# Radiometric Comparison within the Sentinel-1 SAR Constellation over a Wide Backscatter Range
## 1 Introduction  
One of the main reasons for building a space-borne radar constellation instead of operating a single instrument only is the higher revisit rate for acquisitions over the area of interest. For synthetic aperture radar (SAR), a number of such radar constellations are currently in operation [1], e.g., SAR-Lupe (5 satellites), COSMO-SkyMed [2, 3] (4 satellites), or a synchronized operation of the TerraSAR-X/TanDEM-X formation [4] and the PAZ satellite [5]. The Radarsat constellation mission (3 satellites) was lunched recently in June 2019 [6]. Within virtual constellations [7] instruments from different platforms are combined; in particular, multi-temporal products can be derived by merging multiple SAR sources or even combining these with optical instruments [8]. However, SAR constellation missions using small satellites could reduce the mission costs dramatically. Such missions are planned for future developments [9] - related concepts are under investigation or within the development phase [10]. In the frame of the COPERNICUS program, the European Space Agency (ESA) was planned and realized a fleet of Sentinel-1 satellites [11]. The first one, Sentinel-1A (S-1A), is a single SAR satellite which was launched in April 2014. The SAR constellation was completed two years later after starting the operation of Sentinel-1B (S-1B). Both satellites carry a nearly identical payload: a C-band SARoperated at a center frequency of 5.405 GHz and a maximum bandwidth of 100 MHz. The 280 transmit and receive modules (TRMs) of the phased array antenna are able to transmit a nominal peak power of about 5 kW. Both satellites S-1A and S-1B are flying a sun synchronized dusk-dawn orbit with an interferometric revisit time of twelve days each. By flying both satellites in the same orbit plane with 180\({}^{\circ}\) orbital phasing difference, the revisit time is reduced to six days. Current observation scenarios for both satellites are available online: [[https://sentinel.esa.int/web/sentinel/missions/sentinel-1/o](https://sentinel.esa.int/web/sentinel/missions/sentinel-1/o)]([https://sentinel.esa.int/web/sentinel/missions/sentinel-1/o](https://sentinel.esa.int/web/sentinel/missions/sentinel-1/o)) bservation-scenario. Continuing operation of this Sentinel-1 SAR constellation shall be ensured for the next decades: both satellites S-1A and S-1B will be replaced after their end of operation by S-1C and S-1D, respectively. SAR instruments detect the elapsing time of transmitted pulses received at the instrument, as well as the measured backscatter power and phase information acquired from target objects. During SAR data processing, the measured raw data is focused and converted into a two dimensional image mapping the observation area. Radiometric calibration is necessary to allow a valid conversion of the observed pixel intensity into a physical relevant unit, e.g., radar cross section (RCS). Therefore, the whole SAR system has to be calibrated and various geometric and radiometric corrections have to be applied. The internal calibration compensates gain and phase variations of the instrument which may occur due to different temperatures and ensures the specified radiometric stability of the instrument over time. Furthermore, the knowledge of the antenna pointing is important to properly apply the antenna pattern correction during SAR data processing, which defines the relative radiometric accuracy within a SAR image. Absolute radiometric calibration has to be finally applied using reference targets on ground (e.g., corner reflectors and transponders) with well known RCS. On behalf of ESA, the German Aerospace Center (DLR) performed independent calibration campaigns for both Sentinel-1s during their commissioning phases: for S-1A in 2014 [12] and for S-1B in 2016 [13]. Since then, both SAR instruments were permanently checked during routine operations [14] w.r.t. the radiometric accuracy and stability using corner reflectors and C-band transponders remotely controlled and deployed within the DLR calibration site [15].