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СКАЧАТЬ 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 СКАЧАТЬ