[R] non-linear trends in kriging model

[David Pleydell]

Hi 
I am struggling to fit a non-linear trend using the
likfit function in geoR.

Specifically I want a sigmoidal function, something
like SSfpl in the nls package to fit the trend.  But
it seems trend.spatial in geoR only works with lm or
glm type models.

Any ideas how I can specify the model to calculate the
kriging parameters using REML, including the
parameters of a sigmoidal trend function (such as
SSfpl)?

Thanks
David