The potential vulnerability of orbiting GPS satellites is being called into question with the discovery that Russia may have been testing a space-based GPS jammer over the course of many years.

Acting on an anonymous tipoff, Todd Humphreys, an aerospace engineering professor at the University of Texas, began combing through public data from global GPS monitoring stations on specific dates during 2021. While initially skeptical that any GPS anomaly could have gone unreported, Humphreys and his students soon discovered a pattern of brief, sharp drops in signal strength at identical radio frequencies occurring across 75 separate days and affecting Europe. At first, Humphreys thought some natural phenomenon like solar flares might be to blame. Further analysis, however, revealed that the culprit kept regular office hours and didn’t work on weekends.

The continental scale of the disruption ruled out ground-based interference. There also was a great deal of precision. The interference lasted less than 10 seconds and occurred exactly at 1,577.5 megahertz, the frequency used by consumer GPS signals. The interference had to be space-based, Humphreys concluded.

With a finger pointing to a source in orbit, Humphreys and his students devised a sophisticated tracking method that took into account which satellites were visible to affected ground stations, an estimate of the power required to create the disruptions, and the small differences in time it took for the interference to hit different ground antennas. The math led straight to a Russian satellite called Cosmos 2546.

Like a suspect confessing under interrogation, Cosmos 2546 gave up additional culprits. Cosmos 2546’s activity quickly became linked to a wider early-warning satellite network designed to detect incoming ballistic missiles called Edinaya Kosmicheskaya Sistema (EKS). Russia began launching EKS satellites in 2015, with Cosmos 2546 joining the gang in 2020. The evidence suggested that EKS is likely responsible for GPS degradation episodes across Europe dating to 2019. EKS flies an elliptical orbit that keeps its satellites in the northern hemisphere most of the time.

There are also some unsolved riddles like whether one Russian satellite adjusted its jamming signal as it traveled or whether multiple satellites coordinated GPS jamming signals. Some Russian satellites also have exhibited an ability to maneuver in space to alter their orbital paths.

Experts note a continent-wide disruption of GPS signals would have a devastating effect on all manner of commercial activity. It is still unclear what Russia’s tests were meant to accomplish. It’s possible, for instance, Russia may be testing a new-found capability of its EKS satellites. Something similar recently occurred in Iran when it was discovered that NASA satellites designed to track cyclone wind speeds and collapsing ice sheets turned out to have a talent for detecting the approximate location of GPS jammers on the ground. Needless to say, Cosmos 2546 is now being closely watched.