1. A method for calibrating coefficients of an observer of a variable state x k of a physical system from measurements y k of physical quantities of the system, where the index k identifies the instant of measurement, the method comprising: measuring a variable z k of the physical system at N different instants, the variable z k being a function of the state variable x k , and determining a vector p of coefficients that minimizes the following criterion in complying with a predetermined set Δ of one or more constraints: { f ( p ) = ∑ k = 1 N  z k - ( x ^ k , p )  2 } where : {circumflex over (x)} k is the estimation of the variable x k built by the observer calibrated with the coefficients of the vector p at the instant k, φ is a known function that links the estimation {circumflex over (x)} k to an estimation {circumflex over (z)} k of the variable z k , ∥. .. ∥ 2 is a norm of the difference between z k and ({circumflex over (x)} k ,p), and the one or more constraints of the set Δ dictate that the trajectory of the variable z k should be included in a corridor of uncertainty situated on either side of the trajectory of the estimation {circumflex over (z)} k at least for the majority of the instants k.