CPO prediction: accuracy assessment and impact on UT1 intensive results

arXiv:1102.0609v1 [physics.geo-ph] 3 Feb 2011 CPO prediction: accuracy assessment and impact on UT1 intensive results∗ Zinovy Malkin Pulkovo Observatory, St. Petersburg, Russia Abstract The UT1 Intensives results heavily depend on the celestial pole offset (CPO) model used during data processing. Since accurate CPO values are available with delay from two to four weeks, CPO predictions are necessarily applied to the UT1 intensive data analysis, and errors in predictions can influence the operational UT1 accuracy. In this papers the real accuracy of CPO prediction is assessed using the actual IERS and PUL predictions made in 2007-2009. Also, results of operational processing was analyzed to investigate the actual impact of EOP prediction errors on the rapid UT1 results. It was found that the impact of CPO prediction errors is at a level of several microseconds, whereas the impact of the inaccuracy in the polar motion prediction may be about one order larger for ultra-rapid UT1 results. The situation can be amended if the IERS Rapid solution will be updated more frequently. 1 Introduction Rapid VLBI UT1 observations are vital for accuracy of the rapid IERS EOP solution and its prediction. To decrease rapid UT1 latency, the special single-base 1-hour sessions are conducted practically every day with delay of processing from several hours to several days. As shown in previous studies [1–3] UT1 estimates obtained from the single-base intensive programs heavily depend on the celestial pole motion model used during analysis. For the most exacting applications, the celestial pole coordinates are computed as the sum of the theoretical values given by a adopted theory of precession-nutations, IAU2000A nowadays, and corrections called celestial pole offset (CPO) and obtained from observations, exclusively VLBI nowadays. The CPO comprises of trends and (quasi)periodic components, Free Core Nutation (FCN) is the first place, caused by the inaccuracy of Earth Rotation theory. The most accurate CPO can be obtained only from the 24h VLBI sessions and are available, as a rule, with delay from two to four weeks1. Therefore CPO predictions are necessarily applied to the UT1 intensive data analysis, and errors in predictions can influence the rapid UT1 accuracy. In this papers the real accuracy of CPO prediction is assessed ∗In: D. Behrend, K. D. Baver (Eds.), IVS 2010 General Meeting Proc., NASA/CP-2010-215864, 2010, 261–265. 1Strictly speaking, CPO results from individual analysis center are available with lower delay, but we consider the IVS combined CPO series as the most suitable for the EOP service applications 1 using the actual predictions made by IERS (USNO) and PUL IVS Analysis Center (Pulkovo observatory). The required prediction length can be found from analysis of the IVS combination delay, i.e. the time between the date of publication of IVS combined solution and the last EOP epoch in this solution (see Fig 1). For 2009, the median delay was 18 days, and maximum delay was 37 days. Of course, IVS series is then updated with new observations processed, but this changes in the IVS data are small enough to significantly influence rapid UT1 results. One can see that the required length of CPO forecast is about 40 days. We extend our analysis to longer length, which may be interesting for other applications. 0 10 20 30 40 0 50 100 150 200 250 300 350 400 Delay, days Days from 2009.0 Figure 1: Delay of IVS combined EOP. This paper is aimed at accuracy assessment of the CPO predictions computed with different models. As usually, the prediction accuracy is derived from comparison of predicted values with the final ones. For proper interpretation of the results obtained in this study, the following circumstance should be taken into account. Each CPO model is a result of fitting of observed CPO series. The models differ not only by method of fitting, but also by CPO data used for analysis, which makes results of accuracy assessment somewhat ambiguous. One may consider the prediction accuracy with respect to the model itself, which is, in fact, the accuracy of representation of given model. However, we are interested in the accuracy of representation of the actual celestial pole motion, which is most important for majority of users. For this reason we use a comparison of CPO predictions with the IVS combined CPO series, which is intended to be an official standard. 2 Data used In this study we present results of processing of VLBI observations made in the framework of the UT1 intensives IVS observing program with different delay and different CPO models. The following data were used: • INT1 sessions, observed on the workdays on the stations KOKEE (Kk) and WETTZELL (Wz); database is normally available in 2–5 days. 2 • INT2 sessions, observed at weekends on the stations TSUKUB32 (Ts) and WETTZELL; database is normally available in 1–2 days. • INT3 sessions, observed on Monday on the stations NYALES20 (Ny), TSUKUB32, and WETTZE

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