correlogram in spatial producing values outside [-1,1]

Dear all,

I'm using the correlogram function in the spatial library to calculate
spatial correlograms of radar data.  However, I'm finding that the
resulting values are often outside the range [-1,1], usually only at
larger distances of separation.

I'm not sure whether to be overly concerned about this, or dismiss it as
some artefact of the data.  Has anyone had similar experiences?  Or know
why this might be happening?

Thanks

Kechi