# fields, Tools for spatial data
# Copyright 2004-2007, Institute for Mathematics Applied Geosciences
# University Corporation for Atmospheric Research
# Licensed under the GPL -- www.gpl.org/licenses/gpl.html
"poisson.cov" <- function(x1, x2, eta = 0.2) {
    if (missing(x2)) 
        x2 <- x1
    #
    # dot products of direction cosines
    #
    PP1 <- cbind(cos((x1[, 2] * pi)/180) * cos((x1[, 1] * pi)/180), 
        cos((x1[, 2] * pi)/180) * sin((x1[, 1] * pi)/180), sin((x1[, 
            2] * pi)/180))
    PP2 <- cbind(cos((x2[, 2] * pi)/180) * cos((x2[, 1] * pi)/180), 
        cos((x2[, 2] * pi)/180) * sin((x2[, 1] * pi)/180), sin((x2[, 
            2] * pi)/180))
    D <- (PP1 %*% t(PP2))
    con <- (1 - eta^2)^(1.5)
    con/(1 - 2 * eta * D + eta^2)^(1.5)
}