# HISEA-1: The First C-Band SAR Miniaturized Satellite for Ocean and Coastal Observation
## 1 Introduction  
As small satellites and constellations develop rapidly, the new era of Synthetic Aperture Radar (SAR) satellites evolves. Using a low-cost and high-performance CubeSat and small satellite constellation, faster, more frequent, and more flexible data could be acquired. Using microwaves instead of light waves, the SAR signal is able to penetrate cloud and work at nighttime. The technology of pulse compression and synthetic aperture significantly improve the resolution of SAR images. As a result, two-dimensional (2D), all-weather, and all-day high-precision images could be obtained. Moreover, the C-band has been used comprehensively in ocean remote sensing, due to its phase that is convenient for ocean observations. There are many C-band sensors for ocean observation, such as Gaofen-3, Sentinel-1, and Radarsat-1/2, which can observe sea surface winds, sea ice, and ocean waves [1; 2; 3; 4; 5; 6; 7; 8; 9; 10; 11]. With the above background, the first C-band SAR ocean remote-sensing small satellite, named HISEA-1, was launched by the new Long March 8 medium-lift rocket from the coastal Wenchang launch site on 22 December 2020. With a weight of only 185 kg, it can provide high-quality SAR remote-sensed images whose finest spatial resolution and maximum observation width reach 1 m and 100 km, respectively. To introduce more concretely, the specifications of HISEA-1 are shown below. First, the working orbit of HISEA-1 is a sun-synchronous circular orbit with a height of 512 km and an inclination of about 97\({}^{\circ}\), and the mean local solar time is 12:00 (UTC +8:00). The shooting position can also be adjusted flexibly according to the mission; theglobal visiting rate may be up to 78.2% for three days, 95.5% for six days, and 99.78% for ten days. As an example, it observed 8 times within one month based on the simulation results. The attitude of the platform could also be adjusted accordingly, with the attitude control accuracy being 0.03\({}^{\circ}\) and the attitude stability being 0.002 \({}^{\circ}\)/s. As C-band SAR, the center frequency of Hisea-1 is 5.4 GHz, and the phased-array antenna has a size of 4.0 m \(\times\) 0.64 m (azimuth \(\times\) range). Second, the payload of HISEA-1 has three imaging modes: striping, spotlight, and scanning modes (Table 1). The incident angle is from 20\({}^{\circ}\) to 35\({}^{\circ}\) and the polarization is single VV (vertical transmit and vertical receive) polarization, which is vertical transmission and vertical reception. The spatial resolution and observation width are 3 m and 20 km respectively in striping mode, while the spatial resolution can reach as high as 1 m (azimuth) in spotlight mode. The spatial resolution and observation width of scanning mode are 10 m and 50 km, or in wide mode, 20 m and 100 km. As a comparison, Gaofen-3 SAR satellite has 12 image modes and more polarizations by virtue of the larger size (2750 kg to 180 kg), while at a cost of much higher power (15 kW to 2.5 kW) [4; 9; 10; 11; 12]. The small size and low power consumption largely lower the cost of each satellite, making it more suitable for a satellite constellation project, which is being planning and is just starting with HISEA-1. The parameters of the platform and payload preliminarily exhibit the observation ability of HISEA-1 in terms of hardware. The first batch of high-definition (HD) images was received in the first week. External calibrations were made based on 13 Zhongwei Remote Sensing Satellite Calibration Field images (Ningxia, China); the geometric positioning, radiation accuracy and image quality of HISEA-1 were validated. The system sensitivity was tested through ten Amazon rainforest images.