## 2 Data and Methods
### FMPL-2 Brightness Temperature Data from FSSCat  
FSSCat is a tandem mission of two federated 6U CubeSats (\({}^{3}\)Cat-5/A and \({}^{3}\)Cat-5/B) that was proposed by the Polytechnic University of Catalonia (UPC), and designed to retrieve geophysical data on SIE and SIT over polar areas, and low-resolution soil moisture content over land [38]. It was the winner of the 2017 ESA Small Sentinel Satellite Challenge and the overall Copernicus Masters Competition [40]. The two CubeSats were successfully launched on the VEGA Small Spacecraft Mission Service (SSMS) Proof of Concept (PoC) flight on 3 September 2020. Both of them comprise an Radio-Frequency (RF)/optical inter-satellite link payload and the \({}^{3}\)Cat-5/A CubeSat carries the FMPL-2, a dual passive microwave remote sensing instrument that encompasses a Software-Defined Radio (SDR)-based GNSS-Reflectometer and an L-band radiometer with a nadir-pointing antenna and a footprint of \(\sim\)350 \(\times\) 500 km\({}^{2}\).  
The radiometer acquires the antenna temperature, which represents the apparent T\({}_{B}\) at almost global coverage at latitudes that range from \(\sim\)86 \({}^{\circ}\)S to 86 \({}^{\circ}\)N. All of the mission characteristics and the instrument and processing specifications are described in detail in [38; 39]. T\({}_{B}\) maps are constructed from single tracks under evaluation of the antenna pattern, and swath data were projected and regridded onto a 12.5 km Equal-Area Scalable Earth Grid (EASE-Grid 2.0) for both hemispheres using k-d-tree sampling--a fast nearest neighbor interpolation method to resample remote sensing images [41].  
The major part of sea ice in the Northern Ocean occurs at 65 \({}^{\circ}\)N northwards, whereas, in the Southern Ocean, excluding the Antarctic continent, it roughly extends between 60 \({}^{\circ}\)S and 70 \({}^{\circ}\)S, with fluctuating sea ice margins. Because \({}^{3}\)Cat-5/A is a polar orbit satellite, its passes become denser towards the poles and produce more frequent overlaps. T\({}_{B}\) maps can be acquired with sufficient coverage (\(\sim\)80 % after filtering usable tracks) in less than five days and two complete consecutive maps can be obtained within three weeks. This allows for the production of weekly Arctic and monthly Antarctic SIT composite maps of the ice-covered areas, respectively. It is noteworthy that the revisit time could be further decreased if the satellite was able to operate with a larger duty cycle, or in the case more \({}^{3}\)Cat-5/A-like CubeSats were added to the existing constellation. After the commissioning phase, data are available from 1 October to 4 December 2020. The observed period falls into the beginning of the Arctic freeze up, after sea ice having reached its annual minimum extent on 15 September 2020. Hereby, sea ice mainly consists of the remaining multi-year ice with an increasing amount of thin first-year ice. In contrast to the Arctic, the Southern Ocean melts and re-freezes almost completely on a yearly basis and it consists mainly of first-year ice. Sea ice around Antarctica had passed its maximum extent in September 2020, and it is declining during the observation period [42].