Estimating and modeling space-time correlation structures

In this paper, a class of product-sum covariance models has been introduced for estimating and modeling space-time correlation structures. It is shown how the coefficients of this class of models are related to the global sill and "partial" spatial and temporal sills; moreover, some constraints on these sills have been given in order to assure positive definiteness of the product-sum covariance model. A brief comparative study with some other classes of spatial-temporal covariance models has been pointed out.