+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
+Planet Posted October 10, 2003 Share Posted October 10, 2003 How long is the Holland Tunnel? As far as I know the GPS takes readings every 3 seconds or so. Maybe it was just holding the lock, can't help you there. What I can say is I was on Route 7 in Vermont, with a measured elevation spot, and my Garmin Legend was off on the elevation by 6 feet. Planet Quote Link to comment
+The Frantic Cachers Posted October 10, 2003 Share Posted October 10, 2003 If it was only a few seconds long to go through..... Quote Link to comment
+Doc-Dean Posted October 10, 2003 Share Posted October 10, 2003 The Holland tunnel is not a short tunnel, however there are frequent vertical access shafts for ventilation and these will also allow radio signals to go through (don't lose your FM signal) so i would assume the same with the GPS signal... --------------------------------------------------- Free your mind and the rest will follow Quote Link to comment
+Team GPSaxophone Posted October 10, 2003 Share Posted October 10, 2003 The Eisenhower tunnel on I-70 in Colorado is over a mile long and at 11,000 feet elevation. I lost satellite lock after the first hundred yards. Took sun from sky, left world in eternal darkness Quote Link to comment
+KBer Posted October 10, 2003 Author Share Posted October 10, 2003 The Holland Tunnel actually has two tubes. The North tube is 8,568 feet long, the south tube is 8,371 feet long, both over 1 1/2 miles long. There was a bit of traffic, so we did not go through the tunnels quickly. I doubt that the GPS signal from so many different satellites could come through the ventilation shafts, and reach the GPSr. Quote Link to comment
+The Leprechauns Posted October 10, 2003 Share Posted October 10, 2003 My guess -- and it is just that, a guess -- is that the GPS navigation systems built into cars are different from our handheld units so that you don't "lose satellite lock." Perhaps the system is programmed to tell the display "we are entering the Holland Tunnel. Show the driver that he is in the Holland Tunnel, until you get a signal again on the other side." As for the bridge deck, would it similarly be possible for a smart GPS unit to say "since the car is traveling westbound, show his position as being on the upper deck, rather than the eastbound lower deck?" But I have absolutely no idea what I'm talking about, other than to note that my Garmin V loses lock every day in Pittsburgh's many tunnels. x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x-x Next time, instead of getting married, I think I'll just find a woman I don't like and buy her a house. Quote Link to comment
+TX Hokie Posted October 10, 2003 Share Posted October 10, 2003 I believe I've read that GPS units built into cars use more than just the satellites for positioning. They also have a connection to the speedometer (not sure if any take into account changes in steering angle, etc) so they can better approximate your position even when you lose signal lock. Not sure what you mean about it telling you which deck of the bridge you were on. Did you determine this just based on the elevation of that deck vs. the elevation the GPS reported? Quote Link to comment
Dan Vull Posted October 10, 2003 Share Posted October 10, 2003 Unless the GPS unit is augmented with a IPS (inertial positioning system) it is strictly line of sight unlike fm. This is important because a gps unit calculates the psuedo-range based on the phase shift of the signal. After determining this distance from 4 or more it can calculate it's position. So line of sight is very important. I would guess that the latency of your unit just did not determine it was time to display the message for loss of lock by the time you traversed the tunnel. Pat Quote Link to comment
+Stunod Posted October 10, 2003 Share Posted October 10, 2003 Garmin's StreetPilot 2650 (an in-dash unit) has the ability to show position is GPS reception is lost. According to their website: quote:The StreetPilot 2650 also has dead reckoning capabilities, so you’ll continue to get navigation guidance even if you lose GPS reception, such as in intense urban environments with tall buildings and tunnels. To achieve dead reckoning, the 2650 features an internal angular rate sensor, an external speed sensor input, and an external reverse light sensor input. Quote Link to comment
BassoonPilot Posted October 10, 2003 Share Posted October 10, 2003 quote:Originally posted by vtpaddler:Unless the GPS unit is augmented with a IPS (inertial positioning system) it is strictly line of sight unlike fm. I always thought FM was line of sight. AM is not. Anyway, the Port Authority reradiates commercial broadcast frequencies in the tunnels. They have antennas for cellphone frequencies, as well. My handheld gpsrs have never worked in the tunnels. Quote Link to comment
Dan Vull Posted October 10, 2003 Share Posted October 10, 2003 Sorry if I was wrong about the FM thing I'm just a surveyor Jim not a doctor. I got to get outside too much computer time. Quote Link to comment
+parkrrrr Posted October 10, 2003 Share Posted October 10, 2003 quote:Originally posted by BassoonPilot:I always thought FM _was_ line of sight. AM is not. Anyway, the Port Authority reradiates commercial broadcast frequencies in the tunnels. They have antennas for cellphone frequencies, as well. My handheld gpsrs have never worked in the tunnels. Reradiating GPS signals could never work. The best they could do would be to simulate satellites inside the tunnel, but by the time you'd downloaded the ephemeris data for the simulated satellites (assuming you can even express "this satellite is at an altitude of 20 feet and never moves" in orbital-mechanics language) you'd be out of the tunnel. Quote Link to comment
BassoonPilot Posted October 10, 2003 Share Posted October 10, 2003 quote:Originally posted by Warm Fuzzies - Fuzzy:Reradiating GPS signals could never work. I know. Even if it were technically possible, I suspect the agencies responsible for our security would find it highly undesirable. If I was unclear, I was speaking about reradiating the commercial broadcast bands in order that people could listen to their radios in their cars. The authorities also make use of the system to broadcast pertinent information. Quote Link to comment
Kerry. Posted October 10, 2003 Share Posted October 10, 2003 Being one of the built-in car systems most also have inertial as part of the system as well. The micro inertial systems are only about 1/2" square but extremely accurate and included in these car systems to handle the obstruction issues in the urban canyons where GPS reception can be obstructed most of the time. Cheers, Kerry. I never get lost everybody keeps telling me where to go Quote Link to comment
+Doc-Dean Posted October 10, 2003 Share Posted October 10, 2003 quote:Originally posted by vtpaddler:Sorry if I was wrong about the FM thing I'm just a surveyor Jim not a doctor. My excuse would be the opposite I suppose... dadgum it Jim, I'm a doctor not a surveyor! --------------------------------------------------- Free your mind and the rest will follow Quote Link to comment
+Alan2 Posted October 10, 2003 Share Posted October 10, 2003 If you add the feature of "road lock" to the inetial stuff above, it would be fairly easy to compute the distance traveled on a particular road. Actually our hand held units do that for smaller distance now without connection to a speedometer. With road lock and a speedometer reading, the car would stay on the road on the screen and make a slight correction on the first lock after geting out of the tunnel. I suppose one way to test this theory is to make a right or left turn. That's not easy to do in a tunnel, but could be simulated on the streets in the canyons of Manhattan where you lose lock frequently. Alan Quote Link to comment
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