GPS signals are too weak to be accurate deep inside big buildings. In conflict zones like Ukraine and the Strait of Hormuz, adversaries jam and spoof signals. So the hunt has been on for decades for GPS alternatives that might work both indoors and outside, and in even more unconventional places, such as underwater.

One tantalizing option involves using the Earth’s magnetic field. It’s always present, even if it’s weak or subject to noise from nearby local magnetic fields. For more than a decade, some companies have combined maps they’ve previously made of local magnetic fields with other technologies such as Bluetooth or radio-frequency identification (RFID) to offer indoor mapping services.

Now, companies such as AstraNav, Oriient, and SysNav say they are ready to disconnect from other technologies and use the magnetometers installed in every modern mobile device together with magnetic map data. The same technology is starting to be useful outdoors, too. “If we have a magnetic map, then we can provide first-fix absolute positioning in the air, underwater, or underground,” says AstraNav co-founder and CEO Anton Toutov.

AstraNav’s Magnetometer Navigation

AstraNav will be demonstrating in the next couple of months to the U.S. Air Force how aerial drones can use their software under a US $1.8 million Small Business Innovation Research (SBIR) grant. In March, the company announced a separate deal with healthcare real-time location services company Sonitor to provide a magnetometer-based tracking system for workers and medical devices. Sonitor and its healthcare clients will no longer need to install ultrasound, Bluetooth, or other beacons in their facilities to locate staff or hardware.

The moment is ripe for navigational aids relying on magnetometry thanks to advances in signal processing, says Isaac Skog, a communications systems and automatic control professor at KTH Royal Institute of Technology in Stockholm. “In the beginning, people tried using magnetometers just as a compass in a smartphone, but that’s challenging because you have other material in the smartphone and had to do a lot of processing to compensate for that material,” he says. “Then people said, ‘How can we use variations in the magnetic field not only for finding north but also to create a map and determine where we are?’”

Since the 1990s, the U.S. Defense Advanced Research Projects Agency (DARPA) has funded research into alternatives to GPS for urban and indoor environments. Companies have explored repurposing Wi-Fi, RFID beacons, inertial systems, and, more recently, machine vision and phone cameras. But in emergency or combat settings, users can’t rely on any outside infrastructure. The broader goal from both DARPA and various companies has been to mature the tech beyond anything requiring fixed sensors or networks.

The technical challenge is large, though. “There’s a lot of confounding factors,” says David Hanley, a computer scientist at DePaul University, in Chicago. When Hanley started working in this area more than a decade ago, even moving a device a small amount vertically could throw off the positioning models because the magnetic field changed in hard-to-predict ways. The convention among many researchers was that magnetometers were too sensitive to nearby magnetic interference–indeed, that variations even from the electronics inside devices could create enough noise to overwhelm measurements. The field has matured over the last decade and a half, however, as academic laboratories made a sequence of breakthroughs, followed by companies undertaking internal research.

Google saw the potential, and in 2014 hired a team of magnetic location researchers from Germany’s National Aerospace Center. An influential 2017 paper by another research collaboration clarified how researchers could use Gaussian processes to better understand magnetic fields.

A magnetometer aboard a Hidonix rover can precisely map a building’s magnetic field, which can then be used for navigation via mobile devices.Hidonix

By 2020, the technology was mature enough for startup Hidonix, based in Santa Monica, Calif., to use magnetometry for indoor navigation in places such as museums and schools, alongside other available data such Wi-Fi and dead reckoning derived from accelerometers. The company sends a rover or person throughout the buildings to create a magnetic map beforehand. Even doing that required the maturing of adjacent technology, Skog says. “In order to build your map in the exploration phase, you need quite good dead reckoning using accelerometers and gyroscopes with low drift.”

Standalone Magnetic Indoor Navigation

Now, companies are starting to offer standalone magnetic indoor navigation. Hidonix claims that it is able to offer magnetic geolocation without pre-mapping in outdoor settings, which tend to have fewer objects that interfere with the magnetic field. Indoors, a prerecorded map is still necessary to achieve the precision most users demand.

Another approach involves gathering large amounts of data on how magnetic fields vary and then using neural networks to predict local variations in a magnetic field, even in the absence of a prerecorded map. This is what AstraNav has been doing in Ukraine since even before the 2022 Russian invasion, which then turned that region into one of the most contested electromagnetic environments on the planet. An added benefit of all that training data was that they could calibrate data from a wide array of devices, which is useful in indoor settings, too.

“We are agnostic to the hardware,” says AstraNav’s Toutov, though the company gets better location results if they have some time to study the hardware involved beforehand. AstraNav is also seeking to perform all the necessary calculations on the device itself, to avoid the need to maintain communications in places with jamming or physical obstacles like thick walls.

Magnetic information allows AstraNav to precisely locate a device indoors, where GPS is less useful.AstraNav

If magnetometer-only indoor positioning is really ready for commercialization, the demand will certainly be large. The business promise of indoor mapping is that it straddles civilian settings and conflict zones of all kinds. The market size may already be in the tens of billions, and one market researcher forecasts it will grow to more than $150 billion by 2030. Factories need indoor mapping to drive their robots around; retailers want indoor mapping to follow and communicate with customers at an accuracy below 1 meter, which is enough to detect which aisle a customer is in.

Yet given how messy real-world settings are, companies will have their hands full proving that they can handle dynamic scenarios such as people’s own bodies interfering with the local magnetic field or using an elevator, Skog says. “Coming up with a solution that works in a research paper is one thing, but you need robustness,” Hanley says.