# High Performance Computing in Satellite SAR Interferometry: A Critical Perspective
## 3 Operational SAR Systems and Applications
### New-Generation and Forthcoming Spaceborne SAR Sensors  
The major thrust for new advances of InSAR technologies has been represented, in recent years, by the deployment and put in orbit of new constellations of satellites (with SAR sensors onboard) that allow significant spatial coverage and enhanced repetition time acquisition. In this framework, each of the Sentinel-1 (S-1) twin satellites of the European Union Copernicus initiative [101] is equipped with a medium-resolution C-band SAR Sensor (Sentinel-1A and 1B). In particular, the _interferometric wide swath_ (IW) acquisition mode is characterized by short revisit time (currently six days) and large spatial coverage (250 km-wide swath), providing, on a daily basis, a large volume of data associated with SLC images of the order of a few GB each. The free and open access policy of the S-1 data and the weekly repetition frequency of the observations contributed to InSAR technology development. For instance, new developments were carried out to adapt the existing processing codes to handle IW S-1 SAR data, acquired through the TOPS mode. In this context, several studies were performed to demonstrate the potential of S-1 SAR data for global mapping and the effective processing of large sequences of TOPS SAR data (e.g., [102; 103; 104; 105] by fostering the development of new algorithms for effective co-registration TOPS SAR images. Other SAR satellite constellations were also developed in European countries. Among them, we can cite TerraSAR-X (TSX) [106] by Germany, COSMO-SkyMed (CSK) [107] by Italy, and PAZ [108] by Spain. They all map the Earth's surface at X-band with improved spatial resolutions (in the order of 3 m \(\times\) 3 m) and different imaging modes. Furthermore, the second generation of CSK satellites (the first one launched on 18 December 2019) will ensure continuity of acquisitions with CSK [109]. Thanks to the Italian Space Agency and the Argentinean CONAE, the Italian-Argentine SIASGE system is currently operative. The Argentinian SAOCOM (Satellite Argentino de Observacion con Microondas) constellation is composed of two twin satellites (SAOCOM-1A and SAOCOM-1B) equipped with a polarimetric L-band SAR system [110]. Together with CSK data, they exploit multi-band SAR images for joint analyses in an X- and L-band synergistic approach [111]. The Advanced Land Observation Satellite 2 (ALOS-2) is the second generation of ALOS satellite constructed by the Japan Aerospace Exploration Agency (JAXA) and is equipped with a phased array synthetic aperture radar operating at L-band (PALSAR). It was launched in May 2014 and acquires full-polarization SAR data using three different imaging modes: spotlight, stripmap, and ScanSAR. NISAR (NASA-ISRO SAR) is an unprecedented satellite SAR mission that will be capable of acquiring radar data operating at two wavelengths (L- and S-band) simultaneously. It results from a partnership between the National Aeronautics and Space Administration (NASA) and the Indian Space Research Organization (ISRO). The fully polarimetric L-band SAR system operates at a wavelength of 24 cm, and it has several operating modes, including quad-polarimetric ones. The S-band SAR system is Quasi-Quad polarimetric, operating at a wavelength of 12 cm. The new operation mode of NISAR is SweepSAR [112]. Each radar feed aperture generates a narrow strip of energy on the reflector that illuminates the whole swath of the ground. NISAR launch is planned for January 2023 and will have global coverage every 12 days. Radarsat-2 is the second-generation SAR Mission launched in 2007 by the Canadian Space Agency (CSA) and MacDonald Dettwiler Assoc. Ltd. It carries a C-Band SAR sensor capable of acquiring data using different observation modes (spotlight, stripmap, and scanSAR) with swath width ranging from 20 to 500 km. The instrument is fully polarimetric and has left- and right-looking imaging capability. A constellation of three satellites named Radarsat Constellation Mission (RCM) will replace the Radarsat-2.