# Mapping Atmospheric Exposure of the Intertidal Zone with Sentinel-1 CSAR in Northern Norway
## 2 Materials and Methods
### Data
#### 2.1.2 Sentinel-1 C-Band Synthetic Aperture Radar Satellite Data  
The European Copernicus program includes a fleet of satellites called the Sentinels managed by the European Space Agency (ESA). Sentinel-1 (S1) A&B is a constellation of two identical C-band synthetic aperture radar (CSAR) missions (S1A and S1B) with a center frequency at 5.405 GHz, launched separately on 3 April 2014 and 25 April 2016, respectively. These two cloud- and sunlight-independent radar satellites support operational applications of marine monitoring, land monitoring, and emergency management services, and they provide interferometric wide-swath data to map the global landmasses once every 12 days each, with a potential 6 day repeat cycle at the equator [33]. Over Europe,  
Figure 1: Location of field sites in an aerial mosaic (@Norge digitalt, norgeibilder.no) over the Tromso Municipality in Northern Norway: Langnes on Tromsoy and Lille Grindoy, Finnvika, Gretfjord, and Hillesey on Kvaloy. The city of Tromso is on Tromsoy, the small island to the right in the figure. Field data sites are indicated in yellow.  
the Sentinels operate systematically on a continuous basis providing free data to any user, including the public, scientific, and commercial users. The original data format used in this project is Level 1 Ground Range Detected (GRD) data. GRD products consist of focused SAR data that have been detected, multi-looked, and projected to ground range using the Earth ellipsoid model WGS84. Pixel values represent only detected radar backscatter amplitude. Phase information is lost. The resulting product has approximately square pixels of \(\sim\)10 m resolution.  
Initially, all acquired S1 data from 1 January 2018 until 31 December 2019 over Tromse (Municipality) were downloaded through the Copernicus Open-Access Hub (COAH) [34] or the Alaska Satellite Facility (ASF) [35]. As some fjords in the region can be ice-covered during winter, we decided to limit our analysis only to ice-free months June to November. Over Norway, the acquisition scenario reflects the maximum acquisition possibilities, i.e., continuous acquisition of all ascending and descending paths for both satellites S1A and S1B. As S1 is polar-orbiting, the overlap of adjacent paths increases with latitude and is about 2/3 at the latitude of Tromse (69-70\({}^{\circ}\)N). Most pixels are, therefore, covered six times by each satellite during the 12 day cycle, i.e., 12 times using both satellites. As we cut off steep incidence angles below 33.8\({}^{\circ}\), some pixels were only covered eight or 10 times per cycle. The location of the tidal gauge in Hansjordnesbukta on the east side of Tromsey, for example, is covered 10 times per 12 day cycle. Table 1 summarizes the covering paths for one cycle period of 12 days at Hansjordnesbukta, specifying the satellites S1A or S1B, the path number, the flight direction of the satellite (three ascending (ASC) paths and three descending (DES) paths), and the time of overflight. DES paths pass around 5:20 a.m. \(\pm\)10 min, whereas ASC paths pass around 4:10 p.m. \(\pm\)10 min. Figure 2 shows the location of Tromse Municipality and the satellite paths' coverage of the study region.  
Figure 2: Screenshots from the Copernicus Open-Access Hub [34] showing the Sentinel-1 coverage over (**left**) the Tromse Municipality (green line) in (**right**, zoomed out) Northern Norway. Ascending and descending path numbers are indicated in black and white, respectively.