A paper posted in June by a team led by Todd Humphreys of the University of Texas, Austin, traced scores of broad GPS disruptions across Europe and elsewhere since 2019 to a constellation of Russian satellites. THE FIRST casualty of war is truth. But a reliable GPS signal comes a close second. Jamming signals from a global navigation satellite system (GNSS), such as America’s GPS , is so easy it has become common when the shooting starts. Jamming or even inadvertent interference from space, the research suggests, could affect an entire continent.
Stronger signals from newer kit can make jamming harder, but the potential scale of the threat now looks much greater than previously thought.
In a demo on a cruise ship off western Greenland on August 24th 2024, its system, called Matrix, processed transmissions from 21 Starlink and OneWeb broadband satellites passing overhead. Given a starting position and speed, Matrix placed the ship just 27 metres from its true position. SpaceX, the owner of Starlink, now publishes predictions for the orbits of its more than 11,000 satellites.
Dr Humphreys says SpaceX’s forecasts tend to be accurate to within just a couple metres of the satellites’ true positions. Many rocks are weakly magnetic, and their distribution in Earth’s crust results in irregularities in the planet’s magnetic field. The resulting landscape of anomalies is detectable under almost any conditions, and equally well over land and sea. Fit a plane with a magnetometer capable of spotting these anomalies, and a magnetic navigation (MagNav) system that can match them to existing maps, and its position can be accurately plotted. One such firm is SBQuantum, a Canadian outfit. Its device is called an NV-diamond magnetometer which, as the name suggests, has a diamond at its heart. Diamonds are not normally magnetic, but can be made so by creating what are known as nitrogen-vacancy (NV) centres in their crystal lattices. These are places where two of the lattice’s carbon atoms are replaced by a single atom of nitrogen—opening a gap around which electrons gather. Once the diamond is placed in a magnetic field, the regularity of its lattice provides a 3D grid against which the electrons’ behaviour can be measured. Accurate readings of the field’s strength and direction can then be taken. One pioneer is ASPIN Lab, a research outfit at Ohio State University. Precision, though, should improve. Unlike tracking data produced by America’s Space Force, these take into account planned manoeuvres. This should allow receivers to better interpret the Doppler shifts they observe.
Last year Q-CTRL, an Australian firm, demonstrated a MagNav system that was more accurate than a leading GPS alternative in a trial flight over New South Wales.
This will allow each receiver to determine how long the same signal took to reach its location, and, therefore, its distance from the satellite.
The idea, he says, is to use the master receiver’s known position and SpaceX’s predicted position of a Starlink satellite to determine when a signal must have been transmitted. By combining distance measurements from four Starlink satellites with Doppler data, a receiver, says Dr Psiaki, should be able to determine its position to within just two metres. Dr Psiaki’s master receiver could also facilitate another way of exploiting signals of opportunity: working out how long it takes for a signal to travel from source to recipient. That signal’s departure time can then be broadcast to receivers at unknown locations.
