Ionospheric scintillations are the rapid fluctuations in phase and/or amplitude of an electromagnetic wave passing through small scale and time varying electron density irregularities in the ionosphere. Figure-1 provides a graph illustrating scintillation by looking at the incident wave plane (plane of equal phase) which is arriving at a layer with rapidly changing irregularities; the emerging wave front is no longer a plane due to scattering (signal hitting the structures and going off into multiple directions) and diffraction (part of a wave hitting a structure and going forward as a circular wave-front causing an interference pattern). The result for the observer is a signal which is rapidly changing in phase (phase scintillation) and intensity (amplitude scintillation).
Figure 1: Scintillation principle
Figure 2, provides a brief overview of the climatology of scintillation and shows that the main areas of scintillation occurrences are in the equatorial zone (after sunset) and at high latitudes near the auroral oval (at night-time). During solar maximum conditions the scintillation effects are much more pronounced than during solar minimum conditions.
Figure 2: Scintillation occurrence
Figure 3: Scintillation Impact
Figure 4: Affected geometry
Scintillation, GNSS Signals and Positioning Accuracy
The impact of scintillation on GNSS signals and their reception can vary between a complete loss of lock on the signal, to cycle slips and degraded accuracy of the measured ranges, see figure 3. The final influence on the calculated position depends on the available geometry (after lost signals, see figure 4), and how much influence a degraded range (less accurate or with an unrepaired cycle slip) can have on the positioning outcome.
A loss of lock cannot be solved in the positioning computation algorithm; it has to be minimised (together with the occurence of cycle slips and range accuarcy degradation) through a scintillation robust Phase Lock Loop (PLL) design in the receiver. The other effects, increase in cycle slips and degraded range accuracy can also be mitigated through:
- Robust cycle slip detection and repair during scintillation.
- Advanced stochastic modelling to de-weight ranges affected by scintillation.
Looking at figure 2 again, it is evident that where areas of strong scintillation coincide with high precision navigation dependent economic activity, the need for the mitigation of the impact of scintillation becomes important.
Mitigation of the degraded range accuracies
The mitigation strategy (a build up of the stochastic model complexity) followed in this prototype demonstrator is to first improve on the most basic stochastic model. Thus going from a fixed variance to a tailored elevation based model, using variance component estimation (VCE) in an indirect (calibration) way. The mitigation strategy builds from that by fine-tuning the relationship between C/No level and the elevation based stochastic model. Mitigation of scintillation is then further improved by combining screening and a direct implementation of VCE.