# A High-Resolution, Wide-Swath SAR Imaging System Based on Tandem SAR Satellites
## 1 Introduction  
The synthetic aperture radar (SAR) is a powerful sensor that can observe the Earth's surface day and night, regardless of the weather conditions [1; 2; 3]. This sensor has been widely adopted in the military and civilian remote sensing areas, such as oceanographic observations, sea-ice monitoring, and vegetation mapping [4; 5; 6]. For future SAR development, high-resolution wide-swath (HRWS) technology plays an important role in these applications. However, the high resolution and the wide swath contradict each other due to the minimum antenna area constraint [7]. On the one hand, the high resolution requires a high-pulse repetition frequency (PRF) to suppress the Doppler ambiguity. At the same time, the wide swath needs a low PRF to avoid range ambiguity. Therefore, HRWS cannot be obtained in a monostatic SAR system simultaneously [8].  
For achieving monostatic HRWS-SAR imaging, researchers proposed many methods, which can be divided into two categories: 1. The first approach focuses on the range ambiguity issue, where a sufficiently high PRF is selected to avoid the Doppler ambiguity. The frequency diversity array (FDA) technique is proposed in [9]. It introduces a small increment across the all array elements, then the echoes received from different ambiguous regions are separated. Moreover, the multiple-elevation-beam technology is adopted in [10]. It employs real-time beamforming on receipt to form multiple narrow elevation beams, each following the radar echo of a different transmitted pulse; thus, the range ambiguity is suppressed. 2. The second approach is to cope with the Doppler ambiguity issue, where a low PRF is chosen to avoid range ambiguity. For example, the azimuth multichannel technology compensates for the lack of temporal sampling by the spatial sampling of points of multiple channels [11]. Thus, the effective sampling rate is increased, and the Doppler ambiguity is suppressed [12]. Those technologies mentioned above can effectively improve the width of the swath while ensuring the azimuth resolution for monostatic SAR. However, they all have a complex structure, leading to greater challenges in designing a large satellite. To reduce hardware complexity [13; 14; 15], separates the transmitter and receiver of SAR to a multi-satellite network, but the increased launch volume and mass for the deployment of multiple satellites and additional hardware for accurate phase synchronization are indispensable [16]. Furthermore, [17] develops a monostatic SAR\(-\)HRWS imaging systembased on compressed sensing (CS) theory, but its imaging performance depends on the sparsity of the observed scene.  
In order to achieve HRWS-SAR imaging, this paper proposes a novel imaging system based on tandem SAR satellites. The first one works in the scanning mode to obtain the coarse observations of a wide swath. The second one obtains the downsampled echo from the same swath. Then, the high-resolution imaging is achieved by associating the tandem SARs' echo and using the minimum-energy-based algorithm. Since the precise phase synchronization between two satellites is unnecessary, the system cost can be reduced.  
This paper is organized as follows: Section 2 introduces the working modes of the two satellites. Section 3 constructs the signal model and describes the minimum-energy-based algorithm. Section 4 designs an example system and verifies its imaging performance. Finally, Section 5 concludes this paper.