Surface Displacement Measurement from Remote Sensing Images. Olivier Cavalie
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August 1997, which limited its use over direct receiving ground stations, and the spacecraft finally terminated its mission in October 1998. The SAR sensor operated at 1.275 GHz (wavelength of 23.5 cm), with a bandwidth of 15 MHz. Only co-polarization in HH was implemented. The nominal swath was a stripmap mode of 75 km, at a fixed look angle of 35°. Tracks were acquired regularly, minimizing the time between two consecutive swaths, which was a great advantage for large mosaics and radiometric stability. Interferometry showed great coherence for vegetated areas compared to C-band radar, as in ERS, with comparable resolution. As an optical system (OPS) was also used, the satellite was put on a sun-synchronous orbit with a descending node at 10:30 to 11:00.
ALOS was the second Japanese satellite and was able to deliver L-band SAR images through its PALSAR instrument operating at a 1,270 MHz central frequency. The launch took place in January 2006, and the local hour was set at 10:30 descending node, with an onboard optical sensor (PRISM, AVNIR-2). The bandwidth was increased to 28 MHz in single-polarization HH or VV and 14 MHz in dual-polarization HH+HV VV+VH or quad polarization with different fine beams, with 70 km and 40 km swath, respectively. The ScanSAR mode only worked in co-polarization HH or VV, with a 350 km swath sub-divided into five beams. The spacecraft stopped operating in May 2011.
In April 2014, ALOS-2 followed the ALOS program with an L-band radar payload, without the optical part later put on ALOS-3. Compared to ALOS, the ALOS-2 PALSAR instrument improved the resolution (tunable between 14 and 84 MHz), the revisit cycle (14 days instead of 46 for ALOS) and right and left imaging capacities and provided new modes, such as spotlight and ScanSAR with burst synchronization for ScanSAR interferometry.
Data access: We can note that the background mission of these satellites was orchestrated by a basic operational scenario, i.e. a long-term homogeneous and consistent programming of the whole Earth in different modes, with regular updates. It is possible to freely access JERS-1 and ALOS data through the ESA EO portal at https://earth.esa.int/web/guest/-/jers-1-sar-level-1-single-look-complex-image and https://earth.esa.int/web/guest/-/alos-palsar-fbs-fbd-and-plr-products, respectively. The datasets contain all ESA acquisitions over the ADEN zone (Europe, Africa and the Middle East), plus some worldwide products received from JAXA.
1.2.4. SRTM and X-SAR
The Shuttle Radar Topography Mission was aimed at computing a semi-global digital elevation model from the Earth. This was achieved in February 2000 on board the US space shuttle Endeavour carrying the SIR-C/X-SAR payload, which previously flew in 1994 in L-, C- and X-bands. This time, the payload was reconfigured as a single-pass interferometric mission with a fixed baseline as long as the 60 m mast that separated the dual antennas in two bands: the C-band instrument from the Jet Propulsion Laboratory (JPL) and the X-band instrument from the Italian Space Agency (ASI) and German Space Agency (DLR). For the C-band, swath reached 225 km thanks to a four-beam ScanSAR, and for the X-SAR instrument, there was a stripmap swath of 50 km. The mission duration was limited to 11 days: only the C-band swath could assure the semi-global DEM, while the X-band covered a subpart of it. Due to the 57° inclined orbit of the shuttle at an altitude of 223 km, the DEM covered only latitudes ranging from 56° south to 60° north.
Data access: DEMs in C-band at 3 sec (90 m) spacing and then at 1 sec (30 m) spacing were released worldwide in 2003 and 2015, respectively, and are still commonly used 20 years after the mission. They are freely available after registration at https://earthexplorer.usgs.gov/. The more sparse X-SAR DEM is available through the geoportal https://download.geoservice.dlr.de/SRTM_XSAR/.
1.2.5. TerraSAR-X, TanDEM-X and PAZ
TerraSAR-X (TSX), TanDEM-X (TDX; TerraSAR-X add-on for digital elevation measurement) (Germany) and PAZ (Spain) are similar satellites launched in June 2007, June 2010 and February 2018, respectively. They are compact “Toblerone” satellites, including a SAR antenna and a solar panel fixed on the main body. This shape is compliant with an heliosynchronous dawn–dusk orbit and has a very low altitude of 515 km compared to previous SAR missions. The two German satellites produced a worldwide DEM, following the incomplete coverage of the SRTM/X-SAR mission, still with the interferometry technique and simultaneous bistatic acquisitions: flying together, various baselines were chosen along the project to refine a 12 m DEM at a specified 2 m vertical relative accuracy (slopes less than 20°). The SAR mission started with different beam modes, such as four-beam ScanSAR (100 km), stripmap (30 km), spotlight (10 km) and high-resolution spotlight (azimuth extension limited to 5 km instead of 10 km). Then, in 2013, six-beam wide ScanSAR (194–266 km) and Staring Spotlight (7.5 km max and limited azimuth swath of 2.5–2.8 km) modes were added as operational modes. Staring Spotlight has a 0.18 m resolution in the azimuth and can use a 300 MHz bandwidth to afford a 0.6 m slant range resolution. Polarization can be in co-pol HH or VV, and dual polarization is available with different combinations depending on the mode used. Quad polarization is only available as an experimental mode. In 2009, a noticeable experiment assessed the TOPS mode interferometric feasibility, before the launch of Sentinel-1 (see Chapter 2, section 2.8; see also Prats-Iraola et al. (2012)).
PAZ data can be combined with TSX with a four- or seven-day time interval.
Data access: TSX and TDX data can be obtained at a low cost for scientific use after submitting a relevant proposal to the DLR. The 3 sec (90 m) DEM has been freely available with registration since October 2018: https://download.geoservice.dlr.de/TDM90/. The most precise DEM is commercially distributed by Airbus Defence and Space: https://www.intelligence-airbusds.com/imagery/reference-layers/.
1.2.6. COSMO SkyMed constellations
COSMO SkyMed (CSK) is the first Italian SAR constellation. Four satellites operate in the X-band on the same dawn–dusk orbit and the ascending node is at 6:00, whereas other satellites often use 18:00. The platform is inherited from the Radarsat-2 prima platform, with right- and left-looking abilities. The launches of the four satellites occurred in June and December 2007, October 2008 and November 2010.
CSK is a dual civil and military mission, and the submetric spotlight is for defense use only. Other modes are accessible for commercial use, i.e. two different ScanSAR modes (100 and 200 km), one stripmap (40 km) and one civil spotlight mode (10 km). The satellites operate in a single-polarization scheme with HH, HV, VV or VH selectable.
COSMO SkyMed New Generation (CSG) is composed of two satellites: the first one was launched in December 2019. Orbiting at the same altitude as the first generation, this new system has significant improvements in terms of agility, resolution and polarizations. It is possible to obtain simultaneous swaths in stripmap mode and also different spotlight images on the same orbital span. The system also adds quad-polarization possibilities in stripmap. For civilians, three spotlight modes are possible, in single or dual polarization. Stripmap always has the same 40 km swath in single polarization as well as in dual polarization. As for the first generation, a narrow ScanSAR of 100 km and wide ScanSAR of 200 km are possible, again in single or dual polarization.
Data access: Archived products can be seen and commercially ordered through the eGEOS portal at http://catalog.e-geos.it/.
1.2.7. SAOCOM1
Satélite Argentino de Observación con Microondas (SAOCOM) is an L-band SAR program from Argentina’s space agency the Comisión Nacional de Actividades Espaciales (CONAE).