What is ionospheric activity and how do I mitigate against the impact on GNSS?

What is ionospheric activity?

The ionosphere is part of the Earth’s atmosphere and consists of a variable complex physical system located between 60 km to 1000km above the Earth’s surface. This layer contains free electrons, especially at an altitude between 250 to 400 km, that can adversely affect the performance of GNSS receivers. At the time of writing, we are currently within a period of increased ionospheric activity. As such, we should expect adverse effects on tracking and positioning within certain timeframes and environments. Increased ionospheric activity correlates with the following factors:

• Sunspot activity
  Increased ionospheric activity linked with the 11-year solar cycle. The chart below illustrates the current and predicted activity for the current solar cycle 25.

(U.S. Department of Commerce, NOAA, & SWPC, 2022)

• Coronal mass ejections
  Commonly known as solar flares, coronal mass ejections (CMEs) are more likely to occur during increased solar activity. Satellites monitoring the Earth-facing sun’s surface can provide an early warning of earth-bound CMEs.

• Location
  The highest activity is +/- 15 degrees around the geomagnetic equator and, less severely, in auroral (polar) regions.

• Seasonal Variations
  Increased activity occurs during the spring and autumn equinoxes.

• Diurnal (Daily) Variations
  Post-local sunset hours until midnight will typically experience the maximum effects.

How is GNSS impacted?

• The most significant impact is scintillation, caused by small-scale irregularities in the ionosphere. Scintillation effects GNSS signal tracking, causing fluctuations in the amplitude and phase of the carrier phase signal, introducing noise or causing loss of lock to satellite(s). This can result in a reduced number of usable GNSS satellites and occasionally a reduction in the L-band communications link strength, causing intermittent reception of the augmentation data.
  
  Scintillation effects are usually seen in the evening within 6 hours after local sunset, especially within bands surrounding the geomagnetic equator illustrated below:

(Kintner et al., 2009)

• The introduction of significant errors (up to 15 meters) into single frequency position solutions which might also include Differential GNSS (both GPS and GLONASS), because of the potential failure in the differential process to cancel the effects of the ionospheric delay between the reference station and user end.
  
  Dual-frequency systems which measure the ionospheric delay can mitigate these single-frequency DGNSS errors. The VERIPOS Ultra and APEX services are not affected by single-frequency DGNSS errors, as the services are dual or multi-frequency solutions. VERIPOS recommends using Ultra or APEX services in polar and equatorial regions.

How do I mitigate against ionospheric activity?

• Using additional GNSS constellations (GLONASS, BeiDou, Galileo) to increase the number of observations available to the position solution and increase the diversity of the signals tracked.

• Ensure that VERIPOS systems are operating with the latest version of firmware. Where possible, set the GNSS tracking mask to a minimum of 7 degrees.

• Use all available L-band beams to ensure that the loss of lock to one L-band beam does not interrupt the receipt of corrections from others. LD8 and LD900 receivers can track all available L-band beams simultaneously. Using AUTO for L-Band beam choice will ensure uninterrupted corrections should data loss from a single beam occur.

• Increased Ionospheric activity should be considered during risk assessments when planning critical offshore operations. Risks increase when operating near the geomagnetic equator and after sunset. West Africa and Brazil are regions commonly affected by scintillation. DP operators should prepare to select and use non-GNSS positioning reference systems if required.

• Ensure relevant personnel and operations are registered to receive warning emails from Veripos support. Monitor emails carefully for warnings of heightened space weather activity, with the occurrence of large CME events a significant risk. Warning emails will typically give 24 hours warning of CME events.

WARNING
Although these steps may mitigate the adverse effects of scintillation, more extreme scenarios may impact all GNSS and L-band signals, resulting in total loss of GNSS positioning

Please direct any questions to VERIPOS Support.

References

U.S. Department of Commerce, NOAA, & SWPC. (2022, October 7). ISES Solar Cycle Sunspot Number Progression [Chart]. Retrieved from Space Weather Prediction Center (SWPC): https://www.swpc.noaa.gov/products/solar- cycle-progression

Kintner P, Humphreys T, Hinks J (2009) GNSS and ionospheric scintillation. Inside GNSS 4:22–30