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Cellular forums Home > Archive > Garmin GPS > February 2008 > Nuvi and dead-reckoning in tunnels
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Nuvi and dead-reckoning in tunnels
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| Per Weisteen 2008-02-14, 10:33 pm |
| I've had my Nuvi 660 for about a year and travel daily through several
tunnels. The longest one is about 2 miles (3 km) long and the Nuvi have
had no problems showing me my position within the tunnel based on what
must be dead-reckoning using my last known speed or some kind of
average. The manual doesn't say.
Two weeks ago I upgraded to version 2.40 and GPS SW version 2.90. After
that the dead-reckoning performs very erratic. Sometimes I can drive
through 2 miles of the tunnel without any problems, on other occations i
just get halfway when the satellite reception lost warning appears.
Anyone with similar experience or inside info on this ?
Regards,
Per W.
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| sierra 2008-02-15, 4:34 am |
| Can't comment on nuvi, but last week was driving with Streetpilot 2610
through the Madrid inner ring road (M30) tunnel which is about 10km
long. Lost signal soon after entrance, but then near halfway point it
picked up a signal and tracked the tunnel for 1km or so, and then lost
it again. I checked the tracking log, and the signal was picked up in
a section where the tunnel passes under a park - there are no obvious
openings in the roof of the tunnel. The tunnel has mobile phone and
radio station reception throughout, so perhaps this was causing it.
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| Floyd 2008-02-15, 10:34 am |
| The tunnel has mobile phone and radio station reception throughout, so
perhaps this was causing it.
The tunnel must have repeater antennas spaced throuout its length to provide
the cell and radio reception. The repeaters may not carry the frequency
required by the GPS units. As to why a certain section does work, it may be
possible that they have replaced the repeaters in that section with newer
ones that do include the GPS frequencies.
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| Heinrich Pfeifer 2008-02-15, 12:34 pm |
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"Floyd" <Flooydomit@prodigy.net> wrote in
news:bOhtj.597$pl4.596@newssvr22.news.prodigy.net...
> The tunnel has mobile phone and radio station reception throughout, so
> perhaps this was causing it.
> The tunnel must have repeater antennas spaced throuout its length to
> provide the cell and radio reception. The repeaters may not carry the
> frequency required by the GPS units. As to why a certain section does
> work, it may be possible that they have replaced the repeaters in that
> section with newer ones that do include the GPS frequencies.
no, GPS signals cannot pass a repeater like radio signals or mobile phone
signals.
If there was a repeater including GPS frequencies, all satellite signals
would pass the repeater system using the same path, so all sat signals would
get the same delay between the outer antenna and the receiver. This would
look like a common clock error to the receiver. At the end, the recevier
would find the position of the outer antenna, not the position of the car.
GPS requires all sat signals coming through the *direct* path.
In this Madrid tunnel, some sat signals will have passed the ceiling of the
tunnel under the park directly.
--
Heinrich
http://www.gartrip.de
mail: new<at>gartrip.de
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| On Feb 15, 8:25 am, "Heinrich Pfeifer" <n...@gartrip.de> wrote:
> "Floyd" <Flooydo...@prodigy.net> wrote innews:bOhtj.597$pl4.596@newssvr22.news.prodigy.net...
>
>
> no, GPS signals cannot pass a repeater like radio signals or mobile phone
> signals.
>
> If there was a repeater including GPS frequencies, all satellite signals
> would pass the repeater system using the same path, so all sat signals would
> get the same delay between the outer antenna and the receiver. This would
> look like a common clock error to the receiver. At the end, the recevier
> would find the position of the outer antenna, not the position of the car.
True, but it does suggest a possible implementation for the tunnel
authorities if they did want to provide approximate GPS positioning
within the tunnel. Divide the tunnel into sections and in each
section place a retransmitter which is getting GPS signals from an
antenna located outside and directly above the center of that section
of the tunnel. GPS receivers in the cars would then see the position
jump as they passed from one section to the next, but would always
show the car as being in the correct section, albeit at the center of
it rather than at the actual accurate spot. Still better than having
lost position information altogether.
No idea if any tunnels have already implemented such a system.
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| sierra 2008-02-16, 7:33 am |
| Obviously navigation not a problem in most tunnels with one entrance &
exit, but the Madrid tunnel has numerous entrances, exits and
alternative routes, so would be worthwhile.
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| sierra <cfhughes@hotmail.com> wrote:
> Can't comment on nuvi, but last week was driving with Streetpilot 2610
> through the Madrid inner ring road (M30) tunnel which is about 10km
> long. Lost signal soon after entrance, but then near halfway point it
> picked up a signal and tracked the tunnel for 1km or so, and then lost
> it again. I checked the tracking log, and the signal was picked up in
> a section where the tunnel passes under a park - there are no obvious
> openings in the roof of the tunnel. The tunnel has mobile phone and
> radio station reception throughout, so perhaps this was causing it.
I dunno about his/her Nuvi can handle miles on tunnel, but all of my
Garmins including the Nuvi 680 will lose signal within ## feet after
entrance.
