It is possible to extract the IRU information from the ATR or ADA
structures using GT_IRU. The output is in arcseconds. It
is also possible to have the drift in the IRU removed for short
time periods by using the /RESID switch (it fits a line to the drift
and subtracts that drift). Sample calls are:
IDL> iru = gt_iru(atr)
IDL> iru = gt_iru(atr, /resid)
IDL> iru = gt_iru(ada)
IDL> iru = gt_iru(ada, index, /resid)