**7.4.1 Statement and urgency of the problem**

Prediction of the Earth's rotation parameters is an important activity of the International Earth Rotation Service and national services of time and frequency in some progressive countries. The need of ERP prediction is dictated first of all by fast developing space technologies for civil and military areas. The ERP values, calculated on the basis of observations' processing, are related to the time frames when these observations were made. Therefore the ERP data at the current time can get known only on the basis of their precomputed values. The problem is that the precomputed (ephemeris) ERP values belong to the initial conditions by solution of differential equations of the GNSS spacecrafts motion. This is particularly important when it becomes necessary to transfer the GNSS spacecrafts in an autonomous mode. The ERP prediction errors, ultimately, limit the maximum achievable accuracy of the time and coordinate support (TCS), as well as element of exterior orientation (EEO), because all other causes of errors can be minimized by means of new developments and improvements of the measuring instruments, as well as enhancements of physical and mathematical theories of environmental factors' effect on the results of navigation measurements. Besides the handling of the EEO challenges the high-precision ERP predictions are also necessary for solving of the TCS problems of civil and military facilities, which are during a long time located in points, inaccessible for GNSS signals; they are also important for challenges of geophysics and meteorology by building of matching models of

Table 3 shows root-mean-square errors (RMS) of the ERP determinations obtained in 2010 by IERS according to the result of GPS, VLBI and SLR measurements processing in the leading processing centres (Bulletin B 285, 2011). Since there are more than 30 processing centres in the IERS Bulletins, we will state only the RMS values spread limits for ERP

LOD

Analyzing the data in the Tables 2 and 3 we can conclude that the VLBI technique is the most accurate for the universal time parameter definition and is inferior to the GNSS in accuracy of the pole coordinates determination. The SLR method shows the results with a similar accuracy of the pole coordinates determinations in comparison with VLBI and is also inferior to the GNSS technique. However, the SLR and GNSS techniques have similar results by determination of the length of the day. It should also be noted that the data obtained from the Russian processing centres (see Table 2.) conform to an international standard for

Prediction of the Earth's rotation parameters is an important activity of the International Earth Rotation Service and national services of time and frequency in some progressive countries. The need of ERP prediction is dictated first of all by fast developing space technologies for civil and military areas. The ERP values, calculated on the basis of observations' processing, are related to the time frames when these observations were made. Therefore the ERP data at the current time can get known only on the basis of their precomputed values. The problem is that the precomputed (ephemeris) ERP values belong to the initial conditions by solution of differential equations of the GNSS spacecrafts motion. This is particularly important when it becomes necessary to transfer the GNSS spacecrafts in an autonomous mode. The ERP prediction errors, ultimately, limit the maximum achievable accuracy of the time and coordinate support (TCS), as well as element of exterior orientation (EEO), because all other causes of errors can be minimized by means of new developments and improvements of the measuring instruments, as well as enhancements of physical and mathematical theories of environmental factors' effect on the results of navigation measurements. Besides the handling of the EEO challenges the high-precision ERP predictions are also necessary for solving of the TCS problems of civil and military facilities, which are during a long time located in points, inaccessible for GNSS signals; they are also important for challenges of geophysics and meteorology by building of matching models of

VLBI - 24 h 8-15 - 30-100 70-150 VLBI –Intensive 14-17 - - - Satellite Laser - 15-30 100-160 80-200 GNSS (GPS) - 5-30 13-45 13-40 Table 3. Estimated accuracies of the ERP determinations by using different techniques in

(μas) X (0.000001") Y (0.000001")

determinations in these centres.

2010 according to the IERS data.

ERP determination accuracy level.

**7.4.1 Statement and urgency of the problem** 

**7.4 ERP prediction** 

Techniques UT1

(μas)

the internal structure of the Earth and global climate change, as well as for geodynamics and geodesy challenges by the more precise definition of the Earth orientation parameters relative to the inertial sidereal reference system and coordinates determination of points relative to the absolute coordinate system.

Actual requirements of the GNSS EEO are based on the fact that the influence of ERP determination and prediction errors on the overall TCS error is insignificant. For example, pursuant to this condition, the universal time prediction error for the specified 15 days period of the GLONASS spacecraft operation in the autonomous mode must not exceed a value close to 1 ms. Unfortunately, despite the high level of modern scientific and technological developments in the field of the ERP prediction, an achievement of such level of accuracy is still a complicated and unsolved problem.
