# Monitoring the Dynamics of Ephemeral Rivers from Space: An Example of the Kuiseb River in Namibia
## 3 Data Used for This Study
### Sentinel Missions
#### 3.1.2 Synthetic Aperture Radar (SAR) from Sentinel-1  
Sentinel-1 is the first mission of the Copernicus Sentinel series, which is comprised of two identical satellites, 1A and 1B, in the same orbit, and were launched on April 2014 and April 2016, respectively. They provide data every 6 days from the same location in all weather conditions and during day or night. The Synthetic Aperture Radar (SAR) on board the Sentinel-1 mission is an active system that emits its own signal and then records the amount of energy backscattered after interactions with the Earth. A SAR is a coherent radar producing high-resolution images by emitting pulses lateral to the trajectory of the satellite in the centimeter wavelengths (band C-6 cm) and by measuring the backscattered energy of the objects. The main advantage, unlike optical data, is that data acquisition takes place during day and night, and even in overcast weather conditions--since clouds are transparent to the frequencies used. Due to the lateral scanning along the azimuth, the radar returns signals reflected by different objects on Earth that are detected at different distancesand are properly discriminated by measuring return times. A 2D image is then obtained according to the azimuth and the range direction using the synthetic aperture technique. Radar imaging can provide polarimetric information for each pixel in the image, such as HH, VV, VH, and HV. For example, for VH polarization, the signal is transmitted in the vertical polarization and received in the horizontal polarization. The signal intensity from different polarizations yield information about the structure of the imaged surface, depending on the various types of scattering, such as the rough surface, bulk, and double bounce. Since the signal is acquired in a coherent way, the phase information therefore provides interferometric data. Each pixel X of a SAR image is composed of a real part and an imaginary part, which are related to the amplitude A and the phase \(\varphi\) and are determined by the following equation (Equation (3), [31]):  
\[\mathrm{X}=\mathrm{A.cos}\ \varphi+\mathrm{j.\ A.sin}\ \varphi \tag{3}\]  
When considering the information on the amplitude, we use the backscattering coefficient \(\sigma_{0}\), which is defined as following (Equation (4)):  
\[\sigma_{0}=10.\mathrm{logA}\quad\mathrm{(in\ dB)} \tag{4}\]  
As a reminder, the backscattering coefficient \(\sigma_{0}\) can be used as a proxy for soil moisture variations through the impact of water on the dielectric properties. Sentinel-1 carries a single C-band synthetic aperture radar instrument operating at 5.4 GHz in different polarizations, such as HH + HV, VV + VH, and VV and HH, and in four acquisition modes: StripMap (SM), Interferometric Wide Swath (IW), Extra-Wide swath (EW), and Wave Mode (WM). More information is available on [[https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-1-sar](https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-1-sar)]([https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-1-sar](https://sentinel.esa.int/web/sentinel/technical-guides/sentinel-1-sar)) (accessed on 5 January 2021). Data on amplitude were obtained from Ground Range Detected (GRD) images and Single Look Complex (SLC) images for phase differences between two acquisitions--all in IW mode and in dual polarizations VV + VH--from tile Path 29 and Frame 1102, as StripMap mode and other polarizations were not available for our area of interest over Namibia. A total of 153 GRD images and 14 SLC images (28 interferometric couples), acquired October 2016 and December, were downloaded from [[https://search.asf.alaska.edu/#/](https://search.asf.alaska.edu/#/)]([https://search.asf.alaska.edu/#/](https://search.asf.alaska.edu/#/)) (accessed on 25 November 2021). GRD and SLC data were processed using the SNAP (SeNtinel Applications Platform) software, version 8.0.0.