Moklyachuk, M. P. “Robust Interpolation of Random Fields Homogeneous in Time and Isotropic on a Sphere, Which Are Observed With Noise”.
Ukrains’kyi Matematychnyi Zhurnal, vol. 47, no. 7, July 1995, pp. 962–970,
https://umj.imath.kiev.ua/index.php/umj/article/view/5491.