High-accuracy measurements made by using very long baseline interferometry require corrections for the effects of the atmosphere. A ray-tracing analysis shows that the phase path through the troposphere differs from the free-space value by about 3 meters at the zenith. The ionospheric effect predominates at frequencies below 1 GHz. The net atmospheric correction in the experiment can be several tens of meters. To calculate the corrections, atmospheric probing is necessary at the same time and in the same direction as the observation of the radio source. Ionospheric probing methods do not have the desired accuracy, but the ionospheric effect can be taken out by observing at two frequencies. Although tropospheric probing is possible by radiometric measurements, the laser back-scattering approach appears to be more promising.