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Lookup NU author(s): Dr Rajesh Tiwari,
Dr Hal Strangeways
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Radio signals from Global Navigation Satellite Systems (GNSS) are received and processed using appropriate receivers which further provide position, navigation and timing (PNT) services. There are many important GNSS applications including safe aircraft landing and navigation, ship navigation and offshore oil extraction industries which generally, require 24 hour time availability as well as good, accuracy and integrity. However, there are substantial challenges in meeting these requirements during strong ionospheric storms which also depend significantly on solar activity and solar cycle epoch with the occurrence of strong ionospheric storms being significantly more frequent during the peak (solar maximum) of the 11 year cycle than at the solar minimum. The solar maximum of the current cycle is expected around May 2013, but since relatively low SSN (Sun Spot Numbers) are predicted, the SCOSTEP (Scientific Committee and Solar Terrestrial Physics) has pronounced the current solar maximum as the year of “MiniMax24”  and hence GNSS receivers may experience comparatively fewer/weaker ionospheric storm conditions than would be expected for solar maximum conditions. The UK Royal Academy of Engineering recently published a report assessing the possibility of an extreme ionospheric threat to GNSS services in current solar cycle  based on the events from past cycles. During ionospheric storms, the electron density in the ionosphere is significantly affected, generating small and large scale gradients in the ionosphere, and also resulting in small scale structure, in particular ionospheric irregularities which cause rapid random fluctuation in phase and/or amplitude of the received signal. The level of fluctuation can be quantified by phase and amplitude scintillation indices which are the standard deviation of the phase and normalized standard deviation of the intensity respectively. Fast phase changes and/or deep amplitude fades can present a considerable problem to the carrier tracking loop of a GPS receiver’s PLL (Phase Lock Loop) which can result in phase deviations, cycle slips or sometime even a complete loss of lock of the PLL for one or more of the visible satellites leading to a significant positional error. The strength of scintillation generated due to ionospheric irregularities has been extensively studied using the time derivative of TEC (Total Electron Content) and this has been correlated with ionospheric scintillation . In a recent study, Tiwari et al. derived an analogous phase scintillation index (in radians) using high pass filtered RoT at 1 Hz rate  which shows a good correlation with phase scintillation (measured using the conventional phase scintillation index) observed during strong geomagnetic storms in the high latitude region, in addition to correlation of the analytically derived standard deviation of phase jitter of the PLL (Conker et al. formula ). In this study, the phase jitter of the carrier tracking loop of a GPS receiver is investigated under different conditions including PLL loop order, bandwidth and carrier frequency which further, correlated with the estimated phase jitter using TEC data collected at 1 Hz. To validate this approach, experiments were conducted at our Newcastle University laboratory and receiving station employing a generated scintillated signal using output from the SPLN ionospheric scintillation simulator , a NovAtel 4004B GPS receiver for collecting TEC at a 1 Hz data rate and our own developed GPS software receiver coupled to both USRP (Universal Software Radio Peripheral) N210 and STEREO RF front end devices for processing raw GPS signals. The USRP used in this experiment is the second generation USRP front end receiver that can receive a broad range of frequencies (DC to 2GHz) which allow us to receive GNSS signals over a wide bandwidth. The motherboard of this USRP device has a FPGA which processes the complex waveform at a high sampling frequency and high precision ADCs and DACs for further processing. The device is read via a gigabit Ethernet interface using a Linux OS computer. The STEREO RF is a fully dual-board GNSS RF front end device for L band signals. These three receivers were connected to a common GPS antenna using an RF splitter followed by an RF amplifier. The TEC derived phase jitter was further compared with the phase jitter of the carrier tracking loop obtained from the software GPS receiver employing the Conker et al., formula . The preliminary results show that the TEC derived phase jitter is an optimal candidate for modeling ionospheric scintillation effects on carrier tracking loop and could also be used for a regional alarm index to mitigate the scintillation effect. As an application, of such a regional alarm index, it could be in the determination of the phase jitter of carrier tracking loops based on the user’s receiver parameters. Then, in the position estimation, each satellite-receiver link could be weighted inversely according to the determined jitter to improve the position solution as shown by Strangeways et al. .References Gopalswamy, N., MiniMax24 observation campaign, 50th UNCOPUOS/STSC meeting, February 13, 2013.  Royal Academy of Engineering, Extreme space weather: impacts on engineered systems and infrastructure, Anual Report, January, 2013.  Jakowski, N., Borries, C., & Wilken, V. Introducing a disturbance ionosphere index. Radio Sci., 47 , RS0L14, doi:10.1029/2011RS004939, 2012.  Tiwari, R., H. J. Strangeways, S. Tiwari and A. Ahemad, Investigation of ionospheric irregularities and scintillation using TEC at high latitude, Advances in Space Research (submitted January 2013). Conker, R. S., M. B. El-Arini, C. J. Hegarty, and T. Hsiao, Modelling the effects of ionospheric scintillation on GPS/satellite-based augmentation system availability. Radio Sci., 38, 1, 1001, doi: 10.1029/2000RS002604, 2003. Tiwari, R., H. J. Strangeways, S. Tiwari, V. E. Gherm, N. Zernov and S. Skone, Validation of a transionospheric propagation scintillation simulator for strongly scintillated GPS signals using extensive high latitude data sets, 24th ION GNSS, Portland, OR, September 2011, pp. 2561-2571.  Strangeways, H. J., Y. Ho, M. H. O. Aquino, Z. G. Elmas, H. A. Marques, J. F. G. Monico and H. A Silva, On determining spectral parameters tracking jitter and GPS position improvement by scintillation mitigation, Radio Science, vol. 46, doi:10.1029/2010RS004575, 2011.
Author(s): Tiwari R, Strangeways HJ, Skone S
Publication type: Conference Proceedings (inc. Abstract)
Publication status: Published
Conference Name: 26th International Technical Meeting of the Satellite Division of the Institute of Navigation (ION GNSS+ 2013)
Year of Conference: 2013
Online publication date: 16/09/2013
Publisher: Institute of Navigation
Notes: Session A5: GNSS and the Atmosphere 2