+KBer Posted October 10, 2003 Share Posted October 10, 2003 I was in the Holland Tunnel (under the Hudson River, connects NJ with NYC) driving my brothers new car. He had a built in GPSr route direction receiver. I expected the screen to go blank and lose reception. But to my surprise, not only did the GPSr keep working it accurately portrayed my travel through the tunnel. How is this possible? Does anyone know if some sort of GPS transmitter is installed in tunnels? Is this possible? Or is the GPSr simply interpolating the data. A similar question is when I was on the George Washington Bridge, the built in GPSr, knew I was on the upper level of the bridge, instead of the lower level. I always thought that elevation detection in GPSr was not that great. So again I ask are there special transmitters or can the GPSr interpolate the data. Anyone that can offer some insight, it would certainly be appreciated. Quote Link to comment
+MissJenn Posted October 13, 2003 Share Posted October 13, 2003 quote:Originally posted by Kber:I expected the screen to go blank and lose reception. But to my surprise, not only did the GPSr keep working it accurately portrayed my travel through the tunnel. How is this possible? Does anyone know if some sort of GPS transmitter is installed in tunnels? My guess is this: Since this is an automobile application, and since tunnels are a common enough part of US roadways, the programmers have made the display "predict" the upcoming data for the tunnel. It's easy to plug in stuff like length of this tunnel x speed of this vehicle = how long we should fake this display In other words, there was no actual, live GPS reception while in the tunnel, but the display just continued. -- I am looking for cache recommendations for Salem, Mass. Got any? Quote Link to comment
+briansnat Posted October 13, 2003 Share Posted October 13, 2003 Wouldn't it still show that he doesn't have the sat lock? Maybe it has something to do with how we can get AM reception in some tunnels. I think there is some sort of re-transmitter. Perhaps the sat signals come in the same way? I have no clue, it's just a wild guess. "You can't make a man by standing a sheep on his hind legs. But by standing a flock of sheep in that position, you can make a crowd of men" - Max Beerbohm Quote Link to comment
+Crossbar Posted October 13, 2003 Share Posted October 13, 2003 Newer gps systems built into cars have dead reckoning systems. They can tell from sensors in the wheels how fast and what direction you are traveling in. They still need an inital gps lock to figure out where they started from though. Quote Link to comment
+ski Posted October 14, 2003 Share Posted October 14, 2003 BrianSnat is right. Some tunnels have re-transmitters so that firemen,police, and rescue squads can use their radios and cell phone distress calls from motorists can get out to #911. The only way I know this is because Atlantic City delayed the opening of the new connector tunnel when the re-transmitting system wasn't working properly. Quote Link to comment
ucmike Posted October 15, 2003 Share Posted October 15, 2003 in the holland you can get radio through the airshafts, thats where your radio suddenly gets stronger about 1/4 into the tunnel on either side. probably not enough for gps though Quote Link to comment
+lucien Posted October 16, 2003 Share Posted October 16, 2003 I've driven through the holland tunnel many times with my Garmin GPS III+ - it definitely _does_ lose the view of the sattlites almost immediately upon entering the tunnel. Something else must be going on if your GPS continues to show the position - perhaps predicting it based on the last good data. Quote Link to comment
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