## 5 Possible Solutions of Current Limitations--The Sentinel-1 Mission  
The SAR satellites Sentinel-1 A and B launched on 3 April 2014 (satellite A) and planned to be launched 18 month later (B unit), respectively, fulfill the aforementioned required preconditions I to IV very well (see Section 4). Sentinel-1 is a polar orbiting two-satellite-constellation developed by the European Space Agency (ESA). This SAR mission belongs to the Space Component of the Global Monitoring for Environment and Security (Copernicus, former GMES) program. Sentinel-1 operating in C-band (center frequency 5.405 GHz) is a successor of the European C-band SAR missions ERS-1/2 and ENVISAT ASAR [141]. Sentinel-1 will monitor the Earth on a pre-planned and conflict-free observation plan using in most cases a fix incidence angle range. This fulfills the requirement II in Section 4 (\"same imaging geometry of all SAR acquisitions\"). In general case, the so-called Interferometric Wide swath (IW) mode with 250 km swath width and 5 m \(\times\) 20 m spatial resolution will be used to monitor land areas [141]. In the IW and in the Extra Wide swath (EW) mode (extension of IW up to 400 km swath width), Sentinel-1 uses the so-called Terrain Observation by Progressive Scan (TOPSAR) operation [142]. The Strip Map mode (SM) with higher spatial resolution (5 m \(\times\) 5 m) will be used in exceptional cases, e.g., to support emergency services [143]. However, as this imaging mode will not be used regularly, no useful archive data will be available at a global coverage. Therefore, the Strip Map data will only be useful in very exceptional cases (_cf._ requirements I and II in Section 4). Table 4 shows the characteristics of the two Sentinel-1 modes, which are important for damage assessment procedures. The aforementioned spatial resolution of the Sentinel-1 standard imaging mode over land areas (IW mode: 5 m \(\times\) 20 m) limits the application of damage assessment to the block level (encompassing several buildings per block). The damage analysis of single houses would require meter to sub-meter spatial resolution. The repeat cycle of a single satellite is 12 days. This will be reduced to 6 days with the launch of the B unit, as both satellites will fly on the same orbit plane with 180 \({}^{\circ}\) phased to each other [141]. Therefore, Sentinel-1 fulfills the required preconditions of a constant imaging geometry (II) and a short temporal baseline (IV) (_cf._ Section 4). Sentinel-1 enables the coverage of the entire globe (to be more \begin{table}
\begin{tabular}{|c c c|} \hline
**Parameter** & **Interferometric Wide Swath Mode (IW)** & **Strip Map Mode (SM)** \\ \hline Polarization & HH + HV, VV + VH, HH, VV & HH + HV, VV + VH, HH, VV \\ \hline Incidence angle range & 29 \({}^{\circ}\)46 \({}^{\circ}\) & 20 \({}^{\circ}\)45 \({}^{\circ}\) \\ Azimuth resolution & 20 m & 5 m \\ Ground range & & \\ resolution & & \\ Swath width & 250 km & 80 km (6 swathes) \\ \hline \end{tabular}
\end{table} 
Table 4: Characteristics of the Sentinel-1 modes Interferometric Wide swath and Strip Map—the important modes for damage assessment applications (modified after [141,144]). precisely, the entire land mass) within 12 days (only Sentinel-1 A), or even 6 days with Sentinel-1 A and B. For high priority areas, such as Europe and Canada, the coverage will be even more frequent up to 4 to 2 days. However, as the interferometric applications require identical imaging geometries for all SAR images (same orbit, incidence angle, pass direction, _etc._), only the 12 days (or 6 days with two satellites) repeat cycle acquisitions are useful. After the launch of Sentinel-1 A on 3 April 2014 and the In-Orbit Commissioning Review, a 6 months ramp-up phase is planned, for which ESA has developed an observation plan which focuses on following areas of interest (Please note that this list contains only the aspects which are of interest for damage assessment of natural disasters on land areas. They will be monitored using the IW mode [144]):  
* Europe & French oversea territories.