SPOT 4 has a launch mass of 2800 kg. It will be launched by an Ariane 40 version of the Arianespace launch vehicle (without strap-on boosters). The SPOT satellite will be the only passenger on board.
The launch vehicle is not designed to place SPOT 4 on its final orbit.
It is, however, expected to deliver SPOT 4:
The launch vehicle is not designed to place SPOT 4 on its final orbit.
It is, however, expected to deliver SPOT 4:
- onto its near-polar (sun-synchronous) orbital plane, with the descending node at 10:30 local time,
- and in a near-circular orbit at an altitude similar to that of the other SPOT satellites, but different enough to be able to transfer SPOT 4 later onto the required final orbit (see final orbit acquisition phase): SPOT 4 will be placed in an orbit 20 km below that of the other SPOT satellites.
As Ariane is launched from Kourou in French Guiana, the launch must take place when, (given the rotation of the Earth), Kourou crosses SPOT's orbital plane. This occurs twice a day. Theoretically, it is possible to place SPOT in polar orbit either in the ascending part, by launching northwards from Kourou, or in the descending part, by launching southwards. However, the southward launch is rejected for safety reasons (launch towards Brazil).
Taking these considerations into account, the launch is in a northerly direction, into the ascending orbit, when Kourou is near the local time of the orbit's ascending node (22:30). The exact time of the beginning of the launch window is 1:37 (Universal Time Coordinated). The Ariane 40's launch capability and SPOT 4's orbital plane requirement result in a 9-minute launch window.
In fact, three launch times are chosen within this slot:
In fact, three launch times are chosen within this slot:
- Beginning of window: 1 h 37 min 00 s (UTC)
- Middle of window: 1 h 42 min 00 s
- End of window: 1 h 46 min 00 s.
The target time is that of the beginning, so that if there are delays in the countdown (due to unfavorable weather conditions, for example) it is still possible to launch the satellite in the middle or at the end of the window. If the delays make it impossible to launch at the end of the window, the launch is postponed to another day.
SPOT 4 separates from Ariane's third stage 1113 seconds after ignition of the engines of the first stage (H0). By this time it is off Bermuda.