  The GPS performance is impaired in conditions of geomagnetic distrubances.
The rms error of positioning accuracy increases in the case where two-frequency
GPS receivers of three main types (ASHTECH, TRIMBLE, and AOA) are in operation.
For ASHTECH receivers (unlike AOA and TRIMBLE) there is also a clear
correlation between the slip density of the one- and two-frequency modes of
positioning and the level of geomagnetic disturbance.
