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Ionospheric threats and suitable Countermeasures Applicable to satellite Radio systems Under next Solar maximum

Final Report Summary - ICARUS (Ionospheric threats and suitable Countermeasures Applicable to satellite Radio systems Under next Solar maximum)

The ionosphere is the ionized portion of the upper atmosphere, extending from about 60 km to beyond 1000 km and completely encircling the Earth. The main source of plasma for the ionosphere is photo-ionization of neutral molecules via solar EUV and soft X-ray radiation. The ionosphere is very sensitive to solar activity. The maximum phase of the 11-year solar cycle will occur in 2013. In combination with the geomagnetic field, high solar activity drives several phenomena with different origins at high, middle, and low magnetic latitudes. During high solar activity ionospheric phenomena are greatly enhanced.

This fellowship was focused on the study of the ionosphere and its effects on GNSS, such as the American GPS and the European Galileo. It will also contribute to the European space situational awareness (ESSA) service as ionospheric disturbances affect the operation of strategic European space assets, such as telecommunications and Earth Observation satellites (e.g. the European programme global monitoring for environment and security, GMES).

The last preparation phase of the new European system Galileo and the starting phase of the ESSA service will coincide with the next solar maximum. During solar maximum, ionospheric disturbances and consequential effects to GNSS may reach critical levels. The study and thorough understanding of these effects will be essential. GNSS signals represent a very powerful tool for monitoring and assessing the effects of the ionosphere during next solar maximum on a global scale: while the satellites' signals are affected by these effects, the expanding number of terrestrial and space based permanent GNSS receivers allows the continuous sounding of the ionosphere. The amount of free ions and electrons in the ionosphere causes delays in the propagation of radio waves broadcast from artificial satellites, affecting the correct interpretation of the information being transmitted.

Moreover, a radio wave propagating through certain ionospheric structures may experience fluctuations in its amplitude and phase, whose characteristics depend on the radio frequency, magnetic and solar activity, time of day, season of the year and magnetic latitude of the observation point. These fluctuations, known as scintillations, are responsible for signal degradation and service disruption, particularly in the case of satellite navigation applications, such as those relying on GPS and Galileo.

The scientific goals of the fellowship's research project, which go beyond the state of the art, may be summarised as follows:

A. Accurate description of ionospheric disturbances on space assets (such as GNSS and telecommunication satellites, GMES) as well as on GNSS services, during the forthcoming solar maximum;

B. Analysis/description of receiver tracking loops (PLL and DLL) behaviour particularly in a non-linear regime;

C. Description of solar maximum impact on GNSS services accuracy, integrity and availability, which will also contribute to the operation of the European Space Situational Awareness service;

D. Modelling of radio wave propagation through electron density irregularities, particularly in the strong scattering regime (an aspect still not well understood and described);

E. Solutions to mitigate the impact of the ionosphere on GNSS receivers and services during solar maximum.

a description of the work performed since the beginning of the project, a description of the main results achieved so far,

The proposed objectives were tackled and the following results were achieved:

1.The non-linear regime of PLL within standard GNSS receivers was modelled and simulated. The occurrence of losses of lock was explained and described [1]. This is connected to the improvement of the robustness of GNSS receivers in presence of adverse space weather and ionospheric conditions. Space weather adverse events can cause GNSS receivers to lose signals, thus deteriorating the quality of the service reliant on GNSS (e.g. surveying, precise positioning, aviation). The results described the precise occurrence of losses of lock and suggested possible ways to mitigate against this particular vulnerability posed by space weather. Some mitigation strategies were indeed suggested to an industrial partner (Septentrio), within the framework of the Seventh Framework Programme (FP7) GSA CIGALA project (see http://cigala.galileoic.org online).

2.The evidence of strong scattering by means of ionospheric plasma density irregularities was found. The proper modelling of weak and strong scattering regimes is of crucial importance for the characterisation of the effects on GNSS signals and, consequently, on GNSS receivers. The results evidenced a clear distinction between weak and strong scattering regimes. A model for the strong scattering regime was proposed and validated against experimental data.[2]

3.Evidence and description of ionospheric scintillation effects on Galileo signals [1]. The description of ionospheric scintillation effects on Galileo signals and receivers was detailed by means of numerical simulations (through the use of a Spirent signal simulator available at the host Institution) and confirmed by experimental data collected in the framework of the FP7 GSA CIGALA project. Essentially, scintillation effects on Galileo signals are very similar to those experienced by GPS classical and modernised signals, owing to the carrier frequency still in a band very vulnerable to space weather disruptive events. Possible solutions at receiver level (see point 1) were suggested and implemented but the intrinsinc problem connected to the transmission frequency utilised for Galileo still remains (this is connected to the final decision by the EU and ESA on the type of signals)

4.Evidence of scintillation impact on services in the African sector, such as Satellite-Based Augmentation Systems. A study based on experimental data collected in the African sector showed the impact of adverse space weather events (i.e. scintillation) on a typical low-latitude station. The interpretation was done from the point of view of a service based on satellite-based augmentation systems (e.g. EGNOS) [3]. The results showed that extreme care need to be taken in the interpretation of the impact ionospheric scintillation may have on systems such as EGNOS. In these cases, indeed, attention needs to be paid on how scintillation would affect the algorithm at the basis of the service. This analysis has indicated possible ways forward.

The work done during the fellowship allowed a clear improvement in the career of the Fellow through a number of initiatives, such as: participation into projects held at the host Institution, participation into international conferences and workshops, participation into teaching activities within one of the official MSc modules offered at the host Institution, short-term visits at relevant Institutions in the field, long-term visit at the University of Bath (UK).

the expected final results and their potential impact and use (including the socio-economic impact and the wider societal implications of the project so far).

The results achieved during the project fully address the objectives proposed at the beginning. Some of these results (i.e. the description and modelling of tracking errors) supported the submission of a patent application. The studies carried out in ICARUS also assisted the testing and validation of an improved tracking approach developed by SSN and implemented in their receivers in the frame of CIGALA.

Results show that the resilience of the receiver certainly improved in presence of disruptive ionospheric scintillation as the losses of signals were decreased even in presence of harsh ionospheric environments (such as at low latitudes in the South American sector).

A further exploitation and advancement of the studies and results approached during this project will be done in subsequent research projects.

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