One important prerequisite for geodetic Very Long Baseline Interferometry (VLBI) is the use of frequency standards with excellent short term stability, i.e. hydrogen masers. This makes VLBI stations, which are often co-located with Global Navigation Satellite System (GNSS) receiving stations, interesting for studies of time- and frequency-transfer techniques. In this paper we present an assessment of VLBI time-transfer based on the data of the two week long consecutive IVS Cont08 VLBI-campaign by using GPS Carrier Phase (GPSCP). Cont08 was a 15 days long campaign in August 2008 that involved eleven VLBI stations on five continents. For Cont08 we estimated the worst case VLBI time link stability between the station clocks of ONSALA and WETTZELL to about 1.5e-15 at one day. Comparisons with clock differences estimated with GPSCP confirm the VLBI results. The paper also indicates time-transfer related problems of the VLBI technique as used today.

http://publications.lib.chalmers.se/records/fulltext/local_132880.pdf

VLBI time-transfer using CONT08 data

Time and Frequency (T/F) transfer systems using space links developed at the Radio Research Laboratories (RRL) are described. The domestic T/F transfer system uses the Japanese communication satellite: CS-2/broadcasting satellite BS-2 and the Global Positioning System (GPS) in addition to the ordinary links of ground TV and Loran-C The Asian area transfer system makes use of the Geostationary Meteorological Satellite (GMS) and a Very Long Baseline Interferometer (VLBI) as well as Loran-C Other systems for international T/F transfers depend on the space techniques of the GPS and the, VLBI. Spread-spectrum equipments for T/F transfers have recently been built to realize both precision and accuracy of about 1 ns via the CS link, together with a 1-m mobile antenna station. Receivers have been developed at RRL to be used routinely for the GPS and GMS T/F transfers on a daily basis with a precision of 10 ns or more and an accuracy of less than 100 ns. T/F transfer experiments using a VLBI system developed at RRL have been conducted on a routine basis about once a month with the U.S. Naval Observatory since April of 1985. The present goal of accuracy is 10 ns with 10-min or longer observations.

Research activities on time and frequency transfer using space links

It is well known that Very Long Baseline Interferometry (VLBI) is capable of precise time synchronization at subnanosecond levels. This paper deals with a demonstration of clock synchronization using the MKIII VBLI system. The results are compared with clock synchronization by traveling cesium clocks and GPS. The comparison agrees within the errors of the portable clocks (+ 5 ns) and GPS(+ or - 30 ns) systems. The MKIII technology appears to be capable of clock synchronization at subnanosecond levels and appears to be very good benchmark system against which future time synchronization systems can be evaluated.

M. J. Lister

Papers

Precise time transfer using MKIII VLBI technology