Interferometer phase fluctuations produced by path length variations in the troposphere limit the coherence to between 100 and 1000 seconds at a 3 millimeter wavelength. An expression for the coherence is given using an Allan variance to characterize the atmospheric phase fluctuations. Methods of optimizing the fringe detection threshold and calibrating the fringe visibilities under poor conditions of poor coherence are outlined. The astrometric and geodetic accuracy of VLBI is limited by our ability to calibrate the atmospheric path. Atmospheric “self-calibration” techniques which use the elevation dependence of interferometer delay to solve for the highly variable “wet” component of troposphere are discussed. Various models for the elevation dependence of atmospheric path or “mapping function” are reviewed. The accuracy limits imposed by the atmosphere are discussed.

The error in VLBI estimates of baseline length caused by unmodelled variations in the propagation path through the atmosphere is greater for longer baselines. We present and discuss series of estimates of baseline lengths obtained using different methods to correct for the propagation delay caused by atmospheric water vapor. The main methods are use of data from a water-vapor radiometer (WVR) and Kalman-filtering of the VLBI data themselves to estimate the propagation delay. Since the longest timespan of WVR data associated with geodetic VLBI experiments was obtained at the Onsala Space Observatory in Sweden, we present results for the following three baselines: (1) Onsala–Wettzell, FRG (920 km), (2) Onsala–Haystack/Westford, MA (5600 km), and (3) Onsala–Owens Valley (7914 km).

In recent years, the influence of atmospheric irregularities on electronic systems has been recognized. How these irregularities affect electronic systems is related to the propagation environment, the scales of the irregularity, and parameters of the equipment. The phase deviation caused by the atmosphere is important to the interferometer. However, there are some filter effects, namely antenna aperture filter, Fresnel filter, and baseline filter, which must be considered. The first two filter out the effects of small scale irregularities, and the last one filters out larger ones.

Variations in atmospheric water vapor are difficult to correct for in geodetic, astrometric, and mm-wave astronomy VLBI. The use of a water vapor radiometer (WVR) has given promising results. The algorithm which relates the sky brightness temperatures observed by the WVR to the delay caused by atmopheric water vapor is discussed. Examples of WVR measurements made at the Onsala Space Observatory from 1980 to 1987 are presented.

A Chinese VLBI Network Project is under construction in China. The first 25-m antenna and associated VLBI system have just been completed at the She-Shan Station of the Shanghai Observatory. Both geodetic and astrophysical VLBI programs will be undertaken with the Network. A dual frequency water vapor radiometer is being developed to determine the tropospheric path delay correction.

The GRIG-2 geodetic VLBI experiment was conducted in 1985, linking for the first time South America, Europe and Africa. At the single frequency band of 1.66 GHz which was used, the monitoring of the ionosphere is a critical aspect and several predictions of Total Electron Content (TEC) were used. One of them is derived from dual band Doppler observations of TRANSIT satellites, which were simultaneously conducted. The influence of these models on the solution is presented, with comparisons with other VLBI solutions. Decimetric accuracy has been achieved.