint stripmap imaging mode. By introducing the Azimuth Non-Linear Chirp Scaling (ANLCS) filtering, the azimuth variant Doppler Frequency Modulation Rate (DFMR) can then be equalized. Simulation is carried out to validate the effectiveness and efficiency of the proposed algorithm.

Synthetic Aperture Radar (SAR), an effective investigative instrument, is independent of observation day time or environment. In this paper, our team integrates the SAR system with a train platform intended to surveil the environment around the railway by exploiting the powerful detection capacity of SAR and the high frequencies, low cost (relative to aircraft or satellite) of the train set. The system structure of the train-borne SAR is introduced in this paper, and the imaging algorithm is also illustrated. The results of a practical experiment are presented in the end and validate the effectiveness of this idea.

Squint spaceborne synthetic aperture radar (SAR) can significantly improve the ground observation performance of platform sensor through flexible beam pointing; however, it is still challenging for squint spaceborne sliding spotlight SAR data focusing since the significant range cell migration (RCM) and range azimuth coupling (RAC) caused by the squint angles. Whereas the linear range walk correction (LRWC) can deal with these problems, the traditional chirp scaling (CS) algorithm cannot adapt to the signal model after the LRWC operation, especially in the case of sliding spotlight mode. In this letter, a hybrid CS algorithm (HCSA), integrating the modified CS (MCS) in range time domain with the frequency CS (FCS) and frequency nonlinear CS (FNLCS) in azimuth frequency domain, is proposed for squint sliding spotlight SAR system. After LRWC and deramp operations, the hybrid CS (HCS) based on the updated signal model is employed to fulfill whole scene focusing, which can equalize the Doppler frequency modulation rate (DFMR) and handle the back-folding issue in azimuth time domain simultaneously. The geometric correction is then be applied to obtain the focused SAR image without LRWC geometry distortion. Simulation demonstrates that for the spaceborne SAR with 1.0 m resolution in both range and azimuth directions, and a squint angle of 30°, the proposed HCSA can realize effective focusing within the full scene.

High-resolution and high-squint spaceborne synthetic aperture radar (SAR) system has excellent earth observation performance. However, high-squint spaceborne SAR data is more challenging to process than the general broadside counterpart because of the severe range-azimuth coupling (RAC). Whereas the classic linear range walk correction (LRWC) method can handle this problem, it is performed in azimuth time-domain and seriously constrains azimuth swath width. In this article, a decoupled hybrid correlation Algorithm (DHCA), combining the range-azimuth decoupling (RAD) in 2-D frequency domain with the modified hybrid correlation (MHC), is proposed to handle sliding spotlight spaceborne SAR data of high-squint angle case. The decoupled hybrid correlation (DHC) is the main body of the proposed algorithm, and it starts with RAD filtering in 2-D frequency domain, which is designed to eliminate the majority of the range cell migration (RCM) and RAC caused by high-squint angles. A nonlinear chirp scaling (NLCS) in range direction is subsequently performed to equalize the variant range chirp rate caused by residual RCM and RAC. Coarse range compression can then be realized uniformly by the range-matched filtering. The NLCS operation and range coarse compression ensure that the range-Doppler (RD) signals of all targets within the whole scene occupy very narrow range cell scopes. By making full use of the principle of stationary point (POSP) to signals, the range compression position and range frequency mapping relationship after NLCS can be derived. The MHC can therefore be fulfilled by extracting the RD signals along the residual RCM and constructing the reference correlation function after the range NLCS. Thus, both the efficiency and the accuracy of focusing processing are guaranteed. Simulations are carried out to validate the proposed algorithm.

The capability to work on the high-squint mode brings greatly advantages to the Synthetic Aperture Radar (SAR) system. However, the efficient imaging of the high-squint SAR data has not been well settled. Due to the severe Range-Azimuth Coupling (RAC), the conventional imaging algorithms fail to work properly in the high-squint cases. In this paper, a Decoupled Chirp Scaling Algorithm (DCSA) is proposed for the high-squint SAR data imaging, whose key step is a Range-Azimuth Decoupling (RAD) preprocessing in the 2-D frequency domain. After RAD, the majority of the RAC is eliminated, where the high-squint SAR data is simplified into the quasi-broadside mode. The data can be then processed via the chirp-scaling-typed scheme, which is summarized as follows: The data after RAD is firstly transformed into the Range Doppler (RD) domain, in which a Non-Linear Chirp Scaling (NLCS) is performed in the range direction to eliminate the chirp rate variations caused by the residual RAC. The chirp scaling processing is simultaneously performed to equalize the Range Cell Migrations (RCM) for the full scene data. The range compression, bulk RCM correction, as well as a compensation for the cubic and quartic phases introduced by the NLCS are then accomplished in the 2-D frequency domain. Finally, the azimuth compensation is accomplished in the RD domain to obtain the final focused SAR image. Different from the Linear Range Walk Correction (LRWC) accomplished in the azimuth time domain, the RAD method proposed here does not destroy the azimuth-invariant property of the SAR echoes. Therefore, the DCSA can adapt to the wide swath SAR data imaging. Simulation shows that for the airborne SAR with 1m resolution in both range and azimuth directions, and a squint angle of 45°, the DCSA can achieve the bulk focusing of the full-scene data with a range swath of 10km, whereas the azimuth swath is not limited. Compared with the classical CSA, the DCSA requires only one more range FFT/IFFT and one more complex matrix multiplication, which is therefore timely very efficient.

