Penalized Whittle likelihood for spatial data.

Inference for spatial data is challenging because fitting an appropriate parametric model is often difficult. The penalized likelihood-type approach has been successfully developed for various nonparametric function estimation problems in time series analysis. However, it has not been well developed in spatial analysis. In this paper, a penalized Whittle likelihood approach is developed for nonparametric estimation of spectral density functions for regularly spaced spatial data. In particular, the estimated spectral density is the minimizer of a criterion which is developed based on the Whittle likelihood and a penalty for roughness. This approach aggregates several popular nonparametric density estimation methods into a coherent framework. Asymptotic properties of the proposed estimator are derived under mild assumptions without assuming Gaussianity. In addition, a computationally efficient method is developed to optimize the penalized likelihood function. Simulation results and real data examples are also provided to illustrate the finite sample performances of the methodology.