fubie

OK, so this is not specifically related to our cars, but I figured you guys would know the answer to something that's really puzzling me. Recently going through West Virginia we drove through a tunnel at least a mile or so long on. The NAV represented the tunnel as a dashed line on I77 and knew our position in the tunnel, so it kept up with our location. You can hit a GPS satellite from a mile inside a mountain? Nothing else needing radio waves works... Any clues?

ser67

This is a simple one, the nav unit has a telemetry feed of car speed and direction, so when it loses the GPS it still knows where it is going, but after a period of time will eventually be slightly out.

From what I understand the Toyota/Lexus system is especially good at that, causing raised eyebrows with military folks over its use in civilian vehicles.

From a historical perspective, the technology has been around since the Apollo days, when even the rudimentary electronics back then in the space capsule could correctly identify and track their 3D position in space - they used a telescope and a star atlas as their 'GPS' lining up every now and then to confirm their exact position. And if you think back to mariners using sextants and maps its all the same really.

VVTiBob

Data, timing information, from satellite signals is only one of the sources of information used to show your position on the GPS display. The other source are: Data on the map DVD, signals from the car's built in gyro sensor, and data coming from the vehicle speed sensor, i.e. wheel revolutions. All of these data sources are gathered and used to display the information you see on the screen. So when signals from the satellites are lost, such as inside a tunnel, or inside underground parking garage, your car calculates or estimates its position from the speed sensor, wheel turns or revolutions and the data which already exist on the DVD.

In addition the car also gathers information from the Earth's magnetic field so it senses when turns are made, interprets it and displays the information on your GPS screen. The same thing happens if you were to go into a large office building with a few floors of underground parking. The nav system's ECU or brain will approximate your X and Y coordinates based on that information but will have problems with the Z coordinate as it's no longer gathering data on elevation from the satellites.

fubie

Thanks guys. I had figured some feed from the speed sensor, but thanks for the level of detail! Wow, rather than slapping a GPS nav on to say they had one, they really did it right.