Fading timescales associated with GPS signals and potential consequences

The effect of equatorial ionospheric scintillations on the operation of GPS receivers is investigated, with special attention given to the effect of s cintillation timescales on the code division multiple access (CDMA) protoco l used by GPS. We begin by examining the timescales of scintillation fades modeled as a horizontally drifting pattern whose timescales are determined by the Fresnel length and the drift speed. The model is tested by comparing the speed, determined by dividing the Fresnel length by the autocorrelatio n time (width), with the speed estimated using spaced receivers, and the tw o independent estimates of speed are shown to possess a linear relationship , Next we show that the scintillation pattern drift speed is given by the d ifference of the ionospheric drift and the speed of the GPS signal F region puncture point. When the ionosphere and GPS signal puncture point speeds m atch, the fade timescales lengthen. Additionally, if the fade depth is adeq uate, during periods of longer fade times the loss of receiver lock on GPS signals is more likely, as shown in several examples; that is, both larger fade depths and longer fade timescales are required to produce loss of trac king. We conclude by demonstrating that speed matching or resonance between the ionosphere and receiver is most likely when the receiver is moving fro m west to east at speeds of 40-100 m/s (144-360 km/h). This is in the range of typical aircraft speeds.