# Small-Satellite Synthetic Aperture Radar for Continuous Global Biospheric Monitoring: A Review
## 3 Synthetic Aperture Radar Satellite Missions
### Medium/Large Satellite Constellations
#### 3.1.2 Cosmo-SkyMed  
COSMO-SkyMed is the acronym for Constellation of Small Satellites for Mediterranean Basin Observation, conceived by the Italian Space Agency. Funded by the Italian Ministry of Research and the Italian Ministry of Defense, the satellite system has dual-use (civilian and military) applications ranging  
Figure 4: Launch and deployment of SAR-Lupe spacecraft: **(left)** Miniature models of SAR-Lupe and the launch vehicle upper stage. **(middle)** Artist’s concept of SAR-Lupe in space. **(right)** Illustration of SAR-Lupe constellation. Source: OHB-System AG. Figure 5: Nomenclature of electromagnetic spectra per the IEEE and International Telecommunication Union (ITU) standards. P-band, which will be used for the European BIOMASS satellite, is defined as the interval 420-450 MHz beyond the L band. As the frequency increases (wavelength decreases) from left to right, the antenna size decreases while the throughput and the susceptibility to rain increases. Source: KTH [51]. from reconnaissance and risk management to the monitoring of biospheres such as marine/coastal environments, agriculture, and forestry. Because the launch dates of the first satellite (June 2007) and the last satellite (November 2010) are more than three years apart, the constellation size grew rather gradually, operating with intermediate configurations. In each phase, all satellites are in the same orbital plane; the inter-satellite angle decreased from 180\({}^{\circ}\) in two-satellite configuration to 90\({}^{\circ}\) in a four-satellite configuration in Figure 6. The coverage performance during each stage is listed in Tables 3 and 4; the mean revisit time of 3 to 6 h at the full-blown state is still longer than three-plane SAR-Lupe whose revisit time is expected to be about an hour [52; 53]. However, the all-in-one-plane architecture of COSMO-SkyMed enables easy transition of its orbital configuration from the nominal mode to several interferometric modes, namely \"tandem\" and \"tandem-like (one-day interferometry)\" [54]. First, in tandem interferometry, two satellites are separated in two orbit planes slightly differing in the ascending nodes by 0.08\({}^{\circ}\), denoted as AN (blue) in Figure 6, to guarantee exactly overlapping ground tracks. If the satellites are in a single orbit, they will have slightly different ground tracks due to orbital regression, which is called J2 (Earth's oblateness) effects [55]. The phasing of the two satellites is also adjusted such that the resulting leader-follower pair achieves identical incidence angles. In other words, the two satellites acquire imagery of the same scenes with the same geometry, with a delay of 20 s. Second, in the tandem-like configuration, two satellites stay on the same orbit plane at a distance of 67.5\({}^{\circ}\). This specific angle is used instead of 90\({}^{\circ}\) for the tandem-like configuration to achieve \"one-day interferometry\" with 237/16 repeat cycles; a satellite in this orbit completes 14 and 13/16 revolutions after 16 days (denominator), and the remaining 3/16 revolution to complete an integer number corresponds to 67.5\({}^{\circ}\) (=3/16 \(\times\) 360). The two satellites with this phase spacing visit the same ground target with an exact 1-day interval, \begin{table}
\begin{tabular}{c c c c c} \hline \hline
**Configuration** & **Coverage After 12 h, \%** & **Coverage After 24 h, \%** \\ \hline
**\# Satellites** & **Left and Right** & **Right** & **Left and Right** & **Right** \\ \hline
1 & - & - & 67 (85) & 38 (55) \\
2 & 62 \({}^{1}\) (84) \({}^{2}\) & 41 (62) & 81 (92) & 64 (84) \\
3 & 88 (98) & 62 (84) & 96 (99.97) & 84 (98) \\
4 & 97 (100) & 80 (99) & 100 (100) & 95 (100) \\ \hline \end{tabular} \({}^{1}\) Nominal, \({}^{2}\) Extended.  
\end{table} 
Table 4: Areal coverage response of Cosmo-SkyMed during constellation deployment. Figure 6: Deployment steps of the COSMO-SkyMed (Constellation of Small Satellites for Mediterranean Basin Observation) configuration.