Surface Displacement Measurement from Remote Sensing Images. Olivier Cavalie

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СКАЧАТЬ PAN–XS stereoscopic angle: delay 2.25 s 0.018 Pléiades Satellite agility – stereo or tri-stereo 0.10–0.20 WorldView-3 Satellite agility – stereo or tri-stereo B/H = 0.20–0.40

      Spectral resolution or spectral bands: A ground feature or target is defined by its reflectance and spectral emissivity curve. Its response depends on the wavelength ranges. Different classes of objects in an image can thus be distinguished by comparing their responses over distinct wavelength ranges. A large spectral band captures more signal energy and thus enhances the SNR of the image. A panchromatic band defines a wide spectral band, with high SNR and high resolution. In contrast, a multispectral band (XS) captures specific wavelength ranges separated by filters or instruments sensitive to particular wavelengths: the received signal energy decreases as the bandwidth decreases, and therefore the satellite system is designed with a lower resolution to maintain an acceptable SNR. For a given satellite system, regardless of the spectral bands, disparity estimations are performed with the bands that have the highest spatial resolution and the best SNR. Spectral bands may be separated into several families or groups: thermal infrared (TIR) and medium-wavelength infrared (MWIR) depend not only on solar illumination but also on emission from the imaged objects themselves.

      Swath: Swath is the geographical width of the image, which is linked to the optical instrument and the number of detectors in the sensor.

      Viewing angle/incidence angle: The viewing angle (satellite point of view) is the angle between the nadir point and the angle towards the terrestrial point. The incidence angle (region point of view) is the angle between the vertical to a point and the direction of the satellite. Thanks to steering mirrors or the agility of the whole satellite, it is possible to modify acquisition angle configurations. A low incidence angle (region near the satellite nadir) means that, for example, only roofs can be seen. With a high incidence angle, building facades can be seen, but some occlusion occurs (hidden faces, hidden streets, etc.). Spatial resolution decreases as the viewing angle increases. Hence, spatial resolutions are given at the nadir of the satellite.

      Impact of the spatial resolution: The spatial resolution of the satellite directly drives the analysis that can be performed with the images. The definition of optical missions (HR, VHR, etc.) depends on the spatial resolution:

       – 1,000–3,000 m: Analysis of the atmosphere, aerosols and land surface emissivity/temperature. For example: MSG, METOP;

       – 250–1,000 m: Analysis of the atmosphere, aerosols, land surface emissivity/temperature. For example: MODIS, Sentinel-3;

       – 30–60 m: Analysis of land cover (agriculture, forestry, cartography, geology, cryosphere, etc.). For example: Landsat1–7;

       – 15–20 m (medium resolution (MR)): Analysis of large-scale land cover, agriculture, forestry and highway infrastructures and computation of digital terrain models. For example: Sentinel-2 (red edge, shortwave infrared – 20 m);

       – 1.5–10 m (high resolution (HR)): Analysis of land cover, agriculture, regional to city-level coverage, road infrastructures, digital elevation model extraction and medium or large objects, such as boats, and computation of digital terrain models. For example: SPOT-6–7, Sentinel-2 (blue, green, red, near-infrared – 10 m);

       – 0.5–1 m (very high resolution (VHR)): Analysis of many objects in the images. At this resolution, a large number of objects are visible, such as urban elements, houses, vehicles, buildings affected by natural disasters and archeological objects. Extraction of digital surface models can also be undertaken. For example: Pléiades;

       – < 0.5 m (ultrahigh resolution (UHR)): Detailed analysis of elements of the scene. Many objects are visible in the images with more details, including small vehicles, buildings, archeological objects, etc. Computation of digital surface models can be undertaken. For example: WorldView-3, Cartosat-2.

      The main optical satellites that are used for DEM extraction have spatial resolutions better than 10 m. For precise estimation of DEMs, submetric satellites are the best candidates.

      Resolution versus swath versus revisit time: The size of telescope mirrors was the limiting factor in reaching high resolutions for the first generations of optical satellites. With the evolution of space technology, the first civil satellites were launched at the beginning of the 2000s with resolutions lower than 1 m. Ikonos was launched in September 1999 and was the first commercial satellite with a resolution of less than 1 m; it was followed by satellites such as Quickbird-2, Eros B, Kompsat-2 and WorldView-1. A satellite with a very high resolution has a reduced swath, in contrast to decametric resolution satellites. A medium-resolution satellite usually has a wider swath and a shorter revisit time. Conversely, meteorological satellites such as the Meteosat Second Generation (MSG) and Meteorological Operational Satellite (MetOp) family usually have a low spatial resolution around 1 or 3 km, with a wide swath between one and several thousands of kilometers, in order to give a view of the meteorological situation. The number of spectral bands, from 500 nm to 15,000 nm, allows a fine analysis of clouds and aerosols. These satellites have a low spatial resolution but a high temporal resolution. Their spatial resolution is not compatible with DEM extraction.

Choice of the spectral bands: The choice of spectral bands combined with the spatial resolution directly drive the applications of a satellite. Many satellite systems exist with many different configurations (PAN+XS configuration, only PAN configuration, only XS configuration, etc.). A significant number of UHR and VHR satellites have four bands (blue, green, red and near-infrared) associated with a panchromatic band. The panchromatic band (P or PAN) usually has a spatial resolution four times higher than the multispectral ones (XS). Pan-sharpening techniques are employed in order to build panchromatic plus multispectral (P+XS) products, which means fused multispectral products at a panchromatic spatial resolution.

Table 1.5. Description of spectral bands

Band name	Spectral interval
UV band	300–400 nm
Visible bands (VIS)	400–700 nm
Near-infrared (NIR) bands	700–1,000 nm
Short-wavelength infrared (SWIR)	1,000–2,500 nm
Medium-wavelength infrared (MWIR)	3,000–5,000 nm
Thermal infrared (TIR)/long-wavelength (LWIR)	8,000–15,000 nm

      High-resolution (HR) satellites work with lower resolution but include additive infrared bands useful for land cover, water color or atmospheric analysis. To compute digital elevation models and to measure displacements, the spectral band with the highest spatial resolution and the best SNR is generally chosen. For many satellites, this is usually the panchromatic band. For some HR systems with only XS bands, multispectral bands may be used (e.g. Sentinel-2).

1.2. Past and on-going SAR missions

      This section presents spaceborne missions that have СКАЧАТЬ