  Combination of more than two fields provides constraints on the systematic
error of simultaneous observations. The concept is investigated in the context
of the Gravitation Astrometric Measurement Experiment (GAME), which aims at
measurement of the PPN parameter $\gamma$ at the $10^{-7}-10^{-8}$ level.
Robust self-calibration and control of systematic error is crucial to the
achievement of the precision goal.
  The present work is focused on the concept investigation and practical
implementation strategy of systematic error control over four simultaneously
observed fields, implementing a "double differential" measurement technique.
Some basic requirements on geometry, observing and calibration strategy are
derived, discussing the fundamental characteristics of the proposed concept.
