The HIPPARCHOS satellite (High Precision Parallax Collecting Satellite) is designed to determine position, proper motion and parallax for a large number of stars. The precision expected (0.002 arcsec), and the absence of systematic errors with position in the sky arise from the following characteristics of the system:

1. – Measurements of angular distances between stars a long way apart (58°), are by comparison with a very stable angular reference. This reference is formed by an optical block consisting of two plane mirrors, rigidly mounted, forming an angle of 29° between them, and sending two separate stellar fields into the same telescope;

2. – the absence of flexure (thanks to weightlessness) and of thermal deformation (non-expansion material and thermal control) ensure that the angular reference is very stable;

3. – operation outside the atmosphere allows the theoretical resolution to be reached; refraction, dispersion and atmospheric scintillation are avoided; diurnal and seasonal effects that interfere with ground-based measurements are non-existent.