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# PDAF_correlation_function

This page documents the routine `PDAF_correlation_function`

of PDAF. It was introduced with PDAF V2.3.

The routine provides the value of a chosen correlation function. It can be used, e.g. to generate non-diagonal observation error covariance matrices.

The interface is the following:

SUBROUTINE PDAF_correlation_function(ctype, length, distance, value)

with the following arguments:

`ctype`

:`integer, intent(in)`

Type of weight function:

(1) Gaussian function scaled to f(0)=1

(2) 5th order polynomial with support radius sradius (Gaspari&Cohn 1999; 0 for distance>sradius)`length`

:`real, intent(in)`

length scale:

for (1) standard deviation

for (2) cut-off length (f=0 for distance>=length)`distance`

:`real, intent(in)`

Distance at which the function is evaluated`weight`

:`real, intent(out)`

Function value