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| "Floyd" <Flooydomit@prodigy.net> wrote:
> The tunnel has mobile phone and radio station reception throughout, so
> perhaps this was causing it.
> The tunnel must have repeater antennas spaced throuout its length to provide
> the cell and radio reception. The repeaters may not carry the frequency
> required by the GPS units. As to why a certain section does work, it may be
> possible that they have replaced the repeaters in that section with newer
> ones that do include the GPS frequencies.
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
often <bg>
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| Simon Slavin 2008-02-18, 10:33 pm |
| On 15/02/2008, peter wrote in message <54525fbf-1af6-4a33-9630-
fc67d940258d@d21g200
0prf.googlegroups.com>:
> Divide the tunnel into sections and in each
> section place a retransmitter which is getting GPS signals from an
> antenna located outside and directly above the center of that section
> of the tunnel.
Can't be done. Sorry, but GPS receivers depend on extremely precise
timing differences to work out where they are relative to the satellites
providing the signals. The timing differences are used to work out the
distances to each satellite, and thereby the receiver's position on the
earth. It would be impossible to fake any useful positioning information.
Simon.
--
http://www.hearsay.demon.co.uk
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| Edwin Pawlowski 2008-02-18, 10:33 pm |
|
"Simon Slavin" <slavins.delete.these.four.words@hearsay.demon.co.uk> wrote
in message
> . It would be impossible to fake any useful positioning information.
>
It may be impractical, but impossible? Not if you have enough money.
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| peter 2008-02-18, 10:33 pm |
| On Feb 18, 2:28 pm, Simon Slavin
<slavins.delete.these.four.wo...@hearsay.demon.co.uk> wrote:
> On 15/02/2008, peter wrote in message <54525fbf-1af6-4a33-9630-
>
> fc67d9402...@d21g2000prf.googlegroups.com>:
>
>
> Can't be done. Sorry, but GPS receivers depend on extremely precise
> timing differences to work out where they are relative to the satellites
> providing the signals. The timing differences are used to work out the
> distances to each satellite, and thereby the receiver's position on the
> earth. It would be impossible to fake any useful positioning information.
Nothing needs to be "faked." The antennas located directly above
various points of the tunnel would be receiving signals from the GPS
satellites directly, so the timing relationships would be correct for
the exact location of each of these antennas. The received signals
are then carried on coax cables into the interior of the tunnel and re-
broadcast to GPS receivers in cars going through that tunnel section.
Since the signals from the various satellites are all carried on the
same cable from a given antenna, the timing relationships remain
intact. Therefore the receiver inside the car gets the same signals
that it would be getting if it were located at the remote antenna
position with exactly the same timing differences; and so it will
calculate its current position as being that of the antenna. No
special manipulation of the signals is needed or desirable, just
relatively straight-forward placement of a series of antennas above
the tunnel and corresponding placement of rebroadcast transmitters at
a series of locations within the tunnel.
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| larry@fishing.net 2008-02-19, 4:33 am |
| On Mon, 18 Feb 2008 15:27:04 -0800 (PST), peter <prathman@comcast.net>
wrote:
>Nothing needs to be "faked." The antennas located directly above
>various points of the tunnel would be receiving signals from the GPS
>satellites directly, so the timing relationships would be correct for
>the exact location of each of these antennas. The received signals
>are then carried on coax cables into the interior of the tunnel and re-
>broadcast to GPS receivers in cars going through that tunnel section.
>Since the signals from the various satellites are all carried on the
>same cable from a given antenna, the timing relationships remain
>intact. Therefore the receiver inside the car gets the same signals
>that it would be getting if it were located at the remote antenna
>position with exactly the same timing differences; and so it will
>calculate its current position as being that of the antenna. No
>special manipulation of the signals is needed or desirable, just
>relatively straight-forward placement of a series of antennas above
>the tunnel and corresponding placement of rebroadcast transmitters at
>a series of locations within the tunnel.
I don't think that is correct. I am no expert but I do know the
signal takes some amount of time to travel through the wire. It also
takes some time to be rebroadcast.
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| JBDragon 2008-02-19, 4:33 am |
| "peter" <prathman@comcast.net> wrote in message
news:db42a27a-b17a-4d95-acd1- f16274be7748@s8g2000
prg.googlegroups.com...
> On Feb 18, 2:28 pm, Simon Slavin
> <slavins.delete.these.four.wo...@hearsay.demon.co.uk> wrote:
>
> Nothing needs to be "faked." The antennas located directly above
> various points of the tunnel would be receiving signals from the GPS
> satellites directly, so the timing relationships would be correct for
> the exact location of each of these antennas. The received signals
> are then carried on coax cables into the interior of the tunnel and re-
> broadcast to GPS receivers in cars going through that tunnel section.
> Since the signals from the various satellites are all carried on the
> same cable from a given antenna, the timing relationships remain
> intact. Therefore the receiver inside the car gets the same signals
> that it would be getting if it were located at the remote antenna
> position with exactly the same timing differences; and so it will
> calculate its current position as being that of the antenna. No
> special manipulation of the signals is needed or desirable, just
> relatively straight-forward placement of a series of antennas above
> the tunnel and corresponding placement of rebroadcast transmitters at
> a series of locations within the tunnel.
