How your car’s GPS can still work (sort of) in a tunnel

Wilker’s question got me wondering about cellphone and radio reception in tunnels, too. But we’ll start with GPS, short for global positioning system.

Technically speaking - sorry, but with this topic, that’s unavoidable - your car’s GPS does not work in tunnels, said Jake Jacobson, social media manager for Garmin International.

“Your GPS does have to have a clear shot of the sky. Without having that, the conversation is disrupted,’’ he said.

Nor are Big Dig tunnels outfitted with GPS equipment to beam satellite feeds to your car, said Adam Hurtubise, spokesman for the state Department of Transportation.

So why did Wilker’s GPS keep working? The answer is his car’s unit was simply displaying an educated guess about his route while he was in the tunnel, based upon his speed and location at the moment it lost the satellite. Such calculations are generically known as “dead reckoning’’ and are commonly employed throughout the GPS industry.

Garmin uses its own proprietary algorithm to keep its GPS systems running underground, Jacobson said, but in principle it’s the same as dead reckoning.

“It just does a little bit of math in its head and is able to calculate where you’ll be during the loss of the signal,’’ he said. “It assumes you’re going in the same speed and direction toward your destination. But if you were, for some weird action-movie reason, to do a U-turn and come out in the same direction, it would have to recalculate your route.’’

With modern GPS systems, satellite feeds are almost instantaneously restored once you are out of the tunnel, so such recalculations would take just a few seconds, he said.

Any onboard system that relies on satellite signals, from OnStar to SiriusXM Satellite Radio, also will cut out while you are in a tunnel, Hurtubise said. But again, some systems, such as OnStar, have sophisticated backup measures.

“We read up to 1,600 sensors in the car, and with that have more data to pull from when determining the vehicle location,’’ said Vijay Iyer, an OnStar spokesman.