In this study a temperature controlled environment is used in order to quantify the thermal influence on all major parts of state of the art geodetic GPS receiving equipment. Temperature variations, effective as time delay variations, were identified as a dominating error source that degrades the capabilities of carrier phase GPS based time and frequency transfer considerably. For purely code-based measurements with uncertainties in the ns range is temperature rarely an issue. In contrast carrier phase observations offer potentially a two orders of magnitude better accuracy and are therefore suitable for exploiting the characteristics of maser quality clocks. However, the stability of the environment around the receiver equipment defines the achievable accuracy. Four distinct parts of the receiver chain were subject to systematic measurements of the temperature-delay dependency: antenna preamplifier, antenna and clock cables, power distribution devices and geodetic receivers. A temperature controllable climate chamber was deployed with the respective component to follow a long time-constant temperature stepping. Signal through devices were mainly tested in a vector-voltmeter approach. Zero base line GPS processing was used to test receivers. With individual component temperature dependence being far above the expected accuracy of carrier phase based time and frequency transfer it underlines the necessity to include temperature as an important parameter into time/frequency solutions.
We have used code and carrier phase data from the Global Positioning System (GPS) satellites to estimate time differences between atomic clocks in near (< 10 s) real-time. For some sites we have used data transmitted via Internet connections and TCP/IP, while for other sites data were collected in deferred time, but processed by a Kalman filterbased software as if they were available in real time. Satellite orbit and clock data of different quality have been used. The real-time estimates of time differences of the station clocks have been compared to those estimated from regular postprocessing using accurate satellite orbits and clocks from the International GPS Service (IGS). First results show that the standard deviation of the differences between the real-time carrier phasebased and the postprocessing estimates of the clock time differences can be less than 100 ps for baselines of about 1000 km.
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.
SHORT SUMMARY OF THE VLBI TECHNIQUEVLBI is an interferometry concept designed as a radio astronomy tool in the 1950ies. But not before the late sixties the first observational components were developed in the US/Canada resulting in the first transatlantic VLBI session that was carried out as a pioneer experiment between the telescopes at Westford in the USA and the Onsala Space
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