Seems like a complete waste of time and money for really no benefit at all.
You go in one end and out the other. Your not going to get lost. So what if
you loose the signal going in, you'll pick it right back up when you get
out. Personally, the only thing I would really care about in a long tunnel
is Cell service still working so your call isn't interrupted when entering,
but most importantly so you can call in a emergency. Also Satellite Radio.
Both can and have been done. If you really care about that signal loss,
instead buy a GPS with Dead Reckoning built it. Problem solved.
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| On Feb 18, 10:28 pm, la...@fishing.net wrote:
> On Mon, 18 Feb 2008 15:27:04 -0800 (PST), peter <prath...@comcast.net>
> wrote:
>
>
>
>
> I don't think that is correct. I am no expert but I do know the
> signal takes some amount of time to travel through the wire. It also
> takes some time to be rebroadcast.
Yes, it takes some appreciable travel time for the signal through the
cable, but that's not really an issue. The signal as received by the
GPS receiver is still exactly the same and with all the same time
relationships as it had when received by the outside antenna.
Therefore the receiver will do the same calculations as if it were in
the antenna position and will come up with that location as the
solution.
The constant time delay experienced by all the satellite signals
doesn't change that at all. Imagine you could exactly record the sat.
signals received today at a particular spot. Now take your receiver
to some other place where it can't receive any of the satellite
signals and use a transmitter to play back the previously recorded
signals. It might be a day (or a week, or a year) later and the
receiver would still determine the position to be the place where you
originally recorded the signals - and it would also show the time/date
at which you did the recording since those are also determined from
the received satellite signals.
Now if signals from some of the satellites were delayed more or less
than signals from other satellites, that would mess up the position
calculation. But a constant delay that affects all the signals
equally will not.
| |
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| On Feb 18, 10:30 pm, "JBDragon" <JBDragon at someplace dot com> wrote:
> "peter" <prath...@comcast.net> wrote in message
>
> news:db42a27a-b17a-4d95-acd1- f16274be7748@s8g2000
prg.googlegroups.com...
>
>
>
>
>
>
>
>
> Seems like a complete waste of time and money for really no benefit at all.
> You go in one end and out the other. Your not going to get lost.
Certainly true for most tunnels. But 'sierra' already pointed out
that it could be useful in the Madrid tunnel where there are multiple
exits and drivers in the tunnel need to make navigation decisions
while inside. I would expect that implementing the method I described
would only be considered for tunnels of that type where there would be
a benefit.
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| Heinrich Pfeifer 2008-02-19, 3:33 pm |
|
"peter" <prathman@comcast.net> wrote in
news:c2459b25-fed0-4081-8390- 9e2ca52b683f@e23g200
0prf.googlegroups.com...
> Yes, it takes some appreciable travel time for the signal through the
> cable, but that's not really an issue. The signal as received by the
> GPS receiver is still exactly the same and with all the same time
> relationships as it had when received by the outside antenna.
> Therefore the receiver will do the same calculations as if it were in
> the antenna position and will come up with that location as the
> solution.
>
> The constant time delay experienced by all the satellite signals
> doesn't change that at all.
your proposal is very interesting, and it would work, at least in theory. My
only objection ist that some GPS receivers could be confused by the
correction of the internal time base jumping instantly to another value.
--
Heinrich
http://www.gartrip.de
mail: new<at>gartrip.de
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| Simon Slavin 2008-02-21, 10:33 pm |
| On 18/02/2008, peter wrote in message <db42a27a-b17a-4d95-acd1-
f16274be7748@s8g2000
prg.googlegroups.com>:
> <slavins.delete.these.four.wo...@hearsay.demon.co.uk> wrote:
>
> Nothing needs to be "faked." The antennas located directly above
> various points of the tunnel would be receiving signals from the GPS
> satellites directly, so the timing relationships would be correct for
> the exact location of each of these antennas. The received signals
> are then carried on coax cables into the interior of the tunnel and re-
> broadcast to GPS receivers in cars going through that tunnel section.
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
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| Edwin Pawlowski 2008-02-21, 10:33 pm |
|
"Simon Slavin" <slavins.delete.these.four.words@hearsay.demon.co.uk> wrote
in message
> 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.
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.
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| peter 2008-02-21, 10:33 pm |
| On Feb 21, 2:58 pm, Simon Slavin
<slavins.delete.these.four.wo...@hearsay.demon.co.uk> wrote:
> On 18/02/2008, peter wrote in message <db42a27a-b17a-4d95-acd1-
>
> f16274be7...@s8g2000prg.googlegroups.com>:
>
>
>
>
>
>
>
>
> Where are you going to put your emitter ?
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.
> 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.
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.
> And GPS chipsets only get a fix on one satellite in an instant.
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.
> 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.
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.
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| Dan Anderson 2008-02-22, 12:34 pm |
| peter wrote:
> 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.
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)
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