The answer to this question is quite simple: the dense layers of concrete separating the tunnel from the outside block the radio signal from the satellites. GPS devices also use up to four satellites at a time to provide accurate latitude, longitude and altitude information, and without a direct line between the device and the sky, GPS will not work properly.
But the most interesting thing is not the reason for this signal problem, but how Google is implementing a solution and how cities in various parts of the world, including Madrid, are applying this method.
Since it is not possible to get the signal inside a tunnel, Google announced its own solution in January 2024: installing Bluetooth beacons inside tunnels, which can transmit location information to navigators so as not to depend on satellites in these covered or underground sections. These beacons or microcontrollers are low-energy devices (hence the technology is also known as Bluetooth Low Energy or BLE), which operate on batteries and are attached to the internal walls or ceiling of the tunnel.
This is the technology that Madrid City Council will deploy in the tunnels of the M-30 in Madrid so that getting lost on these roads due to lack of directions becomes a thing of the past, as we reported last week.