Lightsquared/GPS [telecom]

GPS does have proper authority, and the frequencies it uses are reserved for the purpose. IIRC, it's a military service, and doesn't need FCC approval. No matter: GPS has never pretended to be anything that it isn't.

What LightSquared has is a license for an "Ancilliary Terrestrial Component", which is supposed to allow a company that has satellite(s) in orbit to provide coverage in the very limited places that satellites can't reach: tunnels under cities, etc.

What LightSquared is trying to do is to convert their "ATC" license into their /only/ service: in other words, they're trying to join the cellular market without ponying up the hundreds of millions of dollars that other cellular data providers contributed to the US treasury in return for /their/ licenses. Harbinger capital was/is betting that LightSquared can sleeze their "LTE" radio transceivers onto every cellular tower that Sprint has erected, and convert it's "Ancilliary" service over to a terrestrial-only network that avoids the expense of orbiting and maintaining sattelites - and the need for their customers to buy satellite-capable cellular data units. It's an end-run around the spectrum allocation auction process, and LightSquared should be shot down for that reason alone.

The fact that LightSquared's ATC frequencies are adjacent to the GPS band is not a coincidence: the whole reason that LightSquared obtained the assignement is because those bands are intended for /sattelite/ transmitters, hundreds of miles above the earth. Had LightSquared planned a /satellite/-based network, they could have used the bands they were assigned without any risk of interfering with GPS, but LightSquared has bet the farm on their lobbyists being able to change the laws of physics.

Bill

Not that limited. Sirius XM has more than a thousand (maybe more than two thousand) such licenses.

-GAWollman

The FCC doesn't license receivers. They license transmitters. The GPS on your dashboard is a receiver, the FCC license (if it applies at all to military devices, as Bill points out) would be for the GPS satellites.

***** Moderator's Note *****

There are broad bands of frequencies set aside for civilian and for military/government use, and the FCC does /not/ have authority to assign licenses that use military bandwidth.

When economies of scale dictate that bands be shared - soldiers need to use "commercial" walkie-talkies while on guard duty, for example - there's a coordination mechanism in place, but the FCC doesn't issue, nor are FCC licenses required for, /any/ military transmitter.

There are, of course, various exceptions, but they're unimportant. The point is that if anyone says "GPS is unlicensed", they're spreading bull manure: GPS doesn't /need/ a license from the FCC.

Bill Horne Moderator

I meant "limited" as in "a place where a receiver is unable to get signals from a satellite", not limited in number. I'm sure there are thousands of highway tunnels, and equally sure that few drivers care if their GPS can't receive while inside them: the navigation choices are, after all, somewhat limited, and the GPS unit's internal logic is adequate to quide drivers to the correct exit, even when the GPS can't "see" the satellite constellation.

In any case, you bring a red herring to the dinner: Sirius operates in

2.31 to 2.36 GHz, but LightSquared wants to bull its way up next to GPS, which is at 1.575 GHz. That's a difference of about 800 MHz, and if LightSquared was trying to set up a terrestrial network 800 MHz away from GPS, we wouldn't be having this debate.

Bill

Incorrect, on several counts.

GPS devices are _receivers_. In the U.S., *no* receiver requires a license.

The GPS satellite *transmitters* are "licensed" devices.

By International agreement, the ITU coordinates spectrum assignments to various nations. For assignments to the United States, 'non government' use is managed by the FCC, while 'government' use (including military) is co-ordinated by the NTIA. The NTIA has assigned certain frequencies for the exclusive use of GPS satellites.

That said, there _is_ a valid element in your question.

All commercial receivers (and *most* other pieces of 'electronic' equipment), as 'incidental' radiators of RF energy (the 'local oscillator, etc.) are required to comply with 'Part 15' of the FCC regulations, before they can be offered for sale. Part 15 limits the amount of 'unintentional' radiation that they can output, and specifies that the device must be shut down _if_ it interferes with a licensed transmission. (note: Part 15 does not apply to _licensed_ transmitters -- they are governed by other, stricter, rules.)

There is a requirement in Part 15 that compliant devices 'accept' legal RF signals that are not part of their operation. BUT, a transmission that interferes with the 'normal operation' of a licensed transmitter (e.g., a GPS satellite) is expressly forbidden as well.

You can't prohibit the lightspeed service because it generaes inteference that is picked up in a Part 15 device. You _can_ prohibit the service because it interferes with the signal from the licensed transmitter.

