  This is the first of three papers describing an `absolute' calibration of the
GONG magnetograph using an end-to-end simulation of its measurement process.
The input to this simulation is a MURaM 3D MHD photospheric simulation and the
output is the corresponding set of simulated data numbers which would be
recorded by the GONG detectors. These simulated data numbers are then used to
produce `synthetic magnetograms' which can be compared with the simulation
inputs. This paper describes the GONG instrument, the MURaM datacube, our
instrument simulator, and calculation of synthetic magnetograms, setting the
stage for the subsequent two papers. These will first lay groundwork for
calibration (and magnetogram comparison in general), then apply them to
calibration of GONG using the simulation results.
