Alternatives to GPS are around the corner

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. Stronger signals from newer kit can make jamming harder, but the potential scale of the threat now looks much greater than previously thought. 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. Jamming or even inadvertent interference from space, the research suggests, could affect an entire continent.

Alternatives to GPS are around the corner (Unsplash)One alternative to GNSS is simply to mount cameras on a craft and use software to keep track of the terrain below. But that only works over land, and can be stymied by cloud cover, haze or darkness. So two other, quite different, methods are under investigation. One employs quantum phenomena—the angular momentum of photons and the spin of electrons—to follow magnetic maps of Earth’s surface. The other establishes an observer’s position by tapping ambient radio waves broadcast for other purposes. Neither method can yet match the precision of GNSS, but both are becoming credible backups.

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. 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.

Q-CTRL’s sensor measures the field’s local strength using a device called an optically pumped magnetometer (OPM). This works by firing a laser into a cloud of rubidium atoms. If the laser beam is tuned correctly, the atoms’ electrons enter a state in which they are extremely sensitive to external magnetic fields. Another laser can then be used to track the behaviour of the electrons and work out the ambient magnetic field, allowing the system’s software to work out where on the map it is.

Other firms, meanwhile, are betting on a different type of sensor capable of showing not only the field’s strength, as Q-CTRL’s sensors already do, but also its direction. They believe these sensors, if used to navigate against the vector maps they help produce, could allow an observer to more accurately pinpoint their position.

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.

For now, the main drawback of both these approaches is a dearth of good maps of the crustal field. Those that are available tend to be inconsistent with each other and often lack the accuracy needed for navigation. Some firms, SBQuantum among them, therefore fly survey missions to create their own maps, while others, such as SandboxAQ, a Californian company, propose improving and joining up the disparate maps that already exist. In June, the firm submitted a paper suggesting that America’s government agencies, armed forces and researchers work together to do just that on a global scale. They may soon get their wish. In June Donald Trump called for advances in quantum sensing and ordered the Department of War to explore applications.

Meanwhile, others are developing ways to derive position from satellite radio signals already out there. America’s Federal Communications Commission has pressed for more work on such “signals of opportunity”, and a few teams are giving it a shot. One technique is to measure Doppler shifts. As a satellite approaches and then recedes, the frequency of its transmissions appear to rise and then drop, much like the whistle of a passing train. The closer an observer is to a satellite’s path, the more rapid the frequency change. Information about satellite orbits can then be combined with frequency measurements to estimate position.

One pioneer is ASPIN Lab, a research outfit at Ohio State University. 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.

Positioning by Doppler, however, has a notable shortcoming: satellites can drift off course or be manoeuvred to avoid potential collisions. This, in turn, can throw off estimates of a receiver’s position worked out from Doppler shifts. Stefano Tomasin, an expert at the University of Padua, in Italy, says errors of between 100 metres and more than a kilometre are common.

Precision, though, should improve. SpaceX, the owner of Starlink, now publishes predictions for the orbits of its more than 11,000 satellites. Unlike tracking data produced by America’s Space Force, these take into account planned manoeuvres. Dr Humphreys says SpaceX’s forecasts tend to be accurate to within just a couple metres of the satellites’ true positions. This should allow receivers to better interpret the Doppler shifts they observe.

Another route to greater accuracy is being pursued by Mark Psiaki of Virginia Tech. The trick, he says, is to place a master receiver at a known location to compare the Doppler shifts expected from a satellite’s forecast trajectory with those it actually observes, and broadcast corrections to receivers at unknown locations. With funding from America’s Department of Transportation, he is developing one such receiver that he hopes will yield positioning accurate to within about ten 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. 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. That signal’s departure time can then be broadcast to receivers at unknown locations. This will allow each receiver to determine how long the same signal took to reach its location, and, therefore, its distance from the satellite. 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.

The quest has even expanded to transmissions from cell towers. In tests with America’s air force, ASPIN Lab has used Matrix to navigate an aircraft by tracking tiny shifts in signals broadcast by mobile-phone towers. Matrix has been licensed to companies and military users, says Zak Kassas, the lab’s director, who expects the kit to be ready for market within two years. More reliable navigation, then, may already be at hand in the natural and man-made signals all around us.