Final orbit acquisition
SPOT 4 is sent into a near-polar sun-synchronous orbit. The orbit must then be adjusted to meet the other requirements of SPOT orbits, particularly phasing. It must also be placed at the right altitude with respect to the other SPOT satellites.
The diagrams below show the relative positions chosen for the three SPOT satellites.
The diagrams below show the relative positions chosen for the three SPOT satellites.
Ariane will send SPOT 4 onto an orbit approximately 20 km below that of the other SPOT satellites. This difference in altitude means that SPOT 4 moves faster than the other SPOT satellites: SPOT 4 drifts with respect to the other satellites.
Advantage is taken of this relative drift as shown in the following example. We wait until SPOT 4 approaches its final position with respect to SPOT 1 and SPOT 2, then raise it to the altitude of the other SPOT satellites (which is also at the nominal phasing altitude), thus stopping the drift.
To optimize altitude correction maneuvers, the raising of the satellite to its nominal orbit is usually split into two orbit-raising maneuvers.
Advantage is taken of this relative drift as shown in the following example. We wait until SPOT 4 approaches its final position with respect to SPOT 1 and SPOT 2, then raise it to the altitude of the other SPOT satellites (which is also at the nominal phasing altitude), thus stopping the drift.
To optimize altitude correction maneuvers, the raising of the satellite to its nominal orbit is usually split into two orbit-raising maneuvers.
Final orbit acquisition depends on three points:
- launch date,
- launch time within the window defined above,
- launch vehicle's deviation with respect to the required altitude, i.e. the difference between target altitude and actual altitude once SPOT 4 has separated from Ariane.
Effect of launch vehicle's altitude dispersion
The launch vehicle aims to place SPOT 4 on an orbit 20 km below the orbit of the other SPOT satellites. The launch is considered nominal (with respect to altitude) if the altitude is within ± 4 km of the target altitude:
- If the orbit reached on separation is exactly that targeted, SPOT 4 gains 25 seconds on the other satellites per orbital revolution.
- If the orbit reached is 4 km below the target orbit, SPOT 4 deviates by 28 seconds per orbital revolution.
- If the orbit reached is 4 km above the target orbit, SPOT 4 deviates by 23 seconds per orbital revolution.
The time taken for final orbit acquisition depends on the drift velocity.
Final orbit acquisition-strategies
SPOT 4 must be placed on its final orbit within 18 days.
The strategy outlined in the paragraph on the basic principle of "final orbit acquisition" will enable final orbit acquisition within 18 days in most cases.
However, there are two exceptions:
- The configuration following separation is such that SPOT 4 is inserted just before its target rendezvous (though 20 km below). This is shown in the diagram for "Effect of launch date" for launches on 7 or 12 March at the beginning of the launch window. In these cases, SPOT 4's drift is too fast to allow the two orbit-raising maneuvers to be performed before the rendezvous, and SPOT 4 will therefore have to complete a whole orbital revolution (with respect to the other satellites).
- The configuration following separation is such that SPOT 4 is inserted at (or just after) its target rendezvous, (case of 17, 22 or 27 March 1998 at the beginning of the window), and the altitude attained leads to a slow drift.
In these two cases, the following strategy will be applied:
SPOT 4's altitude is raised above that of the other SPOT satellites. SPOT 4's velocity will then be lower than that of the other satellites, reversing the drift and returning to the rendezvous.
When SPOT 4 reaches its rendezvous, the drift is stopped by an orbit-lowering maneuver.