SIMBAD references

An alternative method of selection of primary sources, the arc length difference (ALD) method, which is theoretically different from the one implemented at the International Earth Rotation Service (IERS), is proposed in this paper. Remedies are adopted in the realization of the ALD selection method according to the characteristics of arc length differences. The selection process of the 1992's primary sources is shown in detail. Statistical results for years from 1988 to 1993 are listed and some comparisons are performed. Theoretical analyses and real data tests show that the ALD selection method can highlight the characteristics of each source on the premise of the consistency of all the sources. The ALD method is a qualified one to select candidates for primary sources.