GPS reception in tunnels

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Originally Posted by Lotz-A-Landies

OEM In Car nav systems maybe but not after-market ones.

The other problem is that my D4 speedo over-reads by ~ 6KPH so while in the tunnel I won't know whether I was reading GPS +/- 0.5KPH or speedo -6 KPH.

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Actually yes some aftermarket ones do have this feature. My old man has a pioneer flip out screen media/gps system that has a VSS input (Vehicle Speed Sensor). Its over 3 years old! I also would not be suprised if the unit uses GPS to calibrate the VSS input so that it is accurate.


BAE developed a GPS fallback navigation system for military incase GPS satellites get shot out of space or more likely jammed.. it uses known radio frequecy amplitudes or something like that. Mobile phone towers and such. Apparently works very well in built up areas.

Edit- found an article on it: http://atwonline.com/aircraft-engine...ative-gps-0711

[ame="http://en.wikipedia.org/wiki/LORAN"]LORAN - Wikipedia, the free encyclopedia[/ame]

Look up LORAN navigation.
We were using this in Saudi Arabia in 1985.
Still wouldn't work in tunnels though.
Regards Philip A

Fuzz - or Selective Availability was turned off in May 2000 at the direction of Clinton.
Differential GPS was already available to the public by then, and GPS was already well entrenched in civilian systems (particularly aviation).
The US can still deny their satellite constellation to any region of the globe that they wish.

Drilling holes in tunnels so they "leak" GPS signal :) an interesting concept especially for the Sydney Harbour tunnel - oh hang on GPS doesn't work under water anyway :eek::eek::eek:;).


Martyn

LORAN is similar but uses different radio waves and I'm not sure but I don't think it is coded the same. LORAN was fairly poor in accuracy compared to GNSS which is why it has pretty much died out. This is basically gps without the atmospheric and positions errors. I'll search for the article.

actually..

if your gps is good enough you dont need the satelites.

you can put up any number of transmitters (on their dedicated frequency) that transmit as part of their time code their location. This gets done for AGPRS stuff already

The real problem its the distances involved. GPS works better when you have your transmitters randomly located nice and far apart from each other and as far away from you as possible. In a tunnel because of the short distances, the speeds likely to be involved and the relatively symetrical pattern youd wind up having to get the coverage required youd loose the accuracy of the triangulation.

[ame="http://en.wikipedia.org/wiki/GNSS_augmentation"]GNSS augmentation - Wikipedia, the free encyclopedia[/ame]

ELTA : Elt, ground station, military radio transmission, physico-chemical analysis...

Heres some primers for the relative info.

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Originally Posted by tangus89

GPS satelites emit 2 signals on seperate frequencies, the C/A and P waves. the c/a is the wave our handhelds recieve, it repeats its coded signal fairly regularly (I cant remember exactly the time, 12 min comes to mind but may be wrong) and the Yanks put a bit of error into it which is that fuzz.
The P-wave's code is repeated over a long period of time, i think its a number of days from memory, i havent got my notes nearby so cant get actual numbers.
so basically the american military use the c/a code to find out where in the p code they are, then they can use the difference between the two waves to eliminate atmospheric errors quite well as well as use the p code which is error free to get their position. The c/a code is still "fuzzed", there was a brief period of time that they cleared it but its back to its fuzzy ways.
the russians have a satelite system up called Glonass, about as old as GPS, it also has errors put in by the russians. Recently some handheld GNSS units and most surveying GNSS recievers pick up both, giving access to 50 something satelites. making it more likely to get a good fix wherever you are. us surveyors do basically the same thing as the CSIRO test to get accurate positions, fix one receiver over a known location, and use another portable one to do the survey. they are connected by radio link so the position difference can be calculated on the fly by the machine (RTK - real time kinematic) or post processed without a radio link, but it takes time back in the office.

I did a job last week which we did a bit of testing for ****s and giggles with the accuracy difference between our sub centimeter GNSS sytem ($50000) and a handheld ($500) some points were as much as 11 meters different, even though the hand held was saying its accuracy was 5m. We also occupied the same point a number of times and got up to 13m difference just with the hand held. if the fuzz was switched off again handhelds theoretically could have sub meter accuracy.

will have to dig my notes out and freshen up on it, something i am supposed to know.

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The ground based system you saw in Position would be Locata.

12.5 minutes is the length of time it takes to download the entire navigation message (almanac, etc). The C/A and P/Y codes repeat much faster.

The C/A code is not "fuzzed", the signal is at a tenth of the frequency than the P/Y code, so is inherently less precise.
Selective Availability isn't even an option on satellites launched after '09 (Block IIF). Civilian users can get mm precision by using the phase of the L1 and L2 carrier frequencies - which is what quality (surveying) GNSS receivers do.

What is more useful for vehicle applications is the L2C signal (Block IIR-M on), which is intended to improve signal acquisition and reliability, so helpful in cities where multipathing and poor sky view are issues.

Still doesn't help with inside tunnels - there isn't a standard augmentation system, and any augmentation costs extra to implement, so won't ever be a standard feature of receivers.

:)

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Originally Posted by Lotz-A-Landies

But if you read further, what it is actually doing is re-transmitting the GPS data for the location of the initial receiver on the roof which will remain static even though your car is moving within the tunnel. So the reality is that speed measurement will likely fall to zero while you are in the tunnel and then jump as your receiver starts having direct line-of-sight to the actual satellites.

What it will do is reduce the time to acquisition when exiting the tunnel and why it is important for Police, ambulance and fire authorities.

Remember that a GPS requires a minimum of three satellites to calculate position in three dimensional space.

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That was the first example of the technology I was talking about. Google upon the idea and you'll find other companies and other products.

for example....

GPS Source - Commercial - Railways GPS Retransmission, Re-radiator, GPS Repeating, GPS Retran, L1 Antennas, L1/L2 Antennas, GPS Antenas, GPS Amplifiers, GPS Indoors, GPS Power Dividers, GPS Antenna Dividers, DAGR Antenna, Power Splitters, Signal Cond

It's pretty obvious that you can't get GPS signal underground as it requires direct line of sight to the minimum three satellites.

With the first product I linked to it would be pretty daft to repeat a single point to the length of a tunnel. The obvious solution would be to have multiple receivers along the length of the tunnel, or strategically positioned before and at off ramps etc. It wouldn't be real continuous gps coverage but would give warning for turns and a granular indication of position.

cheers
Paul

Simple solution. Don't use tunnels.

I decided years ago to not use toll roads, bridges, or tunnels as an insignificant personal protest against over-taxation of motorists. The only exception I make is the Sydney Harbour Bridge when i am down there. it being a bit of a drag to go over via Gladesville.

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Originally Posted by Brian Hjelm

Simple solution. Don't use tunnels.

I decided years ago to not use toll roads, bridges, or tunnels as an insignificant personal protest against over-taxation of motorists. The only exception I make is the Sydney Harbour Bridge when i am down there. it being a bit of a drag to go over via Gladesville.

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So why not use the M5 East Tunnel in Sydney?

Its the only place in Sydney where you can get free all-day undercover parking! :twisted:

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Originally Posted by Lotz-A-Landies

So why not use the M5 East Tunnel in Sydney?

Its the only place in Sydney where you can get free all-day undercover parking! :twisted:

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Oh, Diana, do I detect just a little sarcasm there. Strike a little traffic getting between POW & home?:D

How is life in Mohammed Alley? Are you still the only lady in the neighbourhood who doesn't wear a bag over her head?:angel: