The role of the Station during launch

The weather unit begins its briefings at T-2 days, and as liftoff approaches, their frequency and detail increases. If Ariane is to successfully leave its launchpad, the lightning criteria both on the ground and in flight must be satisfactory...

Meteorological information during countdown can be divided into two main types: on the one hand key points dealing exclusively with wind conditions at altitude, and on the other hand briefings covering all other parameters. These two categories of information, which may come separately or simultaneously, are exclusively destined for the decision-making authorities at CSG (the Director and Assistant Directors) and Arianespace (Flight Unit management), as well as the mission and operation coordination managers (the Arianespace Mission Director and the Director of Operations). The latter two can then immediately pass the information on to their respective interfaces: the campaign authorities and clients for the Mission Director, and the Ariane Launch Base for the Director of Operations. As for the Weather Deputy, his Weather Status Report represents the renowned 'green' or 'red' weather traffic light.

The weather unit begins its briefings at T-2 days, when the Flight Unit decides whether or not the launcher should be transferred between the BAF (Final Assembly Building) and the launch zone. During the evening of the transfer, it decides whether or not to begin countdown the following day, bearing in mind the time constraints affecting the launcher and the stocks of some fluids (which cannot tolerate too many successive postponements).

As T-0 approaches, the frequency and detail of the weather briefings increases. On launch day, the key ones are as follows:

* T-7 hours: weather briefing preceding cooling down of lines, describes situation and trends for all criteria for countdown and T-0

* T-5 hours: weather briefing prior to filling of cryogenic stages

* T-20 min: weather briefing for trends in C1, C2 and C3 criteria, trends for winds on the beach and launchpad, wind conditions at altitude

* T-10 min: weather briefing to help decide whether to begin synchronised sequence at T-7 min, C1, C2, C3 criteria, criteria for wind on beach and at liftoff

The criteria and the weather reports

Depending on how far the launcher countdown has advanced, CNES, Arianespace and the weather team meet to analyse the meteorological constraints, listing the launch parameters and criteria. These essentially concern lightning, both on the ground and in flight, as well as winds at liftoff, on Kourou Beach (toxicity hazard) and at altitude. If Ariane is to successfully leave its launchpad, the lightning and C1, C2 and C3 criteria must be satisfactory:

* C1: no risk of lightning within a 10 km radius of the launcher.

* C2: no convection clouds with tops at altitudes greater than 6,500 m within a 10 km radius of the launchpad at T-0.

* C3: no 'anvil-shaped' cumulonimbus clouds vertically above the launchpad, if the nearest storm cell is less than 20 km away.

Out of a total of 171 launches, only 23 postponements have been recorded (a rate of 13%), essentially due to the lightning criterion. Apart from four postponements for more than one day, half of the launches were rescheduled within the same launch window and the other half were postponed by 24 hours.

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The CSG Station and Météo-France