This theme considers the external influences on global navigation satellite system (GNSS) signal propagation, prior to reception by a receiver on or near-Earth, including receiver local environment and the atmosphere. The atmosphere remains the dominant error source in applications relying on GNSS, leading to degradation in performance of observation systems and in recovery of geodetic parameters. The atmosphere is therefore the main focus of this theme.
Conversely, from an atmospheric scientist’s perspective, these very same geodetic observational systems provide a unique platform from which to study the Earth’s atmosphere. This synergy creates an ideal opportunity for cross-disciplinary research and collaboration.
At NGI we focus on GNSS atmospheric research to model, monitor, forecast and in particular mitigate the impact on end users. Topics within this remit include monitoring water vapour in the lower, neutral atmosphere (led by Richard Bingley) and background behaviour as well as disturbances, in the upper charged atmosphere plasma (led by Marcio Aquino).
Regarding the neutral atmosphere, since 2002 we have been developing near real-time (NRT) GPS processing systems providing hourly updated estimates of atmospheric water vapour to the UK Met Office. From 2007 these estimates have been included in the Network of European Meteorological Services E-GVAP project, and assimilated into the Met Office operational numerical weather prediction model. This regional hourly updating processing system takes data from about 300 stations, with full coverage of the British Isles and a lower station density for the remainder of Western Europe.
Between 2007 and 2010, this regional system has been complemented by a global hourly processing system, again from a network of about 300 stations, with representation from UK and European station networks integrated with stations of the global International GNSS Service (IGS) network.
The most recent development is a European regional sub-hourly NRT processing system. This cycles every 15 minutes, reducing output latency, so that the estimates are not just useful for assimilation in numerical weather prediction runs that take place every few hours, but can also be used in relation to specific weather events, such as severe thunderstorms. All these developments are complemented by on-going research of the NERC British Isles continuous GNSS Facility (BIGF) which provides products for scientists to study both NRT and long-term changes in atmospheric water vapour.
Regarding the upper atmosphere, we have concentrated significant research effort into monitoring and studying the effects of the disturbed ionosphere, such as irregularities causing signal scintillation. Ionospheric scintillation causes amplitude and phase fluctuation in the signals from GNSS satellites, and current GNSS receivers are not robust against it. Effects range from degradation of positioning accuracy to the complete loss of signal tracking.
We have teamed up with other top UK and international experts, in an effort to tackle this problem through the development of detection, forecasting and mitigation techniques. These include new algorithms that have the potential to be implemented in GNSS receiver firmware, and improved and new receiver tracking models that can make them more robust to ionospheric disturbances.
We have studied in depth, the performance of GNSS receiver tracking loops under conditions of scintillation, with particular regard to the new signals from a modernised GPS and Galileo, exploiting both simulated and real data obtained from monitoring networks at high and low latitudes. It is these regions that are most affected by ionospheric scintillation. As part of this research we analyse long term data, to study the effects of the local environment at individual stations, in order, for example, to isolate multipath from ionospheric effects (image opposite - credit Vincenzo Romano).
An important development in this research area is the recent award to NGI, of the FP7 Marie Curie Initial Training Network TRANSMIT (Training, Research and Applications Network to Support the Mitigation of Ionospheric Threats).
NGI coordinates the TRANSMIT Marie Curie Initial Training Network, with the aim of training young researchers in the topic of ionospheric effects, and the threats they pose to GNSS. TRANSMIT kicked off in early 2011, and involves the recruitment of 16 fellows across 7 top European universities and research centres, such as the German Aerospace Centre (DLR) and the Italian National Institute for Geophysics and Vulcanology (INGV) - group meeting image shown opposite.
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