An FGS in Position Mode measures the relative angular separations of several
objects. The objects generally will be separated by angles on the order of arc minutes, ranging over several magnitudes in brightness (possibly from V = 3 to 16.7), and will have different B-V colors. Because their parallaxes and proper motions need to be determined, HST will visit a given astrometric field several times, spanning perhaps two to four years or longer, over the course of an observing program. During each visit, the FGS will track each target in FineLock, one at a time, according to the instructions and sequence specified in the proposal.