Satellite "slowing" lowers the satellite, which thengets ahead in the phasing compared with a"reference" satellite. This means that the actual orbit's descending nodes are further east than those of the reference orbit.
The Operations Control Centre monitors the difference in longitude between nodes so that at the equator it is kept within ± 3 km (mission requirement).
When this drift reaches the eastern limit allowed by the mission, the Operations Control Centre programs a maneuver to raise the satellite above the theoretical altitude, which makes the descending nodes, drift westward; the effect of atmospheric drag cancels out this drift and brings the satellite down to the theoretical altitude. At this point in time, the descending nodes must be no more than 3 km to the west.
The drift continues eastward until a further maneuver to raise the altitude is scheduled.
The frequency of orbit-raising maneuvers varies between:
o Two months when solar activity is low (the satellite descends about 20 cm a day), and two weeks when solar activity is high (the satellite can descend 5 to 10 meters a day and, occasionally, as much as 20 or 30 meters).
Phasing at upper latitudes (60° North):
· The phasing error at 60° latitude is the sum of the phasing error at the equator due to atmospheric drag and the phasing error due to the decrease in orbital inclination.
· Approximately once every 18 months, an inclination correction maneuvers is scheduled theoretical inclination, whilst respecting the phasing error upper limit of 5 km. The local time of the descending node must be kept at 10:30 ± 10 minutes for mission requirements.The difference between local time and the theoretical local time of 10:30 varies according to a non-linear function of inclination and altitude.
Taking into account the frequency of inclination correction manoeuvres, the local time is in fact kept within one or two minutes of 10:30.