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Orbit and Attitude

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During the nominal 5-year mission lifetime from launch to early 2007 both inclination and altitude were maintained within the limits as listed in Table 2-2. This orbit was also applicable in the first part of the ENVISAT mission extension lasting until October 2010. From late October 2010 on, the ENVISAT orbit was lowered by about 17 km for the second part of the mission extension up to at least end of 2013 including a change in the orbit repeat cycle from 35 days / 501 orbits to 30 days / 431 orbits. In order to use the remaining fuel as efficient as possible it had been decided to abandon the fuel-consuming inclination manoeuvres. This causes the inclination and the mean local solar time at DNX to drift as illustrated in Figure 2-2.

 

 
Orbital Parameters  
Semi-major axis  7159.50 0.07 km 
Inclination  98.55 0.01  
Eccentricity  0.001165 (-0.001165/+0.005) 
Argument of perigee  90.0 3  
Mean altitude  799.8 km 
Orbital period  100.6 min 
Type  Polar, sun-synchronous 
Mean local time at DNX  10:00 a.m. 5 min 
Orbits per day  14 11/35 
Repeat cycle  35 days (501 orbits) 


Table 2-2: Nominal ENVISAT orbit

 

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Fig. 2-2: Nominal and mission extension orbit of ENVISAT (Graphics: DLR-IMF).
 


ENVISAT’s local relative orbital reference coordinate frame forms an orthogonal righthand system with the -Y (roll) axis pointing close to the velocity vector of the platform and the Z (yaw) axis pointing in the direction of the outward local normal of the Earth’s reference ellipsoid (Figure 2-3). For the specification of target directions, azimuth and elevation angles are defined. Azimuth is measured clockwise from the -Y axis around the -Z axis to the projection of the target Line-of-Sight (LoS) in the plane perpendicular to the orbital plane containing the velocity vector. Elevation is the angle between that projection and the target line of sight.

 

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