| One of the two major problems addressed here is the application of the triple-difference technique in the Global Positioning System (GPS) orbit determination. It represents an innovative, high-standard, and optimal approach to the recovery of the GPS trajectory. The implementation of triple-difference requires special consideration of the correlation that is introduced into the data set via subsequent differencing, but it reduces significantly the number of parameters that need to be evaluated.; Another purpose of this research is GPS-based determination of rotational time and pole coordinates (Earth Rotation Parameters, ERPs), under different conditions such as different troposphere modeling, different nutation model, and variation of the ERP model itself. In particular, determination of diurnal and semidiurnal Universal Time (UT1) with satellite technique is discussed.; GPS, similar to other nonabsolute methods of ERP retrieval, suffers from an inherent defect in UT1 determination due to the collinearity between the variations in the right ascension of the satellite's orbital node and UT1. But GPS is sensitive to the change in UT1. Having the rate of UT1 resolved, one needs to connect it to the starting value of UT1-UTC based on an absolute and independent reference (for example, VLBI). GPS-derived ERPs are fairly stable and require VLBI-derived a priori values only once per month without any loss of accuracy.; The triple-difference technique, although not new as a concept of GPS data processing but certainly innovative in the field of orbit determination, has demonstrated remarkable processing capacity, flexibility, efficiency, and reliability. The validity and high precision of the triple-difference solution for GPS orbits and ERPs was confirmed by the consistency and repeatability tests for long baselines, showing the RMS ranging from a few parts in 10{dollar}{bsol}sp8{dollar} to a few parts in 10{dollar}{bsol}sp9{dollar} per coordinate.; Also, the short-periodic UT1 series derived from GPS shows good agreement with the VLBI-derived empirical model of Herring. Due to strong correlation between the long-periodic errors in the orbit and the daily retrograde polar motion, as well as the long-periodic nutation and the daily retrograde polar motion, it is currently impossible to separate the full spectrum of daily and subdaily signatures of polar motion from the satellite technique. |
