A Study of Difference Delay Rates Derived from Processing of Same-Beam VLBI Data

As shown by our analyses difference delay rates from SELENE same-beam VLBI data are almost not influenced by long-term effects on radio signals from the ionosphere and neutral atmosphere. The short-term effects on radio signals from the atmosphere can be mostly eliminated if the separation angle between two satellites is small (<0.1°), but becomes more appreciable and more difficult to be eliminated as the separation angle increases. We further integrate the residuals of correlation phases for a set of data after fits to their systematic trends are subtracted. The data were observed with all baselines of 8 VLBI stations through 2008. Some integrations are over time intervals of 60s or 30s, and each integration generates a data point for detailed analyses. We apply linear fits to every 5 data points to find delay rates and difference delay rates. Statistics of these show that rms values of random errors of delay rates of two satellites are about 0.20ps/s and 0.32ps/s, respectively. Our statistics also show that rms values of random errors of difference delay rates of two satellites are 0.09ps/s to 0.17ps/s and 0.19ps/s to 0.32ps/s, respectively. Random errors of delay rates are statistically small in the spring and larger in the autumn, but those of difference delay rates are stable throughout the year.