# Small-Satellite Synthetic Aperture Radar for Continuous Global Biospheric Monitoring: A Review
## 1 Introduction
Note that the atmosphere is not included in Table 2 because the purpose of radar observation is to minimize the weather influence on its data; ocean wind, for example, is covered by hydrosphere in the context of its surface interactions. Biosheric monitoring herein involves efforts to apply SAR data to agriculture, forestry, and the studies of grasslands and wetlands [26; 27]. When it comes to the interactions between the biosphere and the anthroposphere, SAR data could benefit urban management, resource exploration, biomass monitoring, vulnerability studies (e.g., diseases) to name a few as will be seen in Section 4 [28; 29; 30; 31; 32]. The vulnerability studies may also involve interactions between anthroposphere and other realms such as geological events leading to disasters, which again affect biosphere [33]. This trend testifies the newly increasing importance of biospheric monitoring for the already mature field of SAR-based Earth remote sensing. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Category** & **Application** & \multicolumn{3}{c}{**Temporal Scale**} \\ \hline  & & **Days** & **Weeks** & **Months** & **Years** \\ \hline Biosphere & Deforestation or fire & O & O & O & \\  & Biodiversity & & O & O & O \\ Cryosphere & Sea ice & O & O & O & \\  & Ice cap and glaciers & & O & O & \\ Geosphere & Volcanic activities & O & O & & \\  & Seismic activities & O & O & O & O \\  & landslides & O & O & & \\ Hydrosphere & Floods and soil moisture & O & O & & \\  & Ocean currents and tides & & & O & O \\ \hline \hline \end{tabular}
\end{table} 
Table 2: Areas of research utilizing SAR data. Source: German Aerospace Center (DLR) [25]. Figure 1: Schematic view of synthetic aperture radar (SAR) onboard European Remote-Sensing Satellite (ERS): (**left**) SAR Wave Mode. (**right**) Switching between SAR Wave mode and SAR Image mode along with their ground swaths. Source: ESA. Hitherto, spaceborne SAR instruments have been flown onboard monolithic satellites whose mass is a few tons. On the other side of monolith satellites is the category of small satellites. A small satellite (\"Smallsat\") weighs less than 500 kg and may be called by other names according to its mass, as listed in Figure 2[34; 35]. Small satellites have become an active area of research as the CubeSat platform was standardized by California Polytechnic State University and Stanford University in 1999 [36]. The CubeSat platform consists of multiple units (\"U\" s), which is a \(10\times 10\times 10\) cm cubit volume and is therefore also called \"U-class\" spacecraft. In terms of mass, a 1 to 3U CubeSat weighs between 1-4 kg, falling into the nanosatellite regime in Figure 2[37]. Although some of the optical sensors have reached the extent of miniaturization as well as commercialization to fit into a 3U class (e.g., Planet) that has a size of a shoebox, most of the advanced sensors featuring multiple bandwidths require larger volumes, i.e., 6U or more [38; 39]. Due to its lower operational frequency (longer wavelength), a radar antenna should be larger than satellite payloads that operate in visible, infrared, or microwave spectrums. The radar system also requires higher power throughputs than optical payloads because they must transmit electromagnetic pulses first to the ground before receiving the backscattered echoes [8; 40]. Despite these challenges and constraints, there are several mission plans and startup initiatives for Earth observation missions with SAR-equipped small satellites, which are driven by the aforementioned scientific needs as well as performance-cost advantages of Smallsat constellations [41; 42]. Biosheric monitoring is more explicitly manifested as a scientific objective of large SAR satellites, and the same is expected in Smallsat constellations as well [26; 43; 44]. The organization of this paper is as follows: Section 2 briefly introduces SAR design principles. Section 3 investigates current or planned small satellite missions, paying extra attention to their operational details and constellation design if applicable.