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Remote sensing missions to date have had low-altitude orbits of a few hundred kilometers around the Earth. While optical missions mainly work, except for infrared instruments, along half of the orbit under sun lighting, SAR images can be acquired across the whole orbit, day and night, and also through cloud cover for the frequencies examined later in this chapter.

Sun-synchronous missions: Most remote sensing missions are placed on sun-synchronous orbits. This is important for optical imagery to assure the same looking angle with regard to the sunlight (although it slightly changes anyway with the seasons). These orbits are also used for SAR imagery, sometimes because they are on the same platform as optical sensors (e.g. JERS-1, Envisat or ALOS), and also because of power constraints. Radar instruments have high power consumption and thus recent missions are often on a 6:00 and 18:00 (or dawn–dusk) orbit that maximizes the energy received by a solar panel (which can be fixed and always orientated towards the sun). ERS-1 and ERS-2 inherited a platform heritage from the first French optical satellite (SPOT-1), keeping the local hour at 22:30 (10:30 descending): the panel had a fixed direction towards the sun but had to make one rotation per orbit on the spacecraft.

These orbits are also near polar orbits, which assure a large coverage of the Earth, except some areas near the poles, depending on the look angle and swath, and with differences between the south and north poles depending on whether the radar is left- or right-looking. When crossing the equator, these orbits have a fixed local hour, which must be maintained throughout the mission duration (inclination maneuvers): this is a key parameter, and all satellite passes occur at the same hour if observed under the same incidence angle. Note that the reference value of the local hour is when the orbit crosses the equator from south to north (ascending part).

Revisit: This corresponds to the time taken for a satellite to revisit a given geographical location on the Earth. It depends on the satellite orbit (altitude, inclination, etc.) and agility.

Repeat cycle: This key parameter expresses the number of days separating two data takes under the same orbital point of view, which can be equal to or higher than the revisit. At the beginning of the mission, in relation to the sensor field of view or swath, we must establish a total number of orbits Ntot and also choose the number of days in the cycle Dcycle. Then, we have the following relationship:

[1.1]

where [14(or 15) + p/Dcycle] is the number of orbit revolutions in one day. The orbit duration is about 100 min as the orbit altitudes are in the range of 400–900 km: when under 96 min, there will be at least 15 orbits/day. p is an integer chosen with a close relationship to Dcycle.

It is possible to change the repeat cycle during a mission, which means a change of altitude for the satellite. For example, ERS-1, with a change of altitude of less than 6 km, had a Dcycle that changed from 35 to 3 days. With a constellation where all satellites have the same orbit (but are phased differently), for example, with SPOT, ERS, CSK, Sentinel and TerraSAR-X/TanDEM-X/PAZ constellations, it is possible to combine images from the different satellites and hence reduce the time interval between data takes while keeping the same orbital point of view, called the “repeat time”.

Table 1.1. Main orbital parameters for some sun-synchronous SAR missions

Satellite	Local hour	Entire rev.	p	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.