Surface Displacement Measurement from Remote Sensing Images. Olivier Cavalie
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Название: Surface Displacement Measurement from Remote Sensing Images
Автор: Olivier Cavalie
Издательство: John Wiley & Sons Limited
Жанр: География
isbn: 9781119986836
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The choice of the orbital altitude has an impact on the spatial resolution, as well as on the viewing angle of the terrestrial scenes if a high revisit rate is required. High-resolution satellites are, in general, agile and consequently relax the mission constraints. The altitude of low Earth orbit (LEO) satellites is, in general, chosen to be between 420 km, the altitude of the International Space Station (ISS), and 900 km. The final resolution of the disparity map used to derive elevation (see Chapter 2) or offset map used to derive displacement is directly linked to the initial image resolution. So, the spatial resolution of the pixels is one of the key parameters of an optical mission.
Non-sun-synchronous missions: The famous Shuttle Radar Topography Mission (SRTM) was phased, but not sun-synchronous. With an inclination of 57° at 233 km altitude, this mission produced a worldwide digital elevation model (DEM), but it was limited in latitude.
1.1.2. Parameters specific to SAR missions
This section addresses some parameters specific to SAR systems. Some important parameters, such as the range and azimuth ambiguity ratios, are not listed because they do not really affect phase or ground motion measurements.
Central frequency and bandwidth: In SAR remote sensing, frequency allocation is a key issue. The International Telecommunication Union Radiocommunication Sector (ITU-R) provides recommendations for remote sensing (ITU-R 2009), and particularly for active microwave sensors with regard to Radiocommunication Requirements (RR) Art. 5 (Table 1.3).
Table 1.3. Allocated frequencies and bandwidths for active SAR civil sensors in space as mentioned by the ITU-R (2009)
| Allocated frequencies by RR Art. 5(MHz) | Conventional letter | ITU required bandwidth (MHz) |
| 432–438 | P | 6 |
| 1,215–1,300 | L | 20–85 |
| 3,100–3,300 | S | 20–200 |
| 5,250–5,570 | C | 20–320 |
| 8,550–8,650 | X | 20–100 |
| 9,300–9,900 | X | 20–600 |
There are concerns, however, about other ITU attributions that affect these frequencies, in particular in the C-band (wireless LAN, mobile phones) and L-band (radio location and radio navigation), according to the ITU-R (2010). Sometimes military radars are also active in such frequencies. Both can cause real interferences in image data and make them partly useless. Each frequency has its own properties, and all categories have either flown (L, S, C, X) or are being scheduled (P-band with Biomass).
We can note from Table 1.3 that the highest bandwidth is available in the X-band: as a result of this point combined with the more compact antennas needed for such missions and the inferred cost reduction, many new satellites are aiming at the X-band. In the ITU WRC 2015 meeting, an extension to a 1,200-MHz bandwidth (9,200–10,400 MHz) was approved, but some countries have expressed reservations and stick to 9,300–9,900 MHz (ITU-R 2020). In addition, this table is not applicable everywhere: for instance, the P-band is not accepted over the United States for SAR civil sensors. SAR missions of the interest detailed in this chapter and associated frequency bands are listed in Figure 1.1.
Figure 1.1. Relevant SAR missions and associated frequency bands (future missions in gray italics). For a color version of this figure, see www.iste.co.uk/cavalie/images.zip
Radar swath and pulse repetition frequency (PRF): Stripmap is the simplest working mode of a SAR system. More advanced modes can be explained relative to this mode. In stripmap, the resolution after synthesis in the azimuth direction (along-track) is given by half the antenna length La/2. The Doppler bandwidth in the azimuth is given by BDop = vs/c/(La/2), where vs/c is the satellite velocity. The PRF fP needs to be higher than BDop to avoid spectral layover in the azimuth. In contrast, in the range direction (across-track), the PRF must be limited to avoid range ambiguities or, to put it another way, the swath will be maximized if the PRF is minimized: swath < c/2fP . This explains why stripmap swath is lower in X-band compared to C-band missions (a shorter antenna means higher PRFs and narrower swaths).
If higher resolution is needed, it is possible to expand BDop by electronic steering in the azimuth (the same resolution is kept in range) and thus to increase the length of observation along the orbit: this is called the spotlight mode. The counterpart is that it is no longer possible to obtain a continuous along-track imaging as in stripmap, but only sparse scenes. If a larger swath is needed, then the approach is to use ScanSAR modes: the Doppler bandwidth is shared between different subswaths. By adding an azimuth steering mode like a “reverse” spotlight, it is possible to obtain a Sentinel-1-like mode called the terrain observation by progressive scan (TOPS) mode (Zan and Guarnieri 2006). The range resolution can stay the same as the stripmap mode. In some missions with multi-incidence angles (Envisat or Radarsat-1, for instance), loss of range resolution has been mitigated for some modes by increasing the transmitted bandwidth to assure a better ground-range resolution for the lowest incidence angles.
Yaw steering: Most SAR missions use yaw steering on board, so that the spacecraft compensates for Earth rotation and the point perpendicular to the orbit is effectively at zero Doppler. Radarsat-1 was not yaw steered, which resulted in a Doppler shift of several PRFs.
Incidence angle: In contrast to optical sensors, SAR imaging sensors cannot look to the nadir, as this would create simultaneous echoes in the same pixel from both sides. In addition, the best resolution is obtained at the largest incidence angle, at the expense of lower power received due to the increased distance from the ground. The choice of the incidence angle is critical in mountainous areas due to layover on one side and shadows on the other side of mountains.
Orbit precision: This is an important parameter used to combine SAR images for interferometry. An orbit that is not precisely known will lead to an important phase gradient in the interferogram. There are three kinds of orbit that can be successively delivered:
– the first is a predicted orbit that usually comes along with the product when produced in real time;
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