Accuracy Uniformity of Hipparcos Parallaxes and Proper Motions: Strategy and Current Status

Abstract

Even before the launch difficulties, it was planned to monitor the trend of the accuracy acquisition resulting from the adopted observing process and eventually to modulate the initial values of the observing strategy parameters whenever it may help correcting distorsions in the final uniforme accuracy. Of course this monitoring of accuracy acquisition became more important when, as a result of the revised orbit, large departures from the nominal observations take place. The observing strategy and the associated modulation process behave according to expectation and may smooth out on the long range inhomogeneities. However important effects due to the complete lack of long sequences of observations in the early period in some regions will not be compensated unless the real mission extends substantially beyond the end of 92.