Ionospheric Modelling and Validation for Satellite Based Augmentation Systems: the European EGNOS System Example
Ventura-Traveset, J.1; Prieto-Cerdeira, R.2; Orus, R.2; Brocard, D.1; Arbesser-Rastburg, Bertram2; Charlot, B.3; Debailleux, D.3; Alcantarilla Medina, I.4; Sanz, J.5; Chambre, E.3; Hernández-Pajares, M5; Miguel Juan, J.5
1European Space Agency; 2European Space Agency/ESTEC; 3Thales Alenia Space; 4Groupo de Mechanica del Vuelo; 5Research group of Astronomy and Geomatics (gAGE-UPC)
One of the challenging system requirements of the Satellite Based Augmentation System (SBAS) system is to provide the users with frequent and reliable data to correct the pseudo-range measurements affected by the delay introduced in the propagation of radio waves through the ionized layers of the atmosphere. This estimation shall be sufficiently good so that to allow the provision of safety of life services.
In the SBAS systems, this function is accomplished by the central processing facility through the use of a model of the ionosphere behaviour dynamically driven by real time measurements carried out in the SBAS reference stations.
The ionospheric modelling is a challenging domain since in some cases, sudden variations in the solar activity may lead the ionosphere to behave in a way far different from the nominal behaviour. This happens during severe ionosphere storms. In these cases, it is essential for a safety system, to confirm that the integrity of the computed corrections and the continuity of service are still maintained. Qualification of SBAS performance (both at computing platform and at user level) is done assessing if the system is able to compute the corrections and estimate reliably the residual errors even in the unusual or extremely perturbed propagation conditions of radio waves though the ionosphere.
This paper discusses the approached followed during several years for the ionospheric modelling and validation in the case of the European SBAS system, EGNOS. This have been the results of thorough analysis and discussions involving experts from the European Space Agency, European industry, European ionospheric expert groups and discussions at worldwide level for a better understanding on the limitations of ionosphere modelling and the capture of the true threat models, against which, the system shall demonstrate robustness.
As an example, the modelling on the exceptional ionosphere event of Oct 28 to Nov 1, 2003 (known as Halloween storm) is performed and assessed against implemented and validated EGNOS CPF algorithms. Tow cases are considered: first considering the true modelling of what happened in Europe during that severe storm; and secondly, artificially generating over Europe the same storm conditions that were observed in the Continental USA, which although they were not seen in Europe and probably could never be seen, allows to define a “true-iono” extreme behaviour scenario to test the different EGNOS ionospheric detection thresholds and detection mechanisms.
The paper will discuss detailed problems including, error associated with code carrier ionospheric divergence when applying smoothing at receiver level; the effects of users speed and spatial/temporal simultaneous variation; intrinsic limitations of the SBAS message standards and the strategy to cope with those; etc
Ultimately, the paper will be show that the implemented European EGNOS IONO modelling is sound, providing excellent performances, while at the same time protecting properly the users from an integrity viewpoint under severe ionospheric storm conditions.