VGOS operations and geodetic results

The next generation geodetic VLBI instrument is being developed with a goal of 1 mm position uncertainty in twenty-four hours. The broadband signal chain, which is essential for obtaining the required delay accuracy from a network of relatively small antennas, has been implemented on the 12 meter antenna at Goddard Space Flight Center, Maryland, USA, and on the 18 meter Westford antenna at Haystack Observatory, Massachusetts, USA. Several observing sessions have been carried out to evaluate and demonstrate system performance as new capabilities are implemented. The first twenty-four hour session was conducted in 2013 May during which several new features of the VGOS system were demonstrated. These include automated data acquisition under Field System control, which will be essential for unattended operation. For that session more than 1100 observations were made at a rate of 48 scans per hour, a factor of three higher than for current geodetic sessions. This high density of observations is one of the main goals of the new systems and is expected to lead to better estimation of the atmosphere delays, a primary error source limiting the geodetic accuracy. To compensate for the smaller antennas needed for the high slew rates, the radio-frequency signals were recorded in four bands spanning 3.2 GHz to 8.8 GHz at a total data rate of 8 Gigabits per second. The total data volume at each site was about 40 Terabytes. Because of potential damage to the VLBI receiver from the Satellite Laser Ranging system aircraft avoidance radar at GSFC, observations within a 40 degree cone to the south of the VLBI site in the direction of the SLR were masked out. This loss of sky coverage increases the uncertainty in the site position estimate relative to that which could be made with full sky coverage. Geodetic results from this and subsequent sessions will be reported.