IRS Drift Model
 

Hi everyone!
I'm trying to write a program that simulates IRS drift. I remember seeing a mathematical model that reflected IRS drift rate to an extent. I know you can't predict drifts, as otherwise it wouldn't exist , but nevertheless I'm looking for a model or tables that show the drift (either GS or position offset) after different time intervalls.
Does anyone know of something like this?
Thanks,

Mark