Surface Displacement Measurement from Remote Sensing Images. Olivier Cavalie

10 VRSS-1 2012 - 2.5 450–900 4 + 4 57 10 VRSS-2 2017 - 1 - 4 + 5/6 - - WorldView-1 2007 - 0.46 448–895 0 17.6 11 WorldView-2 2009 - 0.46 464–801 4 + 4 16.4 11 WorldView-3 2014 - 0.31 450–800 8 + 8 + 12 13.1 11 WorldView-4 Zhuhai-1 2016 2019 0.31 450–800 4 13.1 11 Zhuhai-1 OHS-2A-2D/3A-3D 2018, 2019 - 10 - - 150 - Zhuhai-1 OVS-1A/1B/2 2017, 2018 - 0.9–2.0 - - 22.5 - Ziyuan-3/ZY-3 01-03 2012, 2016, 2020 - 2.5 500–800 4 51 10

       – Pléiades: Pléiades represents the new generation of French satellites with submetric resolution. The main characteristics of Pléiades are the high image quality, the high agility of the satellites, high image geolocation, the ability to provide images with a short delay (exemplified with the International Charter “Space and Major Disasters”) and the ability to take stereo or tri-stereo images on a single track.

       – RapidEye: The first constellation of Earth observation satellites, with five identical satellites launched simultaneously with five spectral bands and 5 m resolution. The aim of the constellation was to provide Earth observation imagery for decision-making and geospatial services. In April 2020, Planet Labs decided to end the operation of the RapidEye constellation.

       – Sentinel-2: An Earth observation mission from the Copernicus Program to systematically acquire imagery at a high resolution (10–60 m). It was developed and is being operated by the ESA. Its main characteristics are high swath (290 km), systematic acquisition of land surfaces and a large revisit (five days with two satellites) and a large number of multispectral bands with high SNR. However, the main breakthrough is the free and open data policy. After many years of acquisition of decametric images by several Earth observation satellites, Sentinel-2 has boosted the general use of satellite images at 10 m resolution. The easy access to satellite images opens the door to many new applications: agriculture (crop monitoring), land cover change, detailed vegetation monitoring, ice extent mapping, flood mapping, etc.

       – SPOT-6: SPOT-6 is the proposition by Airbus Defence and Space to ensure the continuity of SPOT-4 and SPOT-5 satellites. The swath is similar, with a resolution of 1.5 m. It is thus able to cover large areas for agriculture, deforestation and environmental monitoring, as well as for industries, coastal surveillance and defense.

       – WorldView-1: This satellite offers a high capacity and high resolution (0.46 m). It is based only on panchromatic acquisitions. WorldView-1 has an excellent geolocation capability and rapid targeting of in-track stereo images.

       – WorldView-2: This very-high-resolution satellite adds multispectral bands to the panchromatic one for enhanced spectral analysis. WorldView-2 has a higher agility and works at a higher altitude. Consequently, the revisit time of any place on Earth is reduced to 1.1 days. Despite its higher altitude, the spatial resolution remains the same as WorldView-1. WorldView-2 has the capacity for direct tasking: some customers can load imaging profiles directly to the satellite and it delivers the data directly to their own ground stations.

       – WorldView-3: While WorldView-3 is similar to WorldView-2 in terms of conception, several multispectral instruments are added: it contains eight multispectral bands (visible to near-infrared), eight short-wave infrared (SWIR) bands and 12 CAVIS bands (at 30 m resolution). It is the first super-spectral high-resolution commercial satellite.

       – WorldView-4: In 2019, WorldView-4 suffered a failure in one of its control moment gyroscopes. It is therefore considered no longer usable.

       – Dove PlanetScope: PlanetScope is one of the satellite constellations operated by the company Planet. It has almost daily coverage worldwide with more than a hundred nanosatellites. Each PlanetScope is a CubeSat 3U with dimensions 10 cm × 10 cm × 30 cm, producing images at 3.7 m resolution at nadir, with four spectral bands (red, green, blue and near-infrared).

      1.4.3. Future optical missions

      Regarding the new missions, it is possible to outline the main strategic drivers as follows:

       1) satellite operators replace the existing missions that provide intensively used products, across the whole range of resolutions (<10 m spatial resolution in general). Examples: WorldView Scout 1-6, ResourceSat-3/3A;

       2) new commercial systems try to reach higher-spatial-resolution markets. Examples: PleiadeNeo, WorldView Legion 1-6;

       3) the value of the data is linked to the acquisition time after an event. Operators want to decrease the satellite revisit time by increasing the number of agile satellites. Thanks to the evolution of technologies, it is possible to reduce satellite dimensions and satellite weights compared to the earlier generations. It is also possible to reduce the cost of a single satellite (and the launch cost). In order to reach a high spatial resolution, a lower altitude is chosen. Examples: new constellations such as Co3D, Vivid-i and PlanetScope. However, many new space constellation projects have also been canceled due to technical and financial constraints;

       4) new technologies open the door to new in-orbit functionalities; for example, new sensors provide new imagery and video capabilities;

       5) new hyperspectral satellites are coming, such as Enmap and Prisma, with 30 m resolution. The hyperspectral images may be used to obtain information about soil characteristics such as humidity and may be very useful for the analysis of soil displacements.