Self-driving cars are like snowbird retirees. Given the freedom to live, or operate, anywhere in the country, they turn their backs on wintry states and flock to the sun. There’s a reason Waymo, Uber, and even grocery store giant, Kroger, are testing their shiny new autonomous vehicles in southwestern cities like Phoenix. Yes, Arizona’s regulations are friendly to them, but the year-round good weather is the major draw. As sophisticated as these machines are becoming, they still struggle when fog reduces visibility, or when snow covers lane markings.

But eventually, AVs will have to learn to navigate the wintry mix. That's where WaveSense, a Boston-based startup, sees an opportunity. The company, launching formally today, wants to take technology developed at MIT for the military, and use it to give self-driving cars an extra sense. It says equipping vehicles with a radar looking down, penetrating the ground, will give them a new way to map exactly where they are in the world without relying on visual clues or GPS.

Autonomous vehicles use a suite of sensors to monitor the world around them. Most of their creators have settled on a combination of lidar laser sensors, radar, cameras, short-range ultrasonic sensors, and GPS, combined with on-board maps. But the final component mix that will turn cars into fully autonomous vehicles is far from finished. (Waymo, for example, added microphones too, to hear sirens, and Tesla says it can do it all with cameras, no lidar.)

For a robo-car to be able to navigate a city, it needs to know where it is, and where it should be, within an inch or so. Before carmakers let their creations loose, they usually send a mapping vehicle around the environment they’re going to drive in, to build up a detailed lidar picture of the signs, the access roads, the bike lanes, and the awkward junctions. Then, when an autonomous vehicle is cruising, it can use that map as a reference, even when GPS satellite navigation signals are blocked by tunnels or tall buildings.

But when a heavy snowstorm obliterates all landmarks and blankets the world in a homogenous layer of white, those reference maps are useless. The same is true when lane markings fade, or get scrubbed off by snowploughs. (Some autonomous engineers say the most cost effective way legislators could help the tech is by spending money on lines of fresh paint.)

“Our solution is to look below the ground, where you don’t have the same issues that optical sensors on the surface have,” says Tarik Bolat, CEO of WaveSense.

His company uses an extra radar sensor on cars, usually mounted just behind the front wheel, pointed down. Even a low-power signal can penetrate up to 10 feet into the road.

“It’s seeing rocks, tree roots, soil density changes, any road infrastructure,” says Byron Stanley, CTO and co-founder.

Those features can be combined as a geographical fingerprint, and just like a lidar map of the world above, give a car a reference to help figure out where it is. Wavesense says it can pinpoint a location to within an inch, even at highway speeds, and that it is weather-independent.

The tech comes out of MIT’s Lincoln Laboratory, a defense R&D center, and was first deployed in 2013 to help troops navigate in Afghanistan, where staying on path and avoiding landmines is a matter of life and death.

But adding sensors to autonomous cars comes at a cost, both literally and computationally. WaveSense says its radar is cheap and doesn’t need too much power to crunch the data, which is crucial when manufacturers are trying to shrink the trunk-sized supercomputers that gobble electricity into something that’s commercially viable.

WaveSense’s radar might not replace the other sensors on a car; its role is more likely as a supplementary verification of location, using an unrelated data stream, which improves redundancy and safety. And it might come with an added bonus. Right now ground-penetrating radar is used by geologists and archeologists to discover underground mineral deposits or the remains of ancient buildings.

“I was driving in Rome a couple of weeks ago, which in addition to having no lane markings and crazy traffic, you could imagine you could find some pretty interesting ruins if you drove WaveSense around there,” says Tarik, jokingly. Ancient ruins, that could help the very latest in automotive technology stay on track.