As with the spatial-only sphere, the sphere surface is initialised using a radius parameter. The default value of the radius R is R=6371, which is the Earth's radius in km, while the measure used to compute distances on the sphere is the great-circle distance on a sphere of radius R. By default Euclidean geometry is used to factor in the time component, so that dist((s1,t1),(s2,t2)) = sqrt(gc_dist(s1,s2)^2 + (t1 - t2)^2). Frequently this distance can be used since separate correlation length scales for space and time are estimated in the EM algorithm (that effectively scale space and time separately).