# Small-Satellite Synthetic Aperture Radar for Continuous Global Biospheric Monitoring: A Review
## 3 Synthetic Aperture Radar Satellite Missions
### Ministatellites
#### 3.2.1 NovaSAR-1 (400 kg)  
Developed by Surrey Satellite Technology Ltd. (SSTL) and Astrium UK, NovaSAR-1 is one of the first low-cost SAR satellites that are both small and commercial [82]. Potential use cases of NovaSAR-1 imagery include tropical forest protection, disaster monitoring, and maritime services to detect ships, icebergs, and oil spills [83,84]. The observation modes of NovaSAR-1 and their characteristics are summarized in Table 5[85]. The originally envisioned constellation had three \"NovaSAR-S\" satellites operating in the S band. Only one satellite, named NovaSAR-S 1 or NovaSAR-1, has been in orbit since 2018. The operational parameters and the revisit performance for each observation mode are provided in Table 5. The expected constellation performance shows that the effect of decreasing average revisit time is around a factor of 3; the effect of decreasing maximum revisit time is larger, meaning that week-long revisits are prevented and global monitoring becomes more continuous. The baseline modes in Table 5 use HH or VV polarization, but the payload of NovaSAR can be configured to operate with dual polarization (any 2 from HH, VV, HV, or VH), tri-polarization (any 3 from HH, VV, HV, or VH), or fully-quad polarization (HH, VV, HV, and VH) as well. As shown in Figure 12A, NovaSAR-1 has a 3 m \(\times\) 1 m microstrip patch array antenna with 18 elements operating at 3.1-3.3 GHz in center frequency. This S band is located between the L band and the C band but has been seldom utilized since USSR's Almaz-1 program whose operations terminated in 1992 [9]. An S-band system has operating wavelengths closest to existing C-band systems in particular, suggesting similarly diverse applications already served by C-band systems. While HJ-1C is another example of exploiting S-band frequencies, NovaSAR-1 concentrated on reducing the satellite mass by employing COTS hardware. In lieu of traveling-wave tube amplifiers that are both bulky and high-voltage, NovaSAR-1 used allium nitride (GaN) solid-state power amplifiers. The radio frequency peak power of NovaSAR-1 is a 1.8 kW, 10% to 15% of a Cosmo-SkyMed satellite, where the power subsystem mass was reduced by limiting the duration of each SAR operation to 2 min per orbit. Although much shorter than the 10-min continuous operation of a COSMO satellite, the duration is still sufficient to cover a nation-state in most cases. Through these miniaturization measures, the interior of NovaSAR-1 still has unoccupied space, as shown in the middle of Figure 14 (left), with further miniaturizablity for an active beam-steering SAR satellite. Figure 14 (right) shows a SAR image capturing the vicinity of the Suez Canal at 30 m resolution. The Commonwealth Scientific and Industrial Research Organisation (CSIRO) of Australia has purchased 10% share of NovaSAR's acquisition time to improve flood forecasting systems [86]. Similar to larger SAR missions such as COSMO-SkyMed and TSX/TDX, NovaSAR adopted Sun-synchronous orbits with LTAN at 10:30 am. This Sun synchronism is a common practice in designing Earth-observing optical satellites because of advantages in power generation and thermal controls, which is the same for SAR satellites. From a \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Observation Mode** & **Resolution (m)** & **Swath (km)** & **Look Angle (\({}^{\circ}\))** & **Av. Revisit** & **Max. Revisit** \\  & & & & **Time (Day)** & **Time (Day)** \\ \hline
1. ScanSAR & 20 m & 50–100 & 16–30 & 3.7 (1.5), 2.5\(\times\) & 14 (3.5), 4\(\times\) \\
2. Maritime surveillance & 30 m & 750 & 48–73 & 0.9 (0.3), 3.0\(\times\) & 8 (0.5), 16\(\times\) \\
3. Stripmap & 6 m & 13–20 & 16–31 & 3.2 (1.1), 2.9\(\times\) & 12.5 (3.5), 3.4\(\times\) \\
4. ScanSAR wide & 30 m & 55–140 & 15–32 & 3.7 (1.2), 3.1\(\times\) & 13 (3.5), 3.7\(\times\) \\ \hline \hline \end{tabular}
\end{table} 
Table 5: NovaSAR observation modes and 1-satellite/3-satellite revisit performance.