  Using the geomagnetic storm of July 15, 2000 as an example, we investigated
the dependence of GPS navigation system performance on the nightside at
mid-latitudes on the level of geomagnetic disturbance. The investigation was
based on the data from the global GPS system available through the Internet. It
was shown that the number of GPS phase slips increases with the increasing
level of disturbance and that there is a good correlation between the rate of
Dst-variation and the frequency of slips. It was further shown that the
relative frequency of slips has also a clearly pronounced aspect dependence.
Phase slips of the GPS signal can be caused by the scattering from small-scale
irregularities of the ionospheric E-layer. Phase slip characteristics are
indicative of Farley-Buneman instabilities as a plausible physical mechanism
that is responsible for the formation of geomagnetic field-aligned
irregularities. Using simultaneous measurements of backscatter signal
characteristics from the Irkutsk incoherent scatter radar and existing models
for such irregularities, we estimated the order of magnitude of the expected
phase fluctuations of the GPS signal at a few degrees.
