  We present the basic elements and first results of an end-to-end simulation
package whose purpose is to test the validity of the Space Interferometer
Mission design. The fundamental simulation time step is one millisecond, with
substructure at 1/8 ms, and the total duration of the simulation is five years.
The end product of a given "wide-angle" astrometry run is an estimated grid
star catalog over the entire sky with an accuracy of about 4 micro-arcseconds.
  SIMsim is divided into five separate modules that communicate via data pipes.
The first generates the 'truth' data on the spacecraft structure and laser
metrology. The second module generates uncorrupted fringes for the three SIM
interferometers, based on the current spacecraft orientation, target stars'
positions, etc. The third module reads out the CCD white light fringe data at
specified times, corrupting that and the metrology data with appropriate
errors. The data stream out of this module represents the basic data stream on
the simulated spacecraft. The fourth module performs fringe-fitting tasks on
this data, recovering the total path delay, and the fifth and final module
inverts the entire metrology/delay dataset to ultimately determine the
instantaneous path delay on a fiducial baseline fixed in space. (Pathlength
feed forward is used every few milliseconds to re-position the interferometer
to keep the fringes in the delay window.) The average of all such delays over
an integration time (typically 30 s) is reported as one of several hundred
thousand measured stellar delays over the five-year period, which are then
inverted to produce the simulated catalog.
  Future plans include taking into account more sources of error from the SIM
error budget and including narrow angle observations in the observing plan.
