# Advancements for Snowmelt Monitoring by Means of Sentinel-1 SAR
## 2 Data Characteristics  
Observational requirements for satellite-based snowmelt area products for operational hydrology and climate research range from 100 m to 500 m spatial resolution and one day to five day revisit time, depending on the application [8; 17]. The data requirements in terms of spatial and temporal resolution can be fulfilled by S1 SAR operating in IW mode. The main characteristics of S1 IW mode data are summarized in Table 1.  
The Sentinel-1 spacecraft, operating in near polar sun-synchronous orbit at 693 km height, provides highly accurate pointing knowledge and real time orbit determination, resulting in very high geolocation accuracy of the SAR products [3; 18]. The orbit repeat cycle is 12 days. Sentinel-1B will have the same reference orbit as Sentinel-1A with a 180-degree orbital phasing difference. Six day exact repeat pass observation will be achieved with the two-satellite constellation. The repeat coverage in mid-latitudes will be two to three days using adjoining swaths.  
The S1 SAR operates at a center frequency of 5.4 GHz. IW mode data over Europe, used for this study, were acquired in dual polarization (_VV_ and _VH_). The IW mode is operated as TOPS (Terrain Observation with Progressive Scans in azimuth) mode, providing a swath width of 250 km at a nominal ground resolution of 5 m \(\times\) 20 m for single look data and offering enhanced image performance compared to the conventional ScanSAR mode [3]. Each image swath in IW mode is composed of three sub-swaths. Individual swaths are subdivided into slices of about 350 km length  
\begin{table}
\begin{tabular}{c c c c c c c} \hline \hline
**Centre** & **Swath** & **Incidence** & **Nominal** & **Pixel Spacing** & **Maximum** & **Radiom.** & **Radiom.** \\
**Frequency** & **Width** & **Angle** & **Resolution** & **LOS \(\times\) AZ** & **NESZ** & **Stability** & **Accuracy** \\ \hline
5.407 GHz & 250 km & 31\({}^{\circ}\)–46\({}^{\circ}\) & 5 m \(\times\) 20 m & 2.4 m \(\times\) 13.9 m & –22 dB & 0.5 dB & 1.0 dB \\ \hline \hline \end{tabular}
\end{table}
Table 1: Main characteristics of Sentinel-1 SAR data in Interferometric Wide swath (IW) mode [3]. Nominal spatial resolution for single look complex (SLC) data. GR: Ground range; AZ: azimuth. LOS: line-of sight. NESZ: noise equivalent sigma-0.  
along track. The evaluation of signals from corner reflectors and transponders yields, for IW mode SLC data, a spatial resolution of 22 m in azimuth and of 2.84 m, 3.10 m, and 3.50 m in LOS for sub-swath IW1, IW2, and IW3, respectively [18].  
Level-1 data (swath-based SAR images) are available in two main categories: Single Look Complex imagery (SLC) and Ground Range Detected imagery (GRD). GRD products consist of focused SAR data that have been detected, multi-looked and projected to ground range using an Earth ellipsoid model [19]. We use SLC images for generating the snow cover product, in order to exploit the original backscatter statistics for speckle filtering and classification, and to perform terrain-corrected geocoding using high resolution digital elevation models (DEMs). The geolocation accuracy of IW SLC products is 1.5 m in LOS and 2.5 m in azimuth [20].  
The analysis of S1A products indicates better radiometric performance than the threshold values specified in the mission requirements [21]. The noise equivalent sigma-0 (\(NESZ\)) values for the cross-polarized channels range from \(-\)24 dB to \(-\)30 dB, depending on the antenna off-boresight angle. The standard deviation of the radar cross section measured over transponders and corner reflectors ranges from 0.34 to 0.56 dB.  
For comparisons and evaluation of the S1 snow products, we derived snow maps from terrain corrected images of the Landsat-7 and Landsat-8 [22] missions using the visible and shortwave infrared channels with 30 m pixel size. The Landsat data were downloaded from USGS as L1T data being radiometrically calibrated and rectified in UTM projection. The absolute geolocation error of the terrain corrected Landsat-8 data is 18 m [23].