“…It is important to estimate the velocity with which the heat front is advancing, for then one can interchange time with location, albeit approximately. In a paper that considers the case of location x on the surface of a sphere, Brillinger () provided a review of various methods to estimate velocity of motion given data { Y ( x , t ), t = 0,…, T − 1, x = 0,…, X − 1}. Projection pursuit regression (PPR) is one method.…”