1. Method to estimate a state of a device using an executable multivariate mathematical model comprising a summation of a plurality of inputs, each input factored by a corresponding weighting factor and by a corresponding parameter, each corresponding parameter determined by: determining each input at a time-certain; deriving a weighted recursive least squares equation from the executable multivariate mathematical model; and, executing the weighted recursive least squares equation based upon the inputs, the weighting factors, and, non-corresponding parameters.