The Effects of the Ionosphere and C/A Frequency on GPS Signal Shape: Considerations for GNSS-2

Jock R. I. Christie, Bradford W. Parkinson, Per K. Enge

Abstract:	By greatly increasing the chipping rate of the pseudo random noise (prn) code, it may be possible to measure the ionosphere (TEC) in near real time, by exploiting the dispersive nature of the ionosphere. The ionosphere is a major source of error for all stand alone GPS based navigation, and spatial decorrelation limits the accuracy of differential GPS users. The ionospheric distortion of the C/A code (1.023 Mbps), and P code (10.23 Mbps) may be too small to measure. However, the use of a faster (40-100 Mbps) chipping frequency for GNSS2, may permit measurement of the ionosphere’s Total Electron Content (TEC) using only a single frequency receiver. Currently, we can only ‘measure’ the ionosphere by using a dual frequency receiver (limited to military users), or with a cross correlating receiver (limited by poor signal to noise) or by observing ‘code-carrier divergence’ (limited by long observation times). The diurnal model of the atmosphere is only capable of removing 50-60% of the error. This investigation is based on the standard model of the ionosphere, which assumes negligible attenuation in the L-band, but does produce a time advance proportional to k/f*2. Expanding in a Taylor Series about Ll, the first two terms of this model lead to code-carrier divergence. The next term is quadratic in frequency and produces both amplitude and phase modulation of the received signal. The higher order terms produce relatively insignificant changes. Although these variations are present in the signal received from GPS, it is quite small and has previously been unmeasured. A computer simulation was developed to analytically determine the shape of the GPS signal after passing through the ionosphere, and to determine the effect of faster chipping frequency on the shape of the received signal. The GPS signal is expressed in the frequency domain, then phase shifts, proportional to k/f, were applied. The signal was then transformed into the time domain, and this signal was compared with the original signal. This paper will present analytical results from the simulation. The benefits of a faster chipping frequency for GNSS2 will be explored. Time domain plots of the modified signal will be presented, illustrating the changes in both amplitude and phase due tot he ionosphere. The limits imposed by the presence of atmospheric noise and receiver noise will not be discussed. This paper will not discuss other aspects of GNSS2 design, such as constellation selection. Although greater spreading requires greater bandwidth, the quadratic dependence of this distortion on frequency may justify the additional bandwidth if it allows us to directly measure, rather than estimate, the ionosphere with a single frequency receiver. GNSS2 should be designed with a much faster code frequency (40-100 Mbps) since it would improve code positioning accuracy, would reduce the carrier ambiguity space, and may permit real- time ionospheric measurements.
Published in:	Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996) September 17 - 20, 1996 Kansas City, MO
Pages:	647 - 653
Cite this article:	Christie, Jock R. I., Parkinson, Bradford W., Enge, Per K., "The Effects of the Ionosphere and C/A Frequency on GPS Signal Shape: Considerations for GNSS-2," Proceedings of the 9th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GPS 1996), Kansas City, MO, September 1996, pp. 647-653.
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