SIM will need to build its own precision reference
frame because no other mission has ever before measured star positions with
such accuracy. The solution is to create a network of relative positions
over the entire sky. By tying together sets of relative measurements of
star separations, we can create a rigid reference frame. The system is much
like laying out a lattice of stiff yardsticks into a frame.
astrometric grid is a model of the positions and motions of a particular
collection of astronomical objects. These objects will mostly be stars, but
some will be quasi-stellar objects (or quasars) that provide an extragalactic
anchor for the astrometric grid. These grid objects will be chosen to uniformly
cover the entire sky.
SIM will observe the grid a few times a year; with the data it
collects, scientists will be able to calibrate the instrument and compute
the positions and motions of the objects in the grid. This calibration is
performed by fitting the grid model to the set of grid observations that
SIM has collected. This calibration can then be used to perform science with
At SIM's level of accuracy, there are no "fixed" stars. All stars will
move in a well-resolved fashion in parallactic ellipses, due to the motion
of Earth around the Sun and the true movement of the stars through space,
For some observations a local grid will be sufficient. The detection
of planetary companions does not require the full knowledge of absolute positions
but can rely on changes in position relative to reference stars close to
the object of interest.
Once sufficient grid observations have been made to build up a reliable
grid model, observations of grid objects made in conjunction with observations
of science objects allow SIM to perform accurate astrometry on science targets.
SIM observes grid objects and uses the grid model to solve for interferometer
parameters (baseline length and orientation). This process provides the proper
calibration for observations of science objects.
It is important to note that it is not necessary for the grid solution
to be completed before science operations begin -- instead combined grid
and science measurement scenarios are made, then properly interpreted in
the context of the grid solution later.
SIM Science Investigations