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In the ALMA project, in order to achieve the specification, very important is the use of a metrology instrument able to correct the non repeatable errors. Errors induced by wind or temperature variations inside the structure that are non systematic have to be predicted and corrected by an active system. During design phase, CFD and thermal analyses have been performed in order to identify the whole behaviour of the structure and the relative corrections formulas. Innovative instruments and layout have been studied from the prototype to the production phase.