## 1 Introduction
### Brief Description of SAR and Its Advantages Compared to Optical Sensors  
The most frequently used EO data for damage mapping after a natural disaster is high spatial resolution optical satellite imagery. However, due to frequent occurrence of clouds, smog, fog, _etc._--especially in the equatorial region--optical EO data is not always useful [8,21]. SAR has \begin{table}
\begin{tabular}{|c c c c|} \hline
**Year** & **Type of Disaster** & **Country** & **Fatalities** \\ \hline
12 January 2010 & Earthquake & Haiti & 222,570 \\
26 December 2004 & Earthquake/Tsunami & Indonesia, Thailand, India, Sri Lanka, & 222,000 \\  & & Myanmar, Malaysia, Maldivians & 222,000 \\
2–5 May 2008 & Cyclone Nargis, storm surge & Myanmar & 140,000 \\
29–30 April 1991 & Tropical cyclone, storm surge & Bangladesh & 139,000 \\
8 October 2005 & Earthquake & Pakistan, India, Afghanistan & 88,000 \\
12 May 2008 & Earthquake & China & 84,000 \\ July/August 2003 & Heat wave, Drought & France, Germany, Italy, Portugal, & 70,000 \\  & & Romania, Spain, UK & \\ \hline July/September 2010 & Heat wave & Russia & 56,000 \\
20 June 1990 & Earthquake & Iran & 40,000 \\ \hline
26 December 2003 & Earthquake & Iran: Bam & 26,200 \\ \hline \end{tabular}
\end{table} 
Table 1: The ten deadliest natural disaster events (1980–2012) [20]. several advantages compared to optical sensors. First, as an active sensor SAR is independent of day and night, while passive optical sensors use the solar radiation. Second, due to its larger wavelength (\(\lambda\): mm to dm) SAR is almost an all-weather technique as only larger atmospheric changes in the time between the radar acquisitions have a big influence on interferometric (InSAR) applications. Compared to this, optical sensors (\(\lambda\): 0.4 um to 1 mm) are using the visible and infrared part of the electromagnetic spectrum which is much more influenced by e.g., atmospheric water vapor [22]. In most cases, SAR EO data of the crisis area is earlier available than cloud free optical imagery. Therefore, SAR damage assessment methods enable faster disaster response. However, for both optical and SAR sensors, the revisit time of the satellite has to be taken into account (see Section 1.1). Table 2 gives an overview of the SAR missions which are relevant in this review article. Furthermore, the larger wavelength of SAR sensors enables the investigation of the physical characteristics of the objects on the ground, _i.e._, SAR is sensitive to the surface roughness, its moisture content and for geometric structures. Especially the last aspect is very critical for damage assessment [8]. Contrary to this, optical sensors are more sensitive to the chemical structure of the ground targets, due to their definitely smaller wavelength. The following example shows another advantage of SAR. A damaged building with destroyed or heavily damaged walls and still more or less intact roof is often difficult to identify when using optical imagery, especially in the near nadir region. Data from nadir-looking aerial and space-borne sensors provides information about the status of the building's roof and the presence/absence of debris next to the building, but not direct information about the status of building walls [24, 25]. A solution to this problem is high spatial resolution side-looking optical sensors [26], but with these sensors ground referencing is more complicated and they are less frequently available than nadir-looking sensors. Moreover, even non-nadir-looking very high spatial resolution sensors enable only the identification of major cracks in the wall. Slighter damage levels can only detected from _in situ_ investigations.