Millimeter accuracy of geodetic VLBI measurements achieved on a 54-km baseline

Millimeter accuracy of geodetic VLBI (very-long-baseline interferometry) measurements was demonstrated in experiments repeated five times from 1984 to 1988 on a 54-km baseline between a 26-m antenna at Kashima and a 5-m transportable antenna at Tsukuba. The average formal errors of the five measurements were 4.6 mm in length, and 4.4 and 6.7 mm in the east and north directions. In the error analysis, effects of atmospheric turbulence and thermal receiver noise were investigated and quantitatively evaluated. As a result, it was found that the stability of the atmosphere was on the order of 10^-14 in the Allan standard deviation, which agrees well with other results obtained in interferometric and radiometric measurements. It was also found that the atmospheric turbulence was a major cause of an error on delay rate observations, whereas the receiver noise is a principal source of error in delay observations