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.