# Development of an L-Band SAR Microsatellite Antenna for Earth Observation
## 1 Introduction  
### Background on SAR Spacecrafts and Small Satellites  
Synthetic Aperture Radar (SAR) technology has been widely used in Earth observation applications, as it can provide unique information independent of cloud coverage and during night time. With the recent miniaturization of technological components, it is nowadays possible to achieve significantly reduced size of payload and bus systems [1]. Constellations of small satellites carrying scientific or commercial payloads provide fast responses and near real-time ground monitoring [2]. Many such satellites carry small payloads as optical cameras or radio receivers, with spacecraft mass ranging from less than a kilogram to few hundred kilograms. However, when it comes to spaceborne SAR, due to mostly the large antenna sizes and high transmission power required, reducing the total spacecrafts mass becomes a very challenging task. Consequently, the bulky payloads and high costs result in SAR missions being commonly sponsored by governmental space agencies. Therefore, up to this date, the number of compact SAR missions with mass of few hundred kilograms are very limited. Moreover,there is a strong social demand to realize small and affordable SAR satellites for fast responses and all-weather monitoring. Such needs are especially important in the Southeast Asian countries, often covered by clouds, which limits observation with optical satellites.  
Figure 1 depicts past, existing and future spaceborne SAR missions, relating total spacecraft mass and respective ground resolution, for Noise Equivalent Delta Sigma \(\sigma_{NE}^{0}=-20\) dB. For each SAR frequency band, there is a visible barrier in size, performance and, consequently, costs to be able to set up affordable SAR constellations. The miniaturization trend in spaceborne SAR is clearly visible in the high frequency bands, but breaking the ton-sized category is still challenging in the low frequencies, namely in the L-band. While low SAR frequencies may provide unique ground surface scattering information, the very few existing compact spaceborne SAR missions operate in X- or S-bands, as such antennas occupy low volume and are easier to stow. Some examples are the X-band TECSAR (Israel, 300 kg) [3], pioneer in showing that a SAR payload could be flown on smaller satellites, and the recent X-band microsat SAR ICEYE-X1 (70 kg, Finland) [4], launched in early 2018 as a promising proof-of-concept prototype for future planned constellation.  
Currently, Chiba University and the Indonesian National Institute of Aeronautics and Space (LAPAN) are developing a monostatic passive L-band SAR microsatellite for Earth observation, as part of a technology demonstrator for small spaceborne SAR systems. Table 1 describes the considered remote sensing applications covered by the mission. A new concept of a 150-kg-class microsatellite circularly polarized SAR is proposed, composed of a compact L-band SAR payload, allowing for piggy-back launch opportunities for the mission. Main strategies for weight and volume reduction are based on a deployable and lightweight SAR antenna, a compact SAR RF system inherited from an airborne-based system, and the use of a low Earth orbit. A primary goal is to demonstrate basic SAR acquisition, showing that an L-band SAR mission can be performed with a microsatellite, and, therefore, making SAR data more affordable for the global remote sensing communities in the future.  
Figure 1: Trends in miniaturization of past, current and future spaceborne SAR missions: spacecraft mass versus range resolution for \(\sigma_{NE}^{0}=-20\) dB.