The Continuously Varying Pulse Repetition Interval (CVPRI) technique is an effective method to acquire Synthetic Aperture Radar (SAR) echo data with significant Range Cell Migration (RCM). The non-uniformity of echo data along the azimuth direction is the primary problem faced in the imaging processing caused by the CVPRI. This paper is focused on the reconstruction of Non-Uniformly Sampled (NUS) signal based on the CVPRI. Then, a modified imaging algorithm combined with the azimuth Non-Uniform Sinc Interpolation (NSI) method is proposed to achieve the NUS echo data focusing. Simulations are conducted to validate the effectiveness of the novel reconstruction method as well as the modified imaging algorithm.

High-resolution and wide-swath (HRWS) imaging is an important research direction of spaceborne SAR. The frequency scanning (F-SCAN) SAR proposed in recent years is an effective way to realize HRWS. Different from transmitting a broad beam in conventional way, the F-SCAN SAR covers wide swath by adjusting the frequency of signal and scanning in the elevation with a pencil beam. This paper focuses on analysis of RASR performance in F-SCAN SAR. Based on the analysis of energy distribution of range ambiguities and parameters of the pencil beam of each sampling point, a RASR analysis method in F-SCAN SAR is proposed. Finally, simulation results are presented to verify the effectiveness of the proposed method.

Azimuth multichannel synthetic aperture radar(SAR) is a form of SAR with one transmitter and several receivers, which is able to obtain both high resolution and wide swath imaging. Azimuth spectrum reconstruction is core operation in the data processing of azimuth multichannel SAR. The spatial variation of Doppler center frequency is non-ignorable in multichannel SAR of squint view, meanwhile, in sliding spotlight SAR imaging mode, the Doppler spectrum of the signal increases greatly due to the rotation of the beam direction, thus, the traditional reconstruction algorithm fails to reconstruct the signal spectrum correctly. To solve above problems, an improved azimuth spectrum reconstruction and imaging processing algorithm for squint multichannel SAR in sliding spotlight mode is proposed. By analyzing the characteristics of multichannel signals in two-dimensional frequency domain, a new spectrum reconstruction matrix is established. Besides, by combining the Deramp operation with the reconstruction algorithm, the Doppler bandwidth caused by the rotation of the beam center is removed. Finally, imaging algorithm of single channel SAR is used to image the reconstructed signal. Simulation experiments verify the effectiveness of the method.

Ultra-high spatial resolution is an important development direction of spaceborne synthetic aperture radar (SAR). The finer the resolution, the more details can be obtained from the image. The range resolution mainly depends on the transmit signal bandwidth. However, due to the limitation of the hardware system, the traditional technologies have difficulties in realizing the wide working bandwidth of the radar. Synthetic bandwidth technology is an effective approach to achieving high range resolution. In actual systems, due to differences in hardware and atmospheric transmission characteristics, there are errors in amplitude, phase, and time delay between the received subband signals. Based on the spaceborne SAR multi-subband system model, this paper analyzes the influence of various errors, and proposes a subband splicing method based on phase gradient autofocus (PGA) and time-domain correlation. The simulation results verify the effectiveness of the algorithm.

With the development and extensive utility of synthetic aperture radar(SAR), SAR systems are required not only to achieve the large-scale imaging but also to continuously stare at focus areas. Accordingly, SAR based on highly elliptical orbit provides a new way for solving these observation problems. This paper presents a method for spaceborne SAR echo simulation in a highly elliptical orbit based on the antenna beam steering control. Due to the effects of the high eccentricity and the apogee of the highly elliptical orbit, squint angle, looking-down angles of antenna beam and other factors are considered to design the beam control of the antenna, so that the satellite can keep illuminating on the earth surface to acquire echo for the whole cycle. In this paper, the proposed method of antenna beam steering implements the SAR echo simulation in highly elliptical orbit. The feasibility of the proposed method was analyzed by the steering control angles' variation characteristics and verified by simulating the lattice targets echo signal and imaging.