Nuvi and dead-reckoning in tunnels

On 18/02/2008, peter wrote in message <db42a27a-b17a-4d95-acd1-


Where are you going to put your emitter ? If the emitter is in the tunnel
ceiling your vehicle will be travelling at right angles to the emitter.
So whatever position you fake, it can be plausible only for an instant.
And GPS chipsets only get a fix on one satellite in an instant. By the
time it has looked for a signal from a second satellite it'll be out of
site of the emitter.

You don't know how fast the vehicle is moving. You don't know which
satellite the GPS unit is sampling at any time. Trying to provide a
useful signal is not going to work.

Simon.
--
http://www.hearsay.demon.co.uk

in message

So YOU say. Just because you don't know how to achieve it, that does not
mean it cannot be done. I'm certain there are ways of faking out a GPS if
you are a good enough electronics engineer. Just look at the technological
goings on around you, like a man on the moon, shooting down an errant
satellites, underwater navigation, and artificially ripening bananas.

Give our scientist enough time and money and it will be working.

On Feb 21, 2:58 pm, Simon Slavin

There would be a series of transmitters at various positions along the
length of the tunnel. Each would be getting the signals from an
antenna with a clear skyview located directly above that particular
section of the tunnel.


No position is being "faked" - the GPS receiver in the car is getting
the regular GPS satellite signals as originally received by the
outside antenna. It therefore calculates the current position as
being the position of that antenna. And yes, that's only exactly
correct at the moment the car is directly under the antenna, but it
wouldn't be off by much. Once the car moves forward enough it'll be
closer to the transmitter for the next tunnel section and will show a
jump to the location of that antenna. So instead of the normal smooth
motion, it'll show a series of short jumps but will always display a
position that's pretty close to the correct one.


All the GPS chipsets that I've seen made in the last 10 years or so
have multiple parallel receivers for listening to at least 12
satellites at once.


No need to know any of that. And it's very easy to provide a useful
signal since it's just the same as the signal provided by the
satellites directly. All this concept would be doing is taking that
set of satellite signals as received by an external antenna above a
particular point in the tunnel and rebroadcasting it unaltered in the
tunnel.

I agree with Heinrich's comment that the GPS receiver would see a
momentary glitch in the timing when it enters the tunnel and starts
receiving the first rebroadcast signals which would be slightly
delayed. But most receivers have some limited 'dead-reckoning'
capability to deal with brief signal interruptions (Garmins usually
wait 30 seconds before declaring the signal lost and assume that
motion is continuing during this time). I think the receiver would
recover from this glitch during the time when it's still dead-
reckoning and the transition would therefore be transparent to the
user. But clearly that would need to be tested with the commonly used
chipsets for verification. The cables from the various antenna
locations could be matched to provide a constant time delay time so
there wouldn't be any such discontinuities when moving between tunnel
sections.

peter wrote:

I doubt the time delay would be much of a problem but
somewhat dependent on the firmware.

I think the biggest problem would come from the receiver
picking up signals from multiple transmitters especially
when you are in between two of them.

--
Dan
(Email: dan at domain below )
(www.gpsmap.net)

        Matter fact, most GPSes can't even get good signal inside the house, so
10-20 miles tunnel with thick steel, concrete etc. is something I don't hear