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Name: Ryan S.
Status: student
Age: 15
Location: N/A 
Country: N/A
Date: 11/27/2004


Question:
What would you need to do in order to "wire" a tunnel for radio waves, such as those of a GPS satellite, to enter the tunnel?


Replies:
Ryan,

That is a very tough problem.

The Global Positioning System network of satellites have accuracy to such a tight tolerance because they really only work when THE END USER (RECEIVER) IS IN DIRECT L.O.S. (line of sight) CONTACT with NO LESS than 4 satellites. Three are used for the triangulation and the 4th I believe is used as a highly accurate time base. (The previous sentence may be a bit of a guess, so you'd have to do a search on the web for the mechanics of GPS triangulation.)

On to your question:

This would not be impossible. But it would be rather difficult. Within this tunnel, you would need at least 4-5 signal repeaters (and this is putting it very simply). While radio repeaters have been around for quite some time now, it would be quite easy to install repeaters that would send the signals that are ABOVE GROUND OR ABOVE THE TUNNEL (from the GPS network of satellites) INTO THE TUNNEL. However, GREAT CARE MUST BE TAKEN to make sure that the SIGNAL LATENCY (this is the time DELAY that the signal experiences while getting repeated into the tunnel from above) is compensated for EXACTLY. If this is not done, than the GPS inside the tunnel may work but give very very INACCURATE readings (LATITUDES / LONGITUDES...a.k.a. LAT'S / LONGS').

While people do not care much about the signal latency's of repeaters in normal use (on top of mountains / buildings etc...), the signal latency of the "GPS repeater system" WOULD BE DIRECTLY PROPORTIONAL TO THE ACCURACY OF THE READING.

That is not all:

You not only have to make sure that all incoming satellite signals are delayed to the exact same amount, you need to somehow get this information to receiver so IT TOO can compensate for the fact that their is a SIGNAL RELAY LATENCY (DELAY). Please keep in mind that whole idea of behind GPS is totally dependent on measuring the very small signal delay DIFFERENCES, ~ 10 nano seconds. This is why it is important that all delays from each satellite ONLY measure the delay associated with the FLIGHT TIME of the signal and NOT any sort of REPEATER DELAY.

I like your idea though. I just wonder if anyone will want to spend the money on such a system that would only give.

Side note: The 28 or so GPS satellites that are in the active GPS "constellation" are at about 12,500 miles altitude ( ~ 67 milliseconds ONE WAY SIGNAL TRAVEL TIME ). But it is not this that you care about ... you must make sure that each satellites signals delay is compensated for exactly (down to the TENSs OR HUNDREDs of PICO seconds ( 0.000,000,000,050 seconds). This is the only way to make sure that the receiver will still be able to have 1-2 feet of accuracy in its readings.

If your "tunnel" is exceptionally long, another solution (that is much cheaper) may be implemented without the need for such a complex repeater array as described above. Survey points along the tunnel (but above ground). Inside the tunnel, install short range RF "signal buoys" every 1/8 or 1/4 mile that can only transmit for 600-700 feet (so as to not interfere with the other "buoys". Each of these signal buoys would act as LAT / LONG "mile markers". Perhaps in future GPS receivers, when there is a loss of signal, the receiver can attempt to look for these signal buoys and still feed the user with data provided by these "21rst century mile markers".

Go to the below web site (you must Java installed on your PC). This site shows most of the satellites in orbit around the earth. Anywhere from LEO (low Earth orbit), MEO (medium Earth orbit), HEO (high Earth, like geo-synchronous) to view these real time satellite

http://science.nasa.gov/RealTime/JTrack/3D/JTrack3D.html

I hope this has helped some.

Regards,
Darin Wagner


Ryan-

I really like this question. I've thought about it some.

A typical short tunnel can be wired for AM and perhaps FM broadcast bands by having an antenna outside one mouth of the tunnel, feeding a broad-band amplifier, which in turn feeds a leaky transmission line running into the tunnel.

A leaky transmission line is not hard to do. Often it is just a single wire running near to the concrete of the ceiling. In this case the ceiling is effectively the "other conductor" of an electric circuit or of a transmission-line, but it is a fairly poor conductor so this kind of line works poorly. It is simple to plan and cheap to install, and I suppose that engineers building these tunnels usually feel they have more important things to be concerned with. A better transmission line would be a 6-inch wide stripe of aluminum flashing glued to the ceiling, with a "fat" conductor suspended 1/4 to 1 inch below it.

The fat conductor could be a 1-2 inch stripe of aluminum tape, or a 1/2 inch refrigeration tube of aluminum or copper. Copper tarnishes, so aluminum might be of lower loss in the long run. Aluminum tape on a Polystyrene foam spacer-strip could be very convenient to install.

A "broad-band amplifier" tries to amplify all the channels at once, a bit hard to do well. So they use amplifiers as narrow as they can get away with. Probably they would use one amplifier that covers only the AM band (0.5-1.6MHz), and another that covers only the FM (88-108 MHz) band, then combine the outputs. You might need a separate amplifier for each radio service you want to add to the tunnel.

These transmission lines are pretty lousy; the signal typically gets halved every 100 feet or so in the FM band. (100MHz) AM (at 1MHz) travels 10x farther because it has a 100x lower frequency. Having no metals more conductive than copper, we cannot make any transmission line that reaches more than a few thousand feet, and usually much less than that.

Cell-phones and GPS have frequencies about 1000MHz, even tougher to propagate than FM. Radio amateurs hate to even run a line up a 50-foot mast at these frequencies. If the tunnel had lots of open space above the cars and trucks, launching a wave through the air, pointed down the tunnel, would be part of the system, though again it would probably only go a few hundred feet.

Going farther would require a fancier system much like cable TV or cellular networks use, with frequent amplifiers or digital signal restorers. Modern fiber-optic technology might be a much easier way to reach about 10 times farther into a tunnel, if you can afford to have a 1% tap (-20dB coupler) and a small intensity-to-voltage converter every hundred feet or so.

Your GPS idea has another, harder problem: it's not one signal but several, from several different directions. Making a valid signal requires combining signals from 3 or 4 satellites simultaneously, not just one. Taking all the signals from one antenna the end of the tunnel, every place in the tunnel would appear to have the exact same location as that antenna, according to your GPS receiver. One can imagine drilling holes to the surface every 100 feet or so for a new antenna and transmission line, but that would be excessively expensive for the benefit. Even that would often confuse your GPS receiver by containing partial signals from multiple places.

I wish civil engineers could leave a place for volunteer efforts to wire in transmission lines long after tunnel construction . You might check out whether some tunnels are already wired for cell-phones. But GPS has big problems for this. Extensions of the GPS format, vaguely similar to differential GPS correction services, plus an in-house location-sensing radio-wave format, would be necessary for this. These have not been defined yet, and when/if they are defined, your present GPS receiver will not understand them.

I have heard that a GPS receiver including an IMU (inertia measurement unit) is available for a few thousand dollars. Soon it will get cheaper still, then your tunnel problem will be approximately solved. The receiver will remember the last signal it "heard" clearly, and add to that the distance and direction it "felt" it traveled since then.

Jim Swenson



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