The largest source of error in GNSS positioning is the delay suffered by the signals as they pass through the ionosphere traveling from the satellites in orbit to receivers on or near Earth’s surface. That is because the ionosphere is full of free electrons stripped from atoms and molecules by ionization and this plasma refracts the signals, changing their speed. Normally, models compensate for this. However, geomagnetic storms wreak havoc on the free electrons in the ionosphere, making it difficult to accurately determine the signal delay.
That is why space weather matters for GNSS and for the myriad human activities that have come to depend on it.
So, here’s the good news. “On a scale of one to five, the geomagnetic storm on April 14 was a three,” Bill Murtagh told me. Murtagh is the Program Coordinator and Space Weather Forecaster at the Space Weather Prediction Center (SWPC) of the National Oceanic and Atmospheric Administration (NOAA). He was referring to the third rung of NOAA’s space weather scales, which were introduced to communicate to the public the current and future space weather conditions and their possible effects on people and systems.
NOAA has three space weather scales, one each for geomagnetic storms (G scale), solar radiation storms (S scale), and radio blackouts (R scale). The steps on the scales, ranging from “minor” to “extreme,” are analogous to those NOAA uses to classify hurricanes, tornadoes and earthquakes. They describe the environmental disturbances for each of these events and list their possible effects at each level.
Solar activity runs in 11-year cycles. A G5 event happens two or three times per cycle, and the last one was in October 2003, Murtagh told me. “I can only remember a handful of occasions over the past 20 years when ionospheric activity has significantly impacted users,” told me Gavin Schrock, PLS, manager of the Washington State Reference Network, a regional cooperative of GPS reference stations and data. According to Rick Hamilton, the GPS Information Analysis Team Lead at the U.S. Coast Guard Navigation Center, it “did not receive any reports of interference related to the geostorm” and “there was no significant increase in reports that we might attribute to geomagnetic activity.”
Now, the bad news. We are heading for a maximum in solar activity, expected to occur in 2025. The Sun is “already quite active,” Murtagh pointed out, and recently there has been an increase in the number of R1 and R2 storms. Solar coronal mass ejections (CMEs), which launch plasma and magnetic fields into space, also have become more frequent. When a CME hits the Earth, its collision with the Earth’s magnetic field causes a geomagnetic storm.
So, the GNSS constellations and the GNSS industry should be preparing now. Fortunately, improvements in GNSS software and receiver technology, plus corrections and integrity information and the much larger number of satellites, make us better prepared than we were during the last cycle. On the other hand, the stakes also are much larger, due to our ever-greater reliance on GNSS.
As a sailor, I rely on NOAA nautical charts and marine weather forecasts. GNSS users can thank NOAA for its space weather forecasts.