Surface Displacement Measurement from Remote Sensing Images. Olivier Cavalie

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СКАЧАТЬ then, a few hours or days after the data acquisition, a restituted orbit is available with more precision;

       – finally, after three or four weeks, the most precise orbit can be delivered to users, thanks to dedicated payloads and specific computing.

      Nowadays, with precise orbit determination payloads onboard, orbit precision can be known at the order of a few centimeters.

Orbital tube: Recent SAR missions have integrated the use of differential interferometry in their specifications (Sentinel-1, ALOS-2, the Radarsat Constellation mission) and have put constraints on orbit housekeeping. Thus, the orbit must stay in a tube called the “orbital tube”, with a radius as small as one hundred or a few hundreds of meters to keep the perpendicular baseline small (see Chapter 4, section 4.2).

      Duty cycle, down-link rate and onboard storage: The duty cycle is equivalent to the percentage of time that the radar will be able to work along the orbit. SAR systems need a lot of energy, making it impossible for them to work permanently. Depending on the design of the satellite power unit, a system can deliver between a few seconds of imaging and about 1/4 of the orbit time. There is a huge difference between very small satellites of a few hundreds of kilograms and big platforms of two or three tons. The down-link rate and onboard storage capacity are often two key elements that usually come together. A data relay satellite is sometimes helpful to cope with all the constraints or avoid multiple ground antennas for down-linking, but this usually needs a laser link and data relay commercial contract, which can also be quite expensive.

      Instrument noise equivalent σ₀: This parameter is very important when looking at amplitude images. It gives the minimum value that can be reached by the system for an elementary pixel, but entails lower values with multi-looking (see Chapter 3). It varies in range along the swath, and the specification must deliver the worst case: in general, the best values are in the middle of the swath where the antenna gain is maximum. This parameter affects measurements for ground motion: if its value is too poor, then some surfaces that have low backscattering coefficients will not be properly estimated, for instance asphalted roads, tarmacs and even sand in the desert.

      Polarization: The design of the antenna sub-system determines what polarization should be implemented. For ground displacement purposes, as the signal-to-noise ratio (SNR) is more favorable, co-polarized data are mainly used, and it is difficult to say whether using HH and VV for offset tracking or InSAR techniques are more advantageous. The use of dual polarization, HH+HV or VV+VH, or quad polarization (HH+HV+VV+VH) is more relevant for other remote sensing applications, such as classification, forest extents and heights, maritime surveillance, pollution at sea or other change detection characterizations. Furthermore, the use of quad polarization generally reduces the swath and azimuth resolution by a factor of two, and thus the size of the archive in this mode is less important.

      1.1.3. Parameters specific to optical missions

      Pushbroom: A pushbroom camera consists of an optical system projecting an image onto a linear array of sensors. Usually, a focal plane is composed of several time delay integration (TDI) image sensors, mounted in a staggered configuration. The image is directly built at the sensor level. Charge-coupled device (CCD) sensors are used where ultra-low noise is preferred, and now complementary metal oxide semiconductor (CMOS) matrix detectors are increasingly used.

Stereo, tri-stereo and more: To build a disparity map (see Chapter 2), at least two stereo images from separate view angles are necessary. It is possible to combine more images (tri-stereo or even more) to build a more precise map. With four images, all faces of buildings may be seen, and it is possible to build a digital elevation model and associated pixel values. The different images over a geographical point are taken from different incidence angles. If they are taken with the same satellite, then they will be asynchronous. If they are taken with a constellation of satellites on the same orbit, then they may be synchronous, such as with the Co3D system. Asynchronous images imply two acquisition instants. If the delay is a few seconds, mobile elements (such as terrestrial vehicles, bots, planes) or clouds will alter the raw displacement measure. If the delay is a few weeks or months, some buildings may appear or disappear. Evolution of agricultural landscape will also alter the match between the images.

      Agile satellites: The agility of satellites is a key function. Agile satellites are able to do attitude maneuvers and focus on one scene. This increases the revisit frequency over the same region, and so reduces the time interval between two acquisitions. Such satellites are able to point towards a geographical point thanks to steering mirrors or by changing their whole attitude (yaw, pitch and roll control).

      Base-to-height (B/H) ratio: The shift between two images creates a stereoscopic parallax in one direction (see Chapter 2). The stereoscopic angle (also called the base-to-height or B/H ratio) is the ratio between:

       – the distance between the two viewing points (base – B);

       – and the distance to the observed scene (height – H).

      In the older generation of satellites, the B/H ratio was directly fixed with the instrument characteristics. For example, the SPOT-5 satellite had high-resolution HRS instruments to provide large-area along-track stereoscopic images (forward and backward of the satellite). With agile satellites, it is possible to choose the best view conditions and define the choice of B/H ratio. A large B/H ratio favors the observation of different points of view of the scene (e.g. two faces of buildings) but creates constraints on the disparity computation, as some elements of one scene are not seen in the other one. With a lower B/H ratio, the delay between two images is reduced and a scene can be observed under nearly the same conditions, but it adds constraints to the precision of the disparity computation algorithms. In the case of SPOT-5, the shift between panchromatic and XS detectors (19.5 mm in the focal plane) creates a slight stereoscopic angle that allows a stereo reconstruction using the P+XS correlation. In the case of Pléiades, the B/H ratio is chosen by the user.

      The key optical parameters of satellite systems are explained in the following:

      Spatial resolution: Spatial resolution is a measure of the smallest angular or linear separation between two objects or two pixels on the ground. It is usually expressed in radians or meters. Spatial resolution decreases as the viewing angle increases. Spatial resolutions are, in general, given at the nadir of the satellite. For example, for Ikonos, the spatial resolution is equal to 0.82 m at the nadir and 1 m at 26^° off-nadir.

      Table 1.4. Examples of satellite missions and B/H ratios

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Satellite	Mission characteristics	B/H ratio
SPOT-5	Steering mirror (MCV). Stereoscopic angle up to 54 deg	1.02
SPOT-5	HRS instrument: angle 40 deg, delay 90 s	0.8
Co3D	Synchronous acquisitions	0.20–0.30