# High-Temporal-Resolution High-Spatial-Resolution Spaceborne SAR Based on Continuously Varying PRF
## 1 Introduction  
The Synthetic Aperture Radar (SAR) is a well-established and efficient imaging technique for acquiring high-spatial-resolution (generally called high-resolution), wide-swath images of the Earth's surface due to its all-time and all-weather imaging ability. Since the first civilian spaceborne SAR, Seasat, was launched in 1978 [1], significant progress has been made in this area. As listed in Table 1, with the launch of the SAR satellites Radarsat-2, TerraSAR-X, TanDEM-X, Sentinel-1a and ALOS-2, the resolution of spaceborne SAR has been upgraded from tens of meters to the meter region [2; 3; 4; 5; 6; 7; 8; 9; 10]. By employing beam steering techniques, TerraSAR-X Next Generation (TerraSAR-X NG) system willachieve a resolution of up to 0.25 meters, thereby being capable of identifying vehicles or objects of a similar size [11]. In addition to the strengthened high-resolution and wide-swath capability, high-temporal-resolution (short repeat-observation interval) would be another key capability for numerous applications [12]. It is necessary to point out that repeat-observation interval means the temporal gap between acquisitions of two images of the same area. Currently, on-orbit SAR systems can provide products with repeat-observation intervals varying from hours to a day, over a day to weeks by employing satellite constellations or left-and-right looking operation. For example, the images obtained by Radarsat-1/2 have been used for monitoring seasonally or permanently ice-covered ocean regions in the Arctic Ocean [13]. Moreover, the COSMO-SkyMed constellation has provided a sequence of 7 strippap images with one day repeat-observation interval to monitor the flooding of the Severn and Avon rivers [14]. [PERSON] and his colleagues from the German Aerospace Center (DLR) have noted that users request time series of high-resolution radar images that are acquired using repetition intervals that are as short as possible to study dynamic processes on the Earth's surface [15]. Additionally, in 2001, [PERSON] and [PERSON] of Jet Propulsion Laboratory (JPL) proposed that the fine temporal sampling or time series of rapidly evolving phenomena would be essential for disaster management for, e.g., flooding, fires, landslides, hurricanes, and earthquakes [12]. Hence, products with high-temporal-resolution are predicted to play a significant role in urgent applications, although current spaceborne systems are proving to be limited in regard to their acquisition capability. The TerraSAR-X and TanDEM-X constellation, with an along-track separation of approximately 3 s, has demonstrated the feasibility of using multiple platforms for the purpose of short repeat-observation interval products, regardless of economic costs [16]. Moreover, geosynchronous SAR would be another potential technique due to its short repeat period. In general, the acquisition capability of SAR systems is limited based on their maximum squint angle (here, the squint angle is defined as the deviation from the broadside). Hence, we propose a novel high-squint-angle system intended to acquire short repeat-observation interval and high-resolution Spaceborne SAR images with a single pass and a single platform. In addition to applications with high-temporal-resolution, which will mainly benefit from the strengthened squint illumination capability, large area applications will also be supported:  
**Comprehensive information analysis of targets**, based on the higher resolution, separate angle and shorter repeat-observation interval images provided by high-squint SAR systems, will enable more precise recognition and identification. **Higher quality SAR images**, with reduced effects from speckle, which can be reduced through the use of multi-look images, can be obtained because high-resolution and short repeat-observation interval images can reduce speckle efficiently and without a loss of resolution. Two key factors should be considered in the proposed concept.