## 4 Preconditions and Current Limitations of Multi-Temporal SAR Methods  
Theoretically, the methods described in Section 2 and reviewed in Section 3 work very well for a fast and accurate damage assessment, which is crucial to a fast crisis response (e.g., to support rescue, humanitarian and reconstruction operations in the crisis area). However, as already mentioned in Section 3.6, for the multi-temporal data based approaches interferometric coherence and SAR intensity correlation several preconditions have to be fulfilled:  
* At least three SAR images, more precisely two pre- and one post-disaster image(s), are required (see also Section 2, Figure 3). * All three images have to be acquired at the same imaging geometry (_i.e._, same pass direction (ascending/descending), incidence angle, relative orbit, imaging mode and SAR wavelength). * As most natural disasters are not predictable (especially earthquakes), a continuous monitoring of the affected AoI is required. * The pre-disaster image pair should be recorded shortly before the event and the post-disaster image shortly after the event. The temporal baseline between all acquisitions should be as small as possible to decrease the influence of temporal de-correlation not caused by the natural disaster. The repeat cycle of the SAR mission is the controlling key factor of the temporal baseline (_cf._ Table 2, Section 1.2). The repeat cycle ranges from 4 to 6 days for modern constellations of 2-4 satellites (e.g., Sentinel-1 and COSMO-SkyMed) to 35-46 days for older SAR missions (e.g., ERS-1/2, ENVISAT, JERS and ALOS PALSAR). To analyze which repeat cycle is useful for the presented damage assessment methods (see Section 2), we also have to consider the wavelength of the SAR sensors. At shorter wavelength, such as X-band (\(\lambda\) = 3.1 cm; e.g., TerraSAR-X and COSMO-SkyMed) or C-band (\(\lambda\) = 5.6 cm), the coherence is more affected by temporal de-correlation than at the longer L-band (\(\lambda\) = 23.6 cm) [79, 81, 118]. Fortunately, this effect was also considered in the design of the SAR missions, as the X-band missions are characterized by shorter repeat cycles (e.g., TerraSAR-X: 11 days, COSMO-SkyMed constellation: 4 days) compared to the L-band missions (e.g., ALOS PALSAR and JERS), which compensate their longer repeat orbit of 44 or 46 days, respectively, with in general a higher coherence over a longer time period. The repeat cycle of the C-band SAR missions (e.g., Radarsat-1/2, ERS-1/2, and ENVISAT ASAR) is with 24-35 days in-between the aforementioned values. As urban area is characterized by long-term high coherence values (e.g., over several months [79, 81]), data from all reported SAR missions is usable for the damage assessment methods presented in Section 2. In general one can recommend to use SAR image pairs of less than 3-5 repeat cycles between the single acquisitions (especially valid for single satellite missions). However, there exists no critical value for a maximum number of repeat cycles. The number of repeat cycles between the pre-disaster and the pre-post-disaster image pair, respectively, should be as low as possible to avoid influences of temporal changes which were not caused by the natural disaster (e.g., clearing of the debris after the event, or construction activities, _etc._). Sentinel-1 with its two-satellite-constellation will image every part of the global landmass every 6 days, providing a solution to the temporal baseline problem--especially for damage assessment at the block level (see Section 5). * The destruction caused by the natural disaster should dominate the temporal de-correlation effect of the AoI. Urban area and especially continuous urban fabric is characterized by long temporal coherence values [79, 81] and therefore very well suited for the application of the damage assessment methods based on the interferometric coherence and the SAR intensity correlation.