GNSS zenith delays and gradients in the analysis of VLBI Intensive sessions

Very Long Baseline Interferometry (VLBI) is the only space geodetic technique which is capable of estimating Universal Time (UT1 = UTC + Delta UT1). So-called VLBI Intensive sessions of the International VLBI Service for Geodesy and Astrometry (IVS) are dedicated to the rapid production of Delta UT1. However, the accuracy achieved with those sessions is still below what could be expected from formal uncertainties of the estimates and one of the reasons is the inappropriate modeling of azimuthal asymmetries of the troposphere delays, because usually no gradients are modeled or estimated. To overcome that deficiency, we introduced troposphere zenith delays and horizontal total gradients estimated from the observations of Global Navigation Satellite Systems (GNSS) i.e. the solution of the Center for Orbit Determination in Europe (CODE) in the analysis of VLBI Intensive sessions carried out from the beginning of 2008 till the end of 2014. We compared our results with the GNSS-derived length-of-day (LOD) estimates of CODE and the International GNSS Service (IGS) and find slight improvements of agreement by up to 1 mu s for both INT1 and INT2 sessions with gradients from CODE. We do not see any additional significant improvement of LOD agreement when GNSS zenith delays are introduced. (C) 2015 COSPAR. Published by Elsevier Ltd. All rights reserved.