Ionospheric scintillation: the GNSS failure that arrives after sunset

It is 20:40 at a site 15° from the equator — Brazil, Nigeria, India, Singapore, northern Australia all behave the same way. The RTK fix that held 1 cm all afternoon starts wandering. Then it drops to float. Then a couple of satellites disappear entirely. Tomorrow morning everything will be perfect again.

Nothing has changed in the hardware, the corrections or the sky view. What changed is the ionosphere. If your deployments include low-latitude sites, this is one of the few GNSS error sources that is predictable in its rhythm and unpredictable in its timing — and it is worth knowing how it works before it shows up in a customer's log.

The signal path goes through a charged layer

GNSS signals travel roughly 20,000 km to reach the antenna, and for most of that journey there is nothing in the way. The last 1,000 km are the exception. The ionosphere — the charged layer between about 100 km and 1,000 km altitude — refracts and diffracts the signal, which produces both a delay and a distortion.

When the ionised particles are smoothly distributed, a receiver can model their effect on the signal. The trouble starts when the ionosphere becomes irregular: local fluctuations in electron density distort the phase and amplitude of the signal. That is ionospheric scintillation, and it is usually described with two indices:

• S4 — the amplitude scintillation index;
• σφ (sigma-phi) — the phase scintillation index.

Figure 1 — S4 and σφ through a scintillation event. The usual "strong" thresholds are S4 > 0.6 and σφ > 0.3. Image courtesy of Septentrio.

Where and when scintillation happens

Scintillation events are most frequent and most intense near the geomagnetic equator, with a weaker occurrence at the poles. They are also documented at mid-latitudes — Western Europe and the United States included — so this is not exclusively a tropical problem.

Timing is driven by the sun. Solar activity follows an 11-year cycle measured by sunspot count: around solar maximum, frequent solar flares release bursts of high-energy protons and X-rays that interact with the atmosphere, and scintillation events become more common. On top of the solar cycle there is a strong daily pattern — sunset triggers a sharp increase in ionospheric activity that can last for hours. That daily pattern is why scintillation so often looks like a "the fix degrades in the evening" bug rather than an environmental one.

Figure 2 — Distribution of high-S4 scintillation events: densest in the equatorial band, weaker at mid and high latitudes. Image courtesy of Septentrio.

Figure 3 — S4 recorded by a static PolaRxS receiver in Brazil (22°S) over 24 hours: activity climbs after sunset and stays elevated for hours. Image courtesy of Septentrio.

What it does to a GNSS solution

For a standard GNSS receiver the failure escalates in stages:

Because the corruption is in the signal itself, not in the satellite geometry, adding constellations does not fix it — the affected measurements have to be identified and excluded, or handled with a receiver that is designed for it.

How IONO+ handles it

Septentrio developed IONO+ as a direct result of working on projects in Brazil, one of the countries most affected by scintillation. Receivers with IONO+ keep tracking satellites under conditions that disrupt a standard receiver, and they identify scintillation events so their effect on position accuracy can be limited. In the static test below, the algorithm correctly identified the scintillation-affected signals and removed them from the position computation.

Figure 4 — Height computed by a static PolaRxS receiver during scintillation: standard positioning (blue) versus scintillation-improved positioning (green). Image courtesy of Septentrio.

The second half of the design is about baseline logistics. With standard RTK you normally need a reference station network to interpolate the ionospheric delay and compensate for it at the rover. With IONO+ the ionospheric delay is estimated inside the receiver, so no network is required: a single reference station at a baseline of up to 40 km is enough, and up to 80 km while the ionosphere is quiet. For integrators working in equatorial regions, that is often the difference between "RTK needs a dense CORS network we do not have" and "one base station on the site".

Practical checklist for low-latitude deployments

1. Log the evidence, not just the symptom. Record C/N0 per band and the receiver's interference/scintillation indicators alongside the position log. A dusk-onset pattern is the signature.
2. Test at the right hour. A site that passes commissioning at 14:00 may fail at 20:00. Plan an evening test during solar maximum.
3. Check the correction architecture. If the site relies on network RTK interpolation, confirm what happens when the ionosphere is disturbed — this is where a receiver-side estimate changes the requirement.
4. Expect cycle slips, not just accuracy loss. Design the recovery path: how fast does your RTK engine re-converge, and what does the autopilot or controller do in the meantime?
5. Pick receivers on capability, not constellation count. Multi-frequency tracking, robust tracking loops and scintillation handling matter more at these latitudes than one extra constellation.

Hardware that ships with this capability

IONO+ is receiver behaviour, not a service. It is part of the Septentrio feature set on the modules and receivers used in drones, robotics and reference stations:

• Septentrio mosaic-X5 module — multi-frequency, all-constellation core for integrators that need raw observables;
• HBEV322 / HBEV322H compact RTK GNSS receiver — mosaic-G5 P3H with dual-antenna heading;
• HB52H / HB52 ultralight RTK module — few-gram mosaic-G5 for mass-limited platforms;
• HB10 dual-antenna RTK receiver — AsteRx-m3 Pro+ where attitude is needed as well as position;
• HB62 rugged GNSS reference station — a turnkey base for a shared correction source, including the 40–80 km baseline cases above.

If you want to work out whether scintillation is what you are seeing, the practical next step is a log review: send us a capture with the C/N0 and position stream and we will read it with you — request a quote or get in touch.

Sources: Septentrio technical note on ionospheric scintillation and IONO+; S4 observations from V. V. Sreeja et al., J. Space Weather Space Clim. 1 (2011); CIGALA project data (Brazil); Septentrio product documentation.