# Remote Sensing of Snow Cover Using Spaceborne SAR: A Review
## 5 Discussion
### The Development of Spaceborne SAR Sensor Design  
The development of SAR sensors advanced remarkably within the last 25 years in terms of instrument design, temporal and spatial coverage, and data distribution policy. Band design was limited to C-band, but has been extended to X-band, as shown in Figure 2. This offers new opportunities to detect and monitor snow cover, because the capabilities of X-band SAR to detect snow are superior to C-band, as outlined in Section 3.2. Moreover, study suggested that Ku-band SAR may be most suitable to detect snow cover as it can detect even shallow and dry snow (87), which was planned (but not realized) for ESA's 7 th Earth Explorer mission candidate, Cold Region Hydrology High-resolution Observatory (CoReH\({}_{2}\)O) satellite, meant to be equipped with X- and Ku-band SAR sensors (190).  
Furthermore, the capabilities to detect snow relying on different polarizations attracted more attention. Among all snow cover detecting studies based on polarimetric information, almost all employed SAR images in full-polarimetry mode including PALSAR-1 (163; 169; 170) and Radarsat-2 (165; 166; 167; 168; 191). Early airborne SAR and recent spaceborne studies already suggested that multi-polarization SAR--and especially fully polarimetric SAR--is more suitable to detect snow and ice than single/dual-polarization [102; 191; 192]. Possible reasons are that multi-polarimetric SAR has a higher sensitivity to the state of snow [35] and that it can greatly eliminate the topographic distortion [30].  
Shortening the revisit time is another advantage of contemporary SAR missions. Practically, the time difference between two repeat-pass SAR images is the most important factor for InSAR-based approaches. Researchers have confirmed that the coherence of InSAR within snow-covered areas would decrease dramatically if the temporal difference is more than one month [142]. In addition, if the revisit time is too long, more than one melting/snow fall event may have occurred in the meantime [144]. In this regard, the value of the COSMO-SkyMed (CSK) constellation is remarkable. Thanks to their orbit design, four satellites (CSK1-4) significantly shorten the revisit time and thus largely eliminate temporal decorrelation [193; 194], which provides a potential to map SCE with a much higher temporal resolution [187].  
The wider swath coverage of contemporary SAR sensors allows snow monitoring in larger spatial scales, which may also reduce costs. [PERSON] and [PERSON] [195] proved that all sensing modes of ASAR images are suitable for processing \"[PERSON]'s method\". Although the definition of wide swath mode varies for different sensors (e.g., ASAR's 150 or 500 km, Radarsat's 300 or 500 km, PALSAR's 250 or 350 km, and Sentinel-1's 250 km), it is clear that they are more efficient than traditional sensing modes providing swath widths below 100 km.  
In addition to sensor's hardware design, another important milestone is the free data policy of ESA. Before Sentinel-1, no long-term, openly accessible SAR datasets were available, which stands in contrast to the free of charge archives of optical sensors such as Landsat and MODIS. As a result, until recently the cost of spaceborne SAR data was much higher compared to optical imagery [68], which inevitably limited the operational use of SAR-based snow cover detection or the subsequent possibility of merging SAR and optical snow cover products.