# Small-Satellite Synthetic Aperture Radar for Continuous Global Biospheric Monitoring: A Review
## 6 Conclusions  
In Earth remote sensing, it is often useful to understand the data generation flows from the very beginning, satellites in this case. The SAR instruments onboard the spaceborne platforms, in particular, have been following a relatively simple flight heritage dating back to Seasat and ERS satellites (Section 1). New space missions represented by small satellites, however, are driving cost reduction  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline \multirow{2}{*}{**Mission**} & **Cost/kg** & **Sat Mass** & **Sat Cost** & **Launch Cost** & **Launch Vehicle** & **Ground** \\  & **(kUSD)** & **(kg)** & **(MUSD)** & **(\%)** & **Launch Vehicle** & **Station** \\ \hline ICEYE & 82 & 85 & 7 & 42 & Falcon 9, Electron & KSAT \\ SAR-Lupe & 117 & 770 & 90 & 24 & Kosmos & DLR \\ TSX/TDX & 143 & 1230 & 117 & 20 & Dnepr & DLR \\ NovaSAR & 146 & 400 & 59 & 23 & PSLV & KSAT \\ Capella & 163 & 48 & 8 & 21 & Falcon 9, PSLV, Electron & AWS \\ X-SAR & & & & & & \\ Cosmo-SkyMed & 214 & 1700 & 364 & 13 & Soyuz & KSAT \\ \hline \hline \end{tabular}
\end{table}
Table 10: NovaSAR observation modes and 1-satellite/3-satellite revisit performance [41].  
and changing the design paradigm of space-based SAR (Section 2) [158]. The deployment of several SAR constellations will ensue from the deployment experience of optical satellite constellations and, at the same time, will be closely followed by the deployment of \"mega-constellations\" by space-internet service providers. Therefore, it would be advisable for the users of satellite imagery data to understand the ever-increasing environmental complexity in the low-Earth orbits and its potential ramifications such as in-orbit collisions or electromagnetic interference (EMI). The Starlink constellation proposed by SpaceX, for example, will consist of 4425 satellites during Phase 1, operating in 10.7-12.7 GHz, 13.85-14.5 GHz, 17.8-18.6 GHz, 18.8-19.3 GHz, 27.5-29.1 GHz, and 29.5-30 GHz partially overlapping with the Ka band [159].  
This paper reviewed some of the medium-to-large SAR platforms that were launched in constellations (Section 3). These SAR constellations achieved technological maturity not only in the individual satellite design but also in the constellation management, as testified by ten years of successful operations of Cosmo-SkyMed [160]. Minisatellites such as NovaSAR-1 have driven the satellite size down to the extent of being able to launch multiple SAR satellites at once. After exploring the utmost miniaturization limits, design iterations for SAR microsatellites are still underway. The advent of big data from SAR satellites creates new opportunities in the field of biospheric monitoring (Sections 4 and 5).  
**Author Contributions:** [PERSON] conceived the scope, surveyed SAR missions, and wrote overviews in corresponding sections; [PERSON] helped to survey SAR missions, drawing figures, and writing the section for SAR design principles; [PERSON] assisted in writing paragraphs regarding power electronics and storage; [PERSON] helped writing paragraphs for satellites orbits and constellations. All authors have read and agreed to the published version of the manuscript.  
**Funding:** This research received no external funding.  
**Acknowledgments:** The first author appreciates helpful discussions with [PERSON] at the Korea Aerospace Research Institute.  
**Conflicts of Interest:** The authors declare no conflict of interest.