Surface Displacement Measurement from Remote Sensing Images. Olivier Cavalie
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SAOCOM-1A and SAOCOM-1B were launched in October 2018 and late August 2020, respectively. Agreements with the ASI made it possible to put SAOCOM satellites on the same orbit as the CSK constellation, providing the opportunity of obtaining X-band and L-band images at a 10-minute interval. CONAE implemented a TOPSAR mode, as in Sentinel-1, for the narrow ScanSAR of 170 km and a wide ScanSAR of 350 km, as well as a stripmap mode of 40 m at 10 m resolution. Single or dual polarization HH, HH+HV, VV or VH or as well as quad polarization, are possible.
Data access: Some archive images can be accessed through the portal at https://catalog.saocom.conae.gov.ar/catalog after registration.
1.2.8. Sentinel-1
Sentinel-1 belongs to the Copernicus European program, but the conception and operations were delegated to the European Space Agency. This is the first long-term program that has really taken InSAR as an operational objective, with a very small orbital tube of 100 m radius and 12-day repeat pass, which comes down to six days with two satellites (S1A and S1B), and possibly four days with three satellites operating, as Sentinel-1C and 1D have already been authorized. Sentinel-1A and 1B were launched in April 2014 and April 2016, respectively.
In contrast to the Envisat mission, a focus on a consistent archive is of prime interest and most emerged land is monitored, with the maximum capacity of any system operating in Europe. There are four basic modes: extra wide (EW) ScanSAR of 400 km in five subswaths, interferometric wide (IW) ScanSAR of 240 km in three subswaths, six stripmap modes about 80 km each, and a wave mode (20 km images every 200 km at incidences of 23° and 36°) for open ocean. IW is the privileged mode for land, avoiding gaps, and sometimes uses single-polarization VV, but most of the time dual polarization has been used since Sentinel-1B was launched.
IW is the main operational mode: the ScanSAR mode includes the TOPS mode successfully tested on TerraSAR-X (see section 1.2.5). Agility for the antenna is introduced along-track (azimuth) from backward to forward, for each burst of about 20 km. Each IW subswath consists of 9–10 bursts delivered with their own auxiliary parameters. Each burst is precisely synchronized between each acquisition so that interferometry is always possible.
The next two Sentinel-1 satellites, S1C and S1D, are quite similar to S1A and S1B and are in a building/storage phase untill launch, which will probably occur in 2022/2023 and 2024/2025, with the same operating modes for SAR images, but with increased capacity for maritime surveillance (AIS payload). This will hopefully insure continuous SAR data from October 2014 to beyond 2035, which has never happened in a space program to date. Hopefully, the system will operate with three satellites and four-day repeat pass interferometry on a large scale, at least over Europe.
Some limitations of the mission with regard to interferometry: In contrast to stripmap, the IW mode introduces additional complexity with regard to interferometry: to avoid phase jumps in the final image, very precise coregistration and phase examination in burst overlaps must be achieved. Products do not always cover the same area on the ground and splitting/gathering bursts in several products may be necessary.
Data access: All Sentinel-1 products are freely accessible via the Copernicus hub (https://scihub.copernicus.eu/dhus/) or on mirror sites such as PEPS (https://peps.cnes.fr/). In contrast to the Copernicus hub, PEPS do not archive RAW data, but only processed single-look complex (SLC), ground range detected (GRD) and wave mode data. The archive began on October 3, 2014. Other products directly derived from the Sentinel-1 data and showing ground motion for European land are freely available through a Copernicus service (https://www.copernicus.eu/en/events/events/onlineeuropean-ground-motion-service-copernicus).
1.2.9. ICEYE
ICEYE-X1, launched in January 2018, is the first SAR microsatellite of Finland’s operational commercial constellation Iceye Ltd, a startup company from Espoo. The ICEYE-X1 cube (70 cm side) weighs under 100 kg, which is less than a twentieth of Sentinel-1. This size reduction produces a strong reduction in the cost too. The global imaging constellation will consist of 18 small satellites, allowing less than a few hours for revisit. ICEYE’s satellite constellation in the X-band is growing, with additional spacecraft being produced and launched each year: in December 2018, ICEYE-X2 was launched. The ICEYE-X3 payload, launched on May 5, 2019, was a demonstration mission, dedicated to the US Army. ICEYE-X4 and -X5 were launched on July 5, 2019, with a Soyuz-2-1b rocket, and ICEYE-X6 and -X7 were launched on September 28, 2020. ICEYE-X8 and -X9 followed in January 2021. The Finnish company launched four additional SAR satellites at the end of 2020 and planned to launch an additional eight in 2021. The company offers X-band SAR data in several imaging modes, including very high resolutions in single-look complex and ground range detected image formats. ICEYE’s satellites are capable of imaging in stripmap, spotlight and ScanSAR modes in VV polarization.
Data access: See https://www.iceye.com. The Finnish New Space Economy company ICEYE is now providing radar imaging data from its commercial SAR satellite constellation to the International Charter “Space and Major Disasters” at no cost for use in monitoring and response activities.
1.3. Future SAR missions
This section presents the main SAR missions that have been decided on and that will use interferometric measurements. We must note that many SAR systems are planned to arrive on the market, more than ever before, with very cheap and light payloads. In most of the cases, the orbit is not maintained, and if interferometry is theoretically possible between two passes, it must be seen more like an opportunity rather than a dedicated application. For these reasons, these missions are not detailed here.
1.3.1. TerraSAR-NG
TerraSAR-NG is an improved system continuing the X-band TSX/TDX/PAZ missions. Initially, one satellite is forecast, and one of the aims is to implement a 1,200 MHz bandwidth and reach a 0.25 m resolution on the ground. ITU regulations incorporated the demand in 2015 (see section 1.1.2 on central frequency and bandwidth). Another improvement is the implementation of a TOPSAR system for different ScanSAR modes.
1.3.2. ALOS-4
ALOS-4, planned to be launched in 2022, is a successor of the Japanese SAR L-band missions ALOS and ALOS-2. Its PALSAR3 instrument will be primarily activated with right-side looking and in a stripmap mode with an increased 200 km swath (compared to 50 km with the ALOS-2 PALSAR2), while maintaining the high temporal resolution with a revisit time of 14 days and covering an incidence angle from 30 to 44°; the other beams (ScanSAR and spotlight modes) and left-side observation are used for quick disaster monitoring. The ALOS-4 will be equipped with an AIS receiver, as with the ALOS-2, for maritime surveillance purposes. To deal with the deforestation issue, ALOS-4 will conduct observations more frequently, with five times more precision, in order to detect smaller deforestation areas than ALOS-2. ALOS-4 will acquire data more frequently than ALOS-2; thus, interferometric data will lead to more efficient infrastructure maintenance.
1.3.3. NISAR
The NASA and Indian Space Research Organisation (ISRO) NISAR mission with both L-band and S-band wavelengths is dedicated to and optimized for studying hazards and global environmental change. NISAR, planned to be launched in 2022, will observe Earth’s land and ice-covered surfaces at 3–10 m resolution globally, with 12-day regularity on ascending and descending passes, sampling Earth on average every six days for a baseline three-year mission. As NISAR will be only left-looking, it will be a major asset for mapping Antarctica, while Sentinel-1, with a similar large