# Small-Satellite Synthetic Aperture Radar for Continuous Global Biospheric Monitoring: A Review
## 3 Synthetic Aperture Radar Satellite Missions
### Medium/Large Satellite Constellations
#### 3.1.2 Cosmo-SkyMed
\begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Configuration** & **Average Revisit Time, h** & **Worst Revisit Time, h** \\ \hline
**\# Satellites** & **Left and Right** & **Right** & **Left and Right** & **Right** \\ \hline
1 & 18 to 35 \({}^{1}\) (12 to 23) \({}^{2}\) & 37 to 64 (25 to 44) & 156 (60) & 252 (120) \\
2 & 9 to 18 (6 to 12) & 19 to 35 (13 to 24) & 60 (36) & 108 (60) \\
3 & 6 to 12 (4 to 8) & 13 to 24 (9 to 16) & 36 (36) & 60 (36) \\
4 & 5 to 9 (3 to 6) & 10 to 18 (6 to 12)as illustrated by COSMO-2 and COSMO-3 in Figure 7 (right). The orbit is a sun-synchronous orbit with an inclination of 97.86\({}^{\circ}\), a nominal altitude of 620 km, and the local time of the ascending node (LTAN) at 6:00 (dawn/dust orbit). The radar transmitter and receiver of each COSMO satellite operate through an electrically steerable multi-beam [56]. This active sensing scheme necessitates a COSMO satellite to manage and store its electrical power more efficiently than other passive SAR systems (e.g., SAR-Lupe). The nominal capacity of a COSMO satellite is 336 AH, nearly three times larger than that of a SAR-Lupe satellite. The battery cells constituting a COSMO satellite are much smaller in size, however, with 18-mm-wide, 650-mm-long cylindrical footprint depicted in Figure 8 (left). A total of 2016 lithium-ion cylindrical cells are connected in a 9s224p configuration, meaning that 9 cells are first connected in series and then 224 such series-strings, distributed across eight modules in Figure 8 (right), are connected in parallel [57,58]. This configuration provides a maximum current of 650 A (460 A for the 10-s maximum duration) in Spotlight mode and 450 A (330 A for the 10-min maximum duration) in Stripmap mode. The power consumption of a SAR payload in the two modes is 13 kW and 7 kW, respectively. The power subsystem supports a fully polarimetric SAR (PolSAR) where only one linear polarization is received at each interrogation [59]. Both SAR-Lupe and COSMO-SkyMed constellations reserve the capability of their satellites to reconfigure their orbits. The resulting launch mass or each satellite was a ton or more, however, electric propulsion seems to be an alternative of chemical propulsion for small satellites to achieve miniaturization, as will be discussed later. Still, medium-to-large satellites will continue to provide responsive access to observation targets, complementing small-satellite constellations' wide coverage [52,53]. The COSMO-SkyMed mission demonstrated another way of cost reduction using commercial cylindrical lithium-ion batteries for power storage, which can be applied to small satellites as well. Figure 8: Energy storage of a COSMO satellite: (**left**) Internal structure of a US18650 hard-carbon lithium-ion cell. (**middle**) Battery packs (8) consisting of 2016 identical batteries. (**right**) SAR-Lupe’s 66 Ah battery packs. Source: SONY, ABSL, COM DEV Ltd. [57]. Figure 7: Orbital configuration of the COSMO-SkyMed constellation: (**left**) Tandem configuration. (**right**) Tandem-like configuration. Source: ASI.