# Small-Satellite Synthetic Aperture Radar for Continuous Global Biospheric Monitoring: A Review
## 1 Introduction
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Mission** & **Country** & **Duration** & **Band** & **Mass (kg)** & **Swath (km)** & **Resolution** \\ \hline SeaSat & USA & 1978 & L & 2290 & 100 & 6–25 \\ SIR-A1/B1 & USA & 1981, 1984 & L & 2460 & 50 & 7–25/6–13 \\ ERS-1/2 & Europe & 1991–2010 & C & 2384 & 100 & 6–26 \\  & & 1995–2011 & & 2516 & 100 & 6–26 \\ ALMAZ-1 & USSR & 1991–1992 & S & 3420 & 280 & 8–15 \\ JERS-1 & Japan & 1992–1998 & L & 1400 & 75 & 18–18 \\ SIR-C/X & Multi 1 & 1994 & C,L/X & 11,000 & 90 & 7.5–13/6–10 \\ RADARSAT-1 & Canada & 1995–2013 & C & 3400 & 500 & 8–8 \\ SRTM1 & Multi 2 & 2000 & CX & 13,600 & 100 & 15–8/8–19 \\ ENVESAT & Europe & 2002–2012 & C & 8210 & 100 & 6–9 (SM) 3 \\  & & & & & 405 & 80–8 (SC) 3 \\  & & & & & & \\ ALOS & Japan & 2006–2011 & L & 3850 & 70 & 10–10 (high) \\  & & & & & 30 & 10–30 (polar) \\  & & & & & 350 & 100–100 (SC) \\ SAR-Lupe & Germany & 2006– & X & 770 4 & - & \\ RADARSAT-2 & Canada & 2007–2020 & C & 2200 & 18 & 0.8–1.6 (SL) 5 \\  & & & & & 100 & 8–9 (std) \\  & & & & & 500 & 70–70 (SC) \\ Cosmo- & Italy & 2007– & X & 1700 & 10 & 1–1 (SL) \\ SkyMed & & & & & 40 & 3–3 (SM) \\  & & & & & 200 & 30–30 (SC) \\ TerraSAR-X & Germany & 2007–2020 6  & X & 1230 & 10 & 1–1 (SL) \\  & & & & & 30 & 3–1 (SM) \\  & & & & & 270 & 40–2 (SC) \\ TecSAR & Israel & 2008– & X & 260 4 & - & - \\ TanDEM-X & Germany & 2009–2020 6  & X & 1230 & - & Same as TSX \\ RISAT-1 & India & 2012–2017 & C & 1860 & 10 & 1–1 \\  & & & & 25 & 3–4 \\  & & & & 220 & 48–8 \\ HI-1C & China & 2012–2016 & S & 890 & 100 & 5–20 \\ KOMPSat-5 & S. Korea & 2013–2020 6  & X & 1400 & 18 & 1–1 (high) \\  & & & & 30 & 3–3 (std) \\  & & & & & 100 & 5–5 (wide) \\ Sentinel 1A/B & Europe & 2014– & C & 2300 & 80 & 4–2 (SM) \\  & 2016– & & & 400 & 43–8 (TS) 7  \\ ALOS 2 & Japan & 2014–2020 6  & L & 2120 & 25 & 1–3 (SL) \\  & & & & & 70 & 5–9 (SM) \\  & & & & & 490 & 60–45 (SC) \\ PAZ & Spain & 2015– & X & 1230 & 10 & 1–1 (SL) \\  & & & & 30 & 3–1 (SM) \\  & & & & 100 & 20–20 (SC) \\ RCM & Canada & 2017– & C & 1430 & 20 & 1–1 (SL) \\  & & & & 30 & 3–10 (SM) \\  & & & & & 500 & 40–40 (SC) \\ SAOCOM & Argentina & 2018– & L & 3000 & 30 & 10 (AZ, SM) \\  & & & & & 350 & 100 (AZ, TS) \\ \hline \hline \end{tabular}
\end{table} 
Table 1: Synthetic aperture radar missions (2018). An important application of SAR is interferometric synthetic aperture radar (InSAR), which was first attempted by time-series analysis of Seasat data. The phase difference in two or more SAR images is measured to yield centimeter- or millimeter-level surface changes over large areas (squares of km) at meter-level resolutions, making InSAR ideal for monitoring surface deformations of Earth [17]. The SAR images capturing the same scene may be acquired at two different times and/or at two slightly different locations. The technique has matured since ERS in the 1990s when the stabilization of satellite orbits allowed for exploiting repeat-ground tracks to visit target areas on a regular basis [18]. Japanese Earth Resources Satellite 1 (JERS-1) also demonstrated two-pass (repeat-pass) cross-track interferometry, meaning that different radar images are obtained from different orbits at different moments [19]. It was the US-Germany joint mission in 2000, called Shuttle Radar Topography Mission (SRTM), which performed single-pass interferometry using one radar antenna onboard a Space Shuttle and another mounted on the end of 60-m mast connected to it [20]. As its name suggests, SRTM was a continuation of the SIR mission in 1994 with a two-antenna configuration. This two-antenna SAR concept is further being developed by German twin-satellite missions as technologies for close-formation flight have become reliable [21]. These missions have new observation modes such as along-track InSAR and SAR polarimetry [22]. Traditional research interests utilizing SAR data focused on geosphere, hydrosphere, and cryosphere among different realms (\"-spheres\") of Earth science [23; 24; 25].