Degradation of GPS performance in geomagnetically disturbed conditions
  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. 
E.L. Afraimovich, V.V. Demyanov, T.N. Kondakova (Institute of   Solar-Terrestrial Physics, Russian)