There is a conflict between those statements, and exactly where the line should be drawn between them is the entire controversy. Available evidence is that Lightsquared 'interferes' with the licensed GPS signal, for the vast majority of receivers deployed today. The cost of modifying or replacing all the susceptible units with one with adequate sensitivity/ selectivity to ameliorate the interference is extremely non-trivial.

Adding gasoline to the fire is the fact that the Lightsquared proposal _is_ for a materially different use of the spectrum -- with regard to transmitter locations and signal power -- than was allowed when existing GPS equipment was designed/implemented/sold.

A case can be made, "in equity", that Lightsquared should bear the costs of replacing all existing equipment built that was 'adequate' for the original spectrum usage specifications, but is -not- adequate in the 'changed rules' environment that Lightsquared is requesting.

Since the premise is 'false to fact' -- GPS does _not_ involve 'unlicensed' operations -- the remainder of the question is meaningless

There are FCC rules -- which apply to Lightsquared's use of transmitters on the frequencies involved -- which forbid either an unlicensed or a licensed transmitter from generating interference with other licensed operations.

Unfortunately, what constitutes 'interference' is not sufficiently precisely defined in the FCC regulations, with regard to causing problems with other licensed operations. E.g. who is responsible for 'intermodulation interference' caused by the mixing of two unrelated RF signals (both from licensed transmitters0, occurring in a passive device (like the 'structural steel' of a building) and being re-radiated on the frequency of a third licensed transmitter? If either of the first two transmitters shuts down the problem 'goes away'. If the 'intermediate' radiator gets modified, the problem 'goes away'.

Suppose the interference is not 'exactly' on the frequency of the third transmitter, just 'close' to it?

_Maybe_ one can, by using sufficiently 'fancy' (read as "large", "bulky", and 'expensive') filtering devices, siphon off enough of the 'interference' to be able to recover the 'licensed' signal, and maybe _NOT_.

Taken to extremes, I wouldn't want to guess as to the cost, or size, of a GPS receiver that required a separate antenna, each with it's own 'cavity resonator' narrow-band filter for each GPS transmitter frequency.

I'm fairly sure you couldn't get it all to fit in a 'hand-held' unit.

Because the premise of your question is not in agreement with reality. :))

***** Moderator's Note *****

Some receivers /are/ regulated: in the U.S., it's unlawful to buy or import a "scanner" receiver capable of receiving cellular calls. Although that may not be a "license" requirement, it's a difference that makes no difference.

Bill Horne Moderator

Sirius XM doesn't transmit from tunnels, they transmit from broadcast towers and skyscrapers. LightSquared, as we know, would be transmitting from cell sites. The ancillary terrestrial component license is not about tunnels, it's about serving broad areas of cities and suburbs where the satellite signals would otherwise get clobbered by multipath. (They still get clobbered by multipath, of course, but the terrestrial signal is strong enough that even in the areas of destructive interference there's enough signal left to decode.) Here in Boston they have a site on the Prudential Tower and another on one of the Newton/Needham towers (I believe 350 Cedar, if I'm remembering the RFR study correctly).

One relevant difference is that Sirius XM is required by the terms of its licenses to transmit a uniform signal nationwide, both from their satellites and from their terrestrial repeaters. Apparently this doesn't apply to LightSquared.

-GAWollman

Actually, it is very precisely defined in the FCC regulations: when one transmitter's specified electric-field "interfering" contour overlaps with another transmitter's specified "protected" contour, interference is deemed to result.

The permittee of the third element to be built, when it is being constructed, is responsible for resolving such interference. If the building is the most recent, then the building owner must resolve the issue. If the second transmitter is the most recent, then the permittee of that transmitter must resolve it.

Broadcasters do this all the time; there is no magic, and the primary difficulty is usually in getting the people who built the new structure

-- often a cell tower! -- to admit their responsibility. Installing a wire skirt to detune a cell tower is pretty simple. (Broadcasters have to deal with this even without intermodulation, because structures like cell towers can reradiate broadcast signals and thus distort the station's directional pattern beyond tolerance.)

There's no reason you would want to do that. *Maybe* you might want to put a GPS passband before the front end, just to reduce the risk of overload. Since consumer GPS receivers seem to be single-chip devices, there may not be any way to improve their IF filtering without redesigning the whole thing. (What IF frequency do they use, anyway? There are good mechanical filters available for the usual choices of communications receivers, but none of them will likely fit in a smartphone, which is the important application here.)

-GAWollman

I don't care if they transmit from old CB sets strapped to the backs of migratory birds! It doesn't matter! Insert