GNSS Receiver Carrier Tracking Loop Impact on Ionosphere Scintillation Signal C/N0 and Carrier Phase Estimation

X. Mao, Y. Morton

Abstract:	Ionosphere scintillation is a nuisance for space-based radio systems such as GNSS. The concurrent deep amplitude fading and rapid phase fluctuation during a strong ionosphere scintillation event impose challenges on GNSS signal carrier tracking loop design. The first objective of our research is to develop robust GNSS receivers that can tolerate the simultaneous amplitude fading and phase fluctuation and generate a navigation solution during strong scintillation. Because of the ionosphere scintillation effects and the well defined GNSS signal structures, GNSS signals can be used as a signal of opportunity to study the ionosphere and space weather phenomena. Any disturbances on the signal parameters can be used to infer the underlying processes occurred in the ionosphere propagation medium. Our second objective is to obtain accurate estimations of scintillating GNSS signal parameters for ionosphere studies. These two different objectives require different strategies in receiver signal processing. To illustrate the need for the different strategies, this paper applies different receiver processing algorithms to IF samples collected during a strong scintillation event. The event occurred on Ascension Island located near the equator in the Atlantic Ocean during the last solar maximum in 2001 [1]. There were a total of eight GPS satellites in direct view of the receiver; seven of them experienced strong scintillation during the 45 minutes data collection experiment. The RF front end used to collect the data has a Rb frequency standard which makes it possible for the receiver carrier tracking loop bandwidth to be determined by the scintillation signal dynamics. A second and a third order PLL have been implemented to successfully track all satellite scintillation signal in this data set. We observed major discrepancies between carrier phase measurements generated by the second and third order carrier tracking loops especially during deep amplitude fading: the third order PLL typically showed much larger and faster phase variations compared to the second order PLL. The deep amplitude fading frequently exceeded over 30 dB to reach a C/N0 level at or below 0 dB-Hz. The carrier phase values obtained during these deep fading periods are not meaningful as there is hardly any energy left in the I channel. In addition to the difficulties in obtain a meaningful measure of carrier phase variations during deep amplitude fading, frequent navigation data bit inversions were also observed. The navigation data bit inversion observation was based on comparisons with navigation data message truth obtained from the same satellite signals prior to the occurrence of scintillations. Our analysis showed that the third order PLL has a much higher rate of data bit inversion compared to the second order PLL. And these inversions occur when deep amplitude fading and large fast phase variations occur. To achieve the objective of robust navigation receivers, the second order PLL appears to be a better choice. Its carrier phase outputs appear to be more filtered to show “smoother” dynamics and its navigation data messages are less prone to bit transition error. The third order PLL, on the other hand, may be more faithful in revealing the true carrier dynamics for the purpose of ionosphere studies. Since the Costas PLL has limitation of phase output only within [-PI/2,+PI/2], carrier phase measured by Costas PLL can not reflect the true phase if the true phase dynamic range go over [-PI/2,+PI/2]. Pure PLL does not have this limitation, but it is sensitive to navigation data transition. To mitigate the navigation data transition impact on the pure PLL, we applied the known navigation message sequence to the input signals and “wiped off” the navigation data bits from the signals before applying the third order pure PLL. This two-stage processing with a third order PLL generates carrier phase outputs that are more accurate representation of the scintillation signal phase variations. Also, by wiping off the data bit, we can apply extended integration time to allow more effective accumulation of signal energy during deep fading. This paper describes the detailed algorithm implementation and analyzes the scintillation signal parameters including amplitude and phase variations, I and Q channel energy history, and navigation data bit transitions generated by the second and third order PLL. [1] Zhang, L., Y. Morton, Q. Zhou, F. van Graas, and T. Beach, “Characterization of GNSS signal parameters under ionosphere scintillation conditions using sequential and batch-based tracking algorithms,” Proc. IEEE PLANS., p264-275, Palm Springs, CA, May 2010.
Published in:	Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013) September 16 - 20, 2013 Nashville Convention Center, Nashville, Tennessee Nashville, TN
Pages:	3248 - 3254
Cite this article:	Mao, X., Morton, Y., "GNSS Receiver Carrier Tracking Loop Impact on Ionosphere Scintillation Signal C/N0 and Carrier Phase Estimation," Proceedings of the 26th International Technical Meeting of the Satellite Division of The Institute of Navigation (ION GNSS+ 2013), Nashville, TN, September 2013, pp. 3248-3254.
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