5. Comparison between the GPM1 and othercatalogues

To test the validity of the new proper motions from the GPM1, we have carried out comparison with those from the PPM (Roeser & Bastian 1993) and ACRS (Corbin & Urban 1990), determined both on the FK5 system. It also allows us to estimate the value of the angular vector velocity originating from spurious rotation of the FK5 system, because proper motions referred to galaxies are assumed to be in a nonrotating system. The proper motion differences PPM-GPM1, ACRS-GPM1 needed for this purpose have been computed for common stars. 837 stars are common with PPM and 743 with ACRS catalogues. The systematic relations between the above catalogues have been derived by least square fits using the known equations, see for instance (Brosche et al. 1995):

Here , are star equatorial coordinates, , are the proper motion differences between PPM (or ACRS) and GPM1 in right ascension and declination respectively, while are the components of the rotation vector. In the relations under consideration the two reference systems are assumed to differ by a rigid rotation, neglecting any kind of systematic errors that may be present in either system. The results that have been found from combined and solution are given in Table 3 (click here), where denotes the standard errors of the unknowns .

  
Table 3: The components of rotation vector

Table 3 (click here) shows that we have significant values from the above solution. Therefore, considering that the accuracy of proper motions is higher than in the PPM or ACRS, the GPM1 contains a sufficient number of stars to yield statistically significant values of the rotation of the system.