# Development of an L-Band SAR Microsatellite Antenna for Earth Observation
## 1 Introduction
### Motivation for Deployable Reflector Antenna  
The SAR antenna is one of the design drivers of SAR platforms. It plays an important role with regards to the final imaging performance, as antenna gain, cross-polarization and side lobe levels (SLL) directly affect the SAR data quality. For the implementation of a small L-band SAR satellite, a lightweight high-gain antenna that occupies low storage space is the main challenge. Such requirements dictate the adoption of deployable antennas. There are promising ongoing developments on deployable inflatable antennas, but difficulties in controlling gas pressure still poses critical challenges with regards to surface accuracy, and thus deemed not suitable for our requirements. Moreover, reflectarray antennas may offer potential as affordable and light SAR antennas, as recently demonstrated in space onboard Isara cubesat mission [5], but still have limitations with regards to antenna aperture sizes. A common choice for SAR antennas is deployable active phased array antennas, as in the 10 m-long L-band antenna onboard ALOS-2 [6]. However, such antenna types are usually heavy, expensive and with complex design.  
On the other hand, reflector antennas are one of the most commonly used solutions for high-gain spacecraft antennas, as they provide high efficiency and can support any polarization. Deployable mesh antenna reflectors have proven to be a venerable product for low frequency communications and radar applications where high gain is required. However, very few scientific missions have successfully adopted deployable reflectors, such as the 6-m large deployable mesh reflector antenna for SMAP (Soil Moisture Active Passive) mission [7; 8], with most being extensively used for commercial or military applications [2; 3]. Up to date, all flown deployable reflectors have been developed for large spacecrafts, which makes scaling down to microsatellite dimensions not possible, since mesh parameters (density, coating, etc.) are driven by the deployment mechanism elements and RF requirements. Table 2 summarizes advantages and drawbacks of various high gain deployable antennas that can be suitable as microsat SAR antennas.  
\begin{table}
\begin{tabular}{c c c} \hline
**Application** & **Detail** \\ \hline \multirow{4}{*}{Land} & \multicolumn{2}{c}{Forest classification} \\  & Land deformation \\  & Paddy field extraction \\  & Metal extraction \\  & Mangrove area mapping \\ \hline \multirow{3}{*}{Ocean} & \multicolumn{2}{c}{ maritime traffic} \\  & oil spill \\  & ocean waves \\ \hline \hline Cryosphere & Icebergs \\ \hline \end{tabular}
\end{table}
Table 1: Remote sensing applications of the L-band microsat mission.  
\begin{table}
\begin{tabular}{c c c c c} \hline \hline  & **Active Phased Array** & **Reflectarray** & **Mesh Reflector** & **Inflatable** \\ \hline \multirow{4}{*}{+} & reconfigurable beam & stowage efficiency & good antenna efficiency & stowage efficiency \\  & controllable SLL & simple deployment mechanism & any polarization & simple deployment \\  & multi-mode scan & low cost & low mass density & large bandwidth \\  & & low SLL & & \\ \hline \multirow{4}{*}{\(-\)} & high cost & low antenna efficiency & medium cost & poor surface accuracy \\  & large stowage & restricted mode & high SLL & high SLL \\  & large mass density & limited antenna aperture & no scan mode & reliability risk \\ \cline{1-1}  & complex design & & complex deployment & \\ \hline \hline \end{tabular}
\end{table}
Table 2: Advantages and disadvantages of potential high gain deployable antenna technology concepts for a 150-kg class microsat SAR.