Correlated and zonal errors of global astrometric missions: a spherical   harmonic solution
  We propose a computer-efficient and accurate method of estimation of spatially correlated errors in astrometric positions, parallaxes and proper motions obtained by space and ground-based astrometry missions. In our method, the simulated observational equations are set up and solved for the coefficients of scalar and vector spherical harmonics representing the output errors, rather than for individual objects in the output catalog. Both accidental and systematic correlated errors of astrometric parameters can be accurately estimated. The method is demonstrated on the example of the JMAPS mission, but can be used for other projects of space astrometry, such as SIM or JASMINE. 
V. V. Makarov, B. N. Dorland, R. A. Gaume, G. S. Hennessy, C. T.   Berghea, R. P. Dudik, H. R. Schmitt
astro-ph.IM