Earth Rotation Results Using Mark 4 VLBI

We present the first results of VLBI experiments recorded in Mark 4 mode at rates of 128 and 256 Mega-sample/sec. Formal uncertainties of polar motion, UT1, and nutation offsets from these experiments are better than uncertainties from concurrent NEOS-A VLBI experiments by a factor of 1.5-2. For the series of 25 Mark 4 experiments over a period of about one year, baseline length repeatability is at the level of 2.5 mm + 0.8 ppb compared with NEOS repeatability of 0.5 mm + 1.4 ppb. We will discuss the optimal use of Mark 4 in the design of VLBI observing networks.