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Dcycle (days) Ntot Altitude (km) ERS-1, ERS-2 routine 22:30 14 11 35 501 785 ERS-1 phases A, B, D 22:30 14 1 3 43 775 Envisat 22:30 14 11 35 501 785 Envisat end of life 22:30 14 11 30 431 783 Radarsat-1,2 18:00 14 7 24 343 798 JERS-1 22:30 14 43 44 659 568 ALOS 22:30 14 27 46 671 692 TerraSAR-X, TanDEM-X, PAZ 18:00 15 2 11 167 515 CSK, CSG 6:00 14 13 16 237 620 Sentinel-1A/B 18:00 14 7 12 175 693 Radarsat CM 18:00 14 11 12 179 593 ALOS-2 18:00 14 3 14 199 628

Table 1.2. Main orbital parameters for some sun-synchronous optical missions

Satellite	Local hour	Ascending/descending	Period of orbit (min)	Dcycle (days)	Altitude (km)
Cartosat-2A/2B	9:32	D	97.4	5	635
Landsat 8	10:00	D	98.8	28	705
SPOT-7	10:30	D	98.8	26	694
Sentinel-2A/2B	10:30	D	100.6	10	786
Kompsat-3A	10:50	A	95.2	28	528
Co3D	11:00	-	94.6	-	502
WorldView-1	13:30	D	94.6	14	496
Eros B	14:00	D	94.8	4	520

      Common parameters for optical missions: For the optical missions, the same orbital parameters are key elements for the mission. The orbits are in general sun-synchronous. The local hour has a direct impact on the viewed scenes. There is a compromise between viewing a scene early in the morning in order to reduce atmospheric effects (water vapor) and near midday in order to reduce the length of shadows in the images. The local hour is in general chosen between 9:30 and 14:00.

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

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