This paper is based on optical observations of 85 instruments (Table 1-a) from January to October 1980. The accuracies of all instruments, 62 for time and 54 for latitude, are shown in Table 1-b, in which:

1. – the roughness ε1i of daily observations is defined by the standard deviation of the observation values of the i-th instrument with respect to the smoothed values of themselves;

2. – the ε2i is the standard deviation of mean observation values during every 0.05 year with respect to the global reference system.

The average accuracy of each type of instrument is given in Table 2, regional averages are given in Table 3.

These tables show that errors in time observations are greater, generally speaking, than those in latitude, and low frequency errors are greater than high frequency ones in time observations.