“Earth’s magnetic field is unjammable”
The solution to overcoming GPS manipulation may lie beneath our feet, based on an ancient method used since the 11th century: navigation using Earth’s own magnetic field the way humans have done for centuries using a compass. What’s new, however, is that you don’t just use the magnetic field to determine a heading (or direction), but also to find your position.
The method is called MagNav, and Professor Nils Olsen from DTU Space is among the leaders in the development of the method and collaborates with ESA (European Space Agency) on this. He sees great potential in MagNav as a backup or in special situations as a real alternative to GPS.
“What’s special about MagNav is that it doesn’t emit signals like GPS does—so it can’t be spoofed or jammed. You can’t jam Earth’s magnetic field,” he says.
Unlike GPS—which requires a satellite connection—MagNav relies on local measurements of magnetic field strength and variation, making it immune to electronic warfare attempting to disrupt signal-based systems.
Mapping invisible landscapes
Around 2,900 km below the surface, most of Earth’s magnetic field is formed, surrounding the entire planet. Also there are contributions from rocks close to the surface, and their magnetic field can be used for positioning.
However, the field is far from even—on the contrary, it varies from place to place like an invisible landscape that on maps resembles mountains and valleys. It is these small variations that MagNav utilizes. Using a magnetometer, typically fitted on an aircraft or drone, the variations are measured with great accuracy, but because the magnetic field can be similar in multiple locations, one measurement is not sufficient. Instead, the system tracks how the field changes as you move, and by comparing that pattern to a magnetic field map, your position can be calculated.
Determining the magnetic field map requires a systematic effort, overflying the entire area you want to cover. The lower you fly, the more detailed and accurate the maps become. Once an area is mapped, the data can be reused for many years as the variations in the part of the magnetic field used for MagNav only changes very slowly over time.
“MagNav will probably not be as accurate as GPS, but it’s not always necessary. The pilots I talk to only require an accuracy within 5 km. That way they can compare the position from the MagNav system with their GPS position and thereby easily detect if it is spoofed,” says Nils Olsen.
Satellites orbiting Earth
In Denmark, the focus of MagNav is on Greenland and the North Atlantic—strategically important areas that are also poorly covered by magnetic maps and Nils Olsen is currently working on a precise magnetic mapping of Greenland using high-altitude drones.
There is considerable international interest in making MagNav a technology that can also be used globally.
The idea is to determine the necessary magnetic field map with satellites in low, elliptical orbits, reaching altitudes as low as 180-200 km for short periods. It’s low enough to detect the very small variations in the magnetic field that MagNav requires.
“We have been asked if it is possible to create an accurate global map of the magnetic field, and it looks like it is—providing that have enough satellites flying sufficiently low. I can well imagine that in the future, we will start mapping larger areas to be able to use MagNav as a robust alternative or supplement to GPS,” says Nils Olsen.
Preliminary simulations show that it is indeed possible to achieve a positional accuracy of 20-50 m in areas with very accurate magnetic field maps and potentially around 5 km globally using satellites. According to the researcher, this is more than sufficient as GPS backup.
