How to observe ionospheric scintillation in GNSS software

Offshore marine navigation and operations rely on highly accurate and reliable GNSS positioning. To achieve the level of precision required, several satellite and atmospheric errors must be resolved, typically through receiving GNSS corrections information, or by algorithms in the receiver.

One of the greatest sources of error is the refraction (bending) and diffraction (scattering) of GNSS signals by charged particles as they pass through the ionosphere (Table 1). To make matters worse, we are currently experiencing the upswing of Solar Cycle 25, resulting in increased ionospheric activity. The level of ionospheric activity depends on your location, time of day and time of year. After sunset in certain equatorial regions, you can almost set your watch to it!

Further, ionospheric scintillation — rapid temporal fluctuations in GNSS signal strength and phase — is caused by small-scale irregularities in electron density called plasma bubbles (Figure 1). Because the time and location of scintillation are highly variable, it’s even more important to understand how the quality of your positioning may be affected to mitigate operational delays, safety risks and downtime.

This short article shows how you can use GNSS visualisation software to observe whether your positioning is being impacted by ionospheric scintillation or radio frequency (RF) interference, enabling you to make informed operational decisions.

Is my position being affected by scintillation?

The United Nations Office for Outer Space Affairs (UNOOSA) International Committee on GNSS (ICG) defines GNSS interference into two categories: RF interference and natural disturbances (Figure 2). While both can result in disruptions to GNSS positioning, how they affect GNSS signals differs, enabling users to distinguish what they are experiencing through software tools. Being able to discern what is affecting GNSS signals, which ultimately impacts positioning accuracy or even the ability to maintain positioning, is paramount to determining the best course of action.  

As an example, Quantum software from Hexagon | Veripos provides an at-a-glance dashboard overview of your receiver’s GNSS status, including satellites by constellation, L-Band corrections, spectral plot, GNSS signal strength (carrier-to-noise ratio, C/N₀) and RF interference detection (Figure 3).  

High-precision GNSS receivers use advanced firmware algorithms to indicate the presence of RF interference. For example, LD8 and LD900 receivers by Veripos have GNSS Resilience and Integrity Technology (GRIT) functionality including the Interference Toolkit (ITK) and Spoofing Detection (SD) modules to identify and detect RF interference. GRIT alerts the user enabling them to mitigate RF interference by applying digital filters. The Quantum dashboard has a simple traffic light indicator for RF interference detection by GRIT. If the light is green, the receiver is not experiencing RF interference.   

As mentioned, ionospheric scintillation presents as a rapid, temporal fluctuation in GNSS signal strength and phase across all GNSS frequencies. We emphasise the latter because it is a distinguishing factor compared to RF interference, which is typically observed in one or two bands.

The bottom graphs (orange box) in Figure 4 show a high variation in GNSS signal strength (light blue) for the GPS L1 and L2 bands. The Quantum dashboard allows the user to toggle through all the GNSS constellations and signals to determine if they all show the same variation in signal strength. Of note, is that the GRIT traffic light is green (Figure 4, blue boxes), indicating the absence of RF interference. Together, these two metrics suggest that the disturbance in GNSS signals is due to ionospheric scintillation.

To corroborate whether the data shown indicates ionospheric scintillation, you can refer to an ionospheric activity forecasting map that monitors the regional and temporal changes in total electron content (TEC) and the rate of change of TEC index (ROTI). Elevated TEC values and in particular high rates of change in TEC are indicative of scintillation conditions, meaning possible impact to GNSS positioning accuracy and availability. If the timing and location where you experienced GNSS signal anomalies coincide with high ionospheric activity, any positioning issues experienced are very likely due to scintillation.

What else could be affecting my position?

As with any interpretation of raw measurements, there is always a chance of false positives and false negatives. In this case, a false positive would conclude that the observed effect on GNSS positioning was due to scintillation, when it was due to other factors. A false negative would conclude that the observed effect on GNSS positioning was not due to scintillation when it was. 

For example, cabling issues can cause the strength of all frequencies to be consistently low versus the fluctuation seen with scintillation. Installation problems such as placing the antenna too close to other electronic devices can cause abnormalities in the received GNSS signals. As discussed, RF interference also typically affects one or two discrete frequencies rather than all signals.

While you cannot completely avoid the possibility of false positive and negative scenarios, observing rapid fluctuations in signal strength for all GNSS constellations and frequencies while your receiver is in a region experiencing high ionospheric activity is a strong indicator of scintillation, particularly when the software does not detect RF interference.

Ultimately, whether you need to be concerned about ionospheric scintillation or not is if your operations are affected by any reduction in GNSS positioning solution accuracy or availability. Having software visualisation and forecasting tools at your disposal can help narrow down if ionospheric scintillation is the issue and guide next steps.

To learn more about ionospheric activity and scintillation, check out our other resources and sign up for updates.

If you need assistance with ionospheric scintillation, please contact us for support.