# Small-Satellite Synthetic Aperture Radar for Continuous Global Biospheric Monitoring: A Review
## 4 Applications of Small Satellite SAR Data
### Sub-Constellation Formation Flight  
Formation flying schemes resembling the TDX mission were also demonstrated by a few non-SAR missions involving small spacecraft: the PRISMA (Prototype Research Instruments and Space Mission technology Advancement) mission, consisting of a 150 kg satellite and a 40 kg satellite, and the PROBA-3 mission consisting of a 300 kg satellite and a 200 kg satellite [126; 127; 128]. Several advanced SAR techniques for biospheric monitoring benefit from having multiple satellites. Digital elevation models of ground surfaces generated by InSAR and forest heights obtained by polarimetric InSAR (Pol-InSAR) together yield the biomass (forest) volume and mass, which are deliverables of future SAR missions such as Tandem-L and BIOMASS as well as TSX/TDX [129; 130]. Representative baseline requirements are summarized in Table 9 for an instance of the Tandem-L mission [25]. Now that Pol-InSAR was enabled  
\begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Mission** & **NISAR** & **Tandem-L** & \multicolumn{3}{c}{**NovaSAR**} \\ \hline
**\# Sats (\(\rightarrow\))** & **1 SSO** & **2 SSO** & **1 SSO** & **EqO (15\({}^{\circ}\))** & **3 SSO** \\
**Orbit (inc)** & **(98.4\({}^{\circ}\))** & **(98.4\({}^{\circ}\))** & **(97.5\({}^{\circ}\))** & **(97.5\({}^{\circ}\))** & **EqO (15\({}^{\circ}\))** \\ \hline Altitude & 747 km & 745 km & 580 km & (\(\leftarrow\)) & (\(\leftarrow\)) & (\(\leftarrow\)) \\ LATAN & 18:00 h & - & 10:30 h & (\(\leftarrow\)) & (\(\leftarrow\)) & (\(\leftarrow\)) \\ Repeat cycle & 12 day & 16 days & - & - & - & - \\ Revisit time & 5–7 day & 3–4 day & 1–3 day & 0.5–1.5 day & 0.5–1.5 day & 0.3 day \\ Band & S, L & L & S & (\(\leftarrow\)) & (\(\leftarrow\)) & (\(\leftarrow\)) \\ Resolution & 7 m (Az), & 7 m, & 6–30 m & (\(\leftarrow\)) & (\(\leftarrow\)) & (\(\leftarrow\)) \\  & 3–24 m (SR) & 1 m (Spot) & & (\(\leftarrow\)) & (\(\leftarrow\)) & (\(\leftarrow\)) \\ Swath & 240 + km & 350 km & 20–150 km & (\(\leftarrow\)) & (\(\leftarrow\)) & (\(\leftarrow\)) \\ \hline \hline \end{tabular}
\end{table}
Table 8: Comparison of NISAR, Tandem-L, and NovaSAR missions (–2018).  
Figure 22: NISAR spacecraft and mission concept: (**left**) Deployed configuration and internal subsystems including 24 L-band Tx/Rx modules and 48 S-band modules (12 per polarization). (**right**) Areas and topics of interest in India and neighboring regions. Source: JPL, ISRO.  
in the TDX mission whose mass is slight over 1 metric ton, more reduction in mass could be attempted within the medium satellite (500-1000 kg) range; prior to TDX, Pol-InSAR was only feasible in large satellites such as ENVISAT (C-band, 8 ton) and ALOS (L-band, 3 ton). The large-to-medium-sized spacecraft could be supported by a swarm of small spacecraft that can only receive radar echoes (no transmitter) and communicate indirectly with ground stations for command and data handling through their master spacecraft, as envisioned in a concept called \"mirror-SAR\" [131; 132]. In support of formation flying, drifting orbits are used for SAR tomography (vertical forest profile) and the teardrop patterns of a \"Cobra\" array or constellation might be used for holographic SAR tomography (full 3-D forest structure) [53; 55]. The desired satellite movement for SAR tomography and holographic SAR tomography are shown in Figure 23[25; 133; 134]. The Cobra constellation, originally intended for telecommunications, uses medium-Earth orbits but smaller heights (with more satellites) could be a possibility for SAR observation given the low-cost drivers today in both satellite manufacturing and launch services. While this concept uses two elliptical orbits with different arguments of perigee to create two sides of a teardrop pattern, multitudes of adjacent circular orbits may also be used to create the circular formation, which could be viewed as extensions of TDX/TSW helix orbits [135].