SpectrumTalk has moved!

25th Anniversary of FCC Decision Enabling Wi-Fi and Bluetooth

25th Anniversary of FCC Decision Enabling Wi-Fi and Bluetooth
A series of posts describing how this all came about. (Click on picture above)

Saturday, November 07, 2009



The Big Dig:

1) If GPS amplifier use in underground highways causes no problems, why is it illegal?

2) If it is illegal, why don't FCC and NTIA care about enforcement?

A few months ago I was in Boston staying with relatives in the Northern suburbs. I was invited to give a talk at the Massachusetts Department of Telecommunications and Cable, located at South Station in the center of Boston. Without thinking too much I punched the address into my trusty GPS and headed South. I am a native of Boston but hadn't thought of the impact of the "Big Dig", the major highway reconstruction in Boston that moved several key highways to underground bringing the harbor back to the cityscape. As I entered Boston from Cambridge it was an "Oh, sh.." moment as I realized that I was entering a Big Dig tunnel and my GPS would lose signal. However, it did not and even followed me through the tunnel and told me what exit to take. Amazing?

Recently I had a similar occurrence in the I-395 tunnel in downtown Washington.
Not unrelated is noticing that my GPS works in rental car parking garages at certain airports such as Phoenix. Why do these things happen?

The most likely explanation is the use of
GPS amplifiers - amplifiers with an outdoor antenna that receives a GPS signal and a second antenna in an area that has no GPS signal which it "illuminates". THESE ARE ILLEGAL (except with a license this can only be obtained in special circumstances.) Why are they illegal? The main GPS frequency, 1575 MHz, is an exclusive Federal Government band and is listed in 15.205 as a "restricted band" - forbidding all unlicensed systems. NTIA has provisions in its "Red Book", the analog to FCC Rules, for use of this band. This gives terms for federal agencies to use systems other than GPS in this band. FCC and NTIA have an understanding that is documented nowhere on the public record that nonfederal users, e.g. FCC regulatees, may apply to FCC for an experimental license for use of this band and it will be reviewed by NTIA in accordance with Red Book Section 8.3.28. If NTIA agrees with the request, an experimental license can be issued by FCC. While 8.3.28 does not say so, senior staff at NTIA have said that they will only approve GPS amplifiers in connection with testing GPS receivers - not for use in tunnels and garages. It appears that this unwritten policy is applied inconsistently.

In any case, a search of the FCC database shows no valid experimental licenses for this band in Boston. It would appear that the GPS signals are coming from an illegal transmitter.
(I thought the GPS amplifiers in question might have been put there to support cellular base stations in the tunnel as cellular base stations usually need GPS input for both frequency and time reference. However, some inquiries revealed that several cellular carriers in Boston collaborated in a fiber optic system that brings the GPS information to their underground base stations without reradiating them in the tunnel. Do all the cellular carriers in the Big Dig use this system? I don't know.)
As you can see from the map at the top of this page, the Big Dig is close to Logan Airport.

  • Has the illegal system in the tunnel caused any problems? It certainly has helped the public traveling through the Big Dig.
  • If it hasn't caused any problems, why does NTIA continue to forbid this type of spectrum use?
  • Since it is illegal and NTIA insists it is a safety-related issue, why doesn't FCC take some enforcement action?

Perhaps NTIA and FCC could explain their views on this issue for the benefit of readers.


Mike said...

Are you using a standalone GPS receiver, or a car's Navigation system. Car navigation systems can use both GPS and dead reckoning to determine location. For example, my car's nav system shows our progress through either of the tunnels through Baltimore's harbors and likewise through the Holland and Lincoln tunnels into New York.

Also, some navigation devices use Wi-Fi geolocation. Don't assume it's using only cellular/PCS.

Mike said...

Are you sure your unit is GPS-only? Many units use other technologies in addition. For example, some cars have nav systems that use not only GPS but also dead reckoning. This lets the nav system work just fine at lower levels of the garage, as well as in the I-95 and I-895 tunnels in Baltimore (and in the NY tunnels). Another possibility is that the unit is using Wi-Fi.

MJM said...

I use a bottom of the line Garmin Nuvi unit which is only connected to the car using the cigarette lighter power cord.

Also in the Phoenix rental car parking garage it was able to figure out it was in Phoenix, something impossible with dead reckoning.

vik said...

Valuable post, can take knowledge as well upon the subject.

Anonymous said...

I just had exactly the same experience last night (Feb 27 2010) . . .

My wife and I were driving to Boston for an event near Boston's waterfront using my bare-bones Garmin 255 to navigate. As we entered the Big Dig tunnel I said to my wife, "we better memorize the exit because now we'll lose the satellites and it won't know when to tell us to get off".

But it never lost anything! All through the tunnel it knew exactly where we were and as the exit approached it gave us all the correct instructions for exiting.

I posted this question to a GPS discussion forum where people speculated that maybe the Nuvi 255 has dead-reckoning built in (where it assumes your same course and speed as when it lost the satellite signal) but others confirmed that it does not.