DASP Blog: Rethinking the Fresnel Frequency – A Better Way to Decode Ionospheric Scintillation

 In this DASP blog, Kaili Song (University of New Brunswick) discusses their recent paper on evaluating different techniques for deriving the Fresnel Frequency of radio waves in the ionosphere. If you have any questions, reach out to Kaili at kl.song@unb.ca.

When GNSS signals pass through the ionosphere, they encounter fluctuations in electron density that distort their phase and amplitude—a phenomenon known as ionospheric scintillation. These fluctuations can interfere with satellite-based positioning and navigation systems, especially in high-latitude regions where irregularities are more dynamic and structured.

    In our recent JGR Space Physics article, we investigate a key quantity in wave propagation theory: the Fresnel frequency (), which identifies the spectral boundary between refractive and diffractive effects in signal propagation. Although traditionally approximated by the rollover frequency in amplitude spectra, we demonstrate that this approach often underestimates the true Fresnel frequency.

We propose and compare three different analysis methods to estimate more accurately:

• The Ionosphere-Free Linear Combination (IFLC) filters out the refractive contributions by combining L1 and L2 GPS phase measurements, isolating the diffractive effects.
• The Normalised Cross Spectrum (NCS) between amplitude measurements at L1 and L2 detects zero-crossings that coincide with Fresnel-scale structure.
• The Two Components Phase Spectral Method identifies the transition between refraction-dominated and diffraction-dominated spectral regimes.

We validated our results using over 150 scintillation events recorded by the Canadian High Arctic Ionospheric Network (CHAIN). Figure 1 (left) shows strong agreement between Fresnel frequencies derived from IFLC and from NCS, with a best-fit line slope of 1.17. In contrast, Figure 1 (right) reveals that the rollover frequencies derived from amplitude spectra are systematically lower than the corresponding Fresnel frequencies.

Figure 2 shows the relationship between Fresnel frequencies obtained from IFLC and the spectral knee frequency determined from the phase spectra. These results are consistent and highlight that Fresnel effects become evident well before traditional rollover markers.

By using techniques like IFLC and NCS that isolate diffraction from refraction, we show that the true Fresnel frequency can be reliably identified, improving the characterization of ionospheric irregularities. This approach enhances our understanding of scintillation mechanisms and supports better modelling of GNSS signal degradation in space weather events.

Figure 1 - Statistical comparison across 152 weak scintillation events (CHAIN dataset). Left: Fresnel frequency from IFLC vs. NCS. Right: Rollover frequency vs. Fresnel frequency from NCS.

Figure 2 - Comparison between Fresnel frequency derived from IFLC and knee frequency from dual-slope phase spectra. Most data points lie close to the identity line.

Interested in writing a blog post for DASP about a recent paper? Email DASP at dasp.dpae@gmail.com, or Dan Billett at daniel.billett@usask.ca.