Tuesday, May 31, 2011

Can LightSquared 4G and GPS Coexist?

There's a common line of thinking among some folks in the aviation community that the federal government doesn't restrict cellphone use in an airborne aircraft, that cellular telephones do not interfere with radio navigation, and that it's fine to use a cellphone or have it powered on in flight. The usual reasoning I hear from pilots who leave their cellphones on or use them in flight is that since they've never seen any problems, no such problems exist. The fact is that we are surrounded by an ever-growing sea of radio frequencies and detecting interference requires a more rigorous approach than casual observations made by individuals. If you need more convincing that radio frequency (RF) interference can be serious, look no further than the proposed LightSquared 4G broadband network and the impact it may have on GPS users.

Never Seen a Leprechaun

It's understandable how some pilot's come to the conclusion that cellphone use is okay in an airborne aircraft since the relevant aviation regulation, 14 CFR 91.21, appears to permit cellphone use if the operator or pilot-in-command has determined there's no interference or if the flight is conducted under visual flight rules. The regulation provides no guidance on how one is to test for interference and that could lead one to conclude that the FAA doesn't care if you use your cellphone in flight. Consult the portion of the Code of Federal Regulations that deals with the FCC to find the regulation, 47 CFR 22.925, that clearly and specifically forbids the use of cellular telephones in flight. The FAA may not care, but the FCC clearly does care.

The common follow-up argument is that the FCC ban is out-of-date and that there's absolutely no problem with leaving your cellphone on or using it in flight. Folks who make this assertion usually do so based on their personal experience in their own aircraft. An important element in this line of thinking is a strong desire to do what is convenient, not so much on any rigorous measurements of radio frequency (RF) interference. While I haven't made any specific measurements with sophisticated equipment, I have noticed several basic types of cellphone interference in aircraft.

In one aircraft I occasionally fly, one of the two VOR receivers will show a 15 degree error at a ground VOR checkpoint whenever an iPhone 4 is turned on and placed in the console directly below the radio stack. Put the iPhone 4 in airplane mode and the VOR error goes away.

I've also demonstrated interference between cellphones and my portable Zaon PCAS MRX traffic detector that manifests as false traffic alerts. It took me a while to correlate this, but on several occasions the Zaon MRX gave continuous traffic alerts for an aircraft within a mile and at the same altitude. Asking ATC if they saw any traffic in my area always resulted in the same response: "Negative." On one such occasion, we heard the unmistakable sound of cellphone data transmissions over the intercom (dit-da-dit-dit-dit-dit-dit ...). When we located the offending cellphone and put it into airplane mode, the intercom interference went away and so did the phantom aircraft that the Zaon MRX had said was within 0.3 miles and at the same altitude.

Just because you've never seen a leprechaun, that doesn't mean they don't exist.

4G vs GPS

While I've never been able to correlate cellphone interference with GPS receivers, at least one next generation 4G network system appears to pose a widespread threat to GPS accuracy. The FAA has been issuing NOTAMs over the past several months warning of potential GPS unreliability related to the testing of new 4G network equipment created by LightSquared.

In January of this year, the FCC issued a waiver to LightSquared allowing them to move forward with plans to deploy transmitters that uses the L band 1 spectrum to provide a high-power terrestrial broadband service. While this could be great news for people in remote areas that want high-speed data transfer on their mobile device, GPS experts and users are concerned. The 1525 MHz-1559 Mhz band is very close to the 1575.42 Mhz band used by GPS.

Recent tests of LightSquared's 4G equipment has inconvenienced many users of GPS. On one occasion, I was unable to get a clearance to fly an RNAV approach because the NOTAM prevented ATC from allowing such approaches during the testing. Imagine aerial survey companies who rely on WAAS GPS to do their mapping or ships that rely on GPS for maritime navigation. Aside from acquiring the necessary equipment and databases, GPS is provided without charge. The LightSquared 4G product will undoubtedly be a commercial product and user's will understandably have to pay for data access. Whether it's the profit motive or a large number of lobbyists, the implementation of LightSquared's network is continuing at a rapid rate.

Hope you like Jammin' Too

Testing done by Garmin showed that an automotive nuvi 265W GPS receiver was jammed when within a 3 to 4 mile distance of a LightSquared transmitter. Garmin's aviation GPS receiver tests were even more sobering with jamming of a 430W occurring within 9 to 13 miles of a LightSquared transmitter and a total loss of position occurring within 5.6 miles.

The waiver granted to LightSquared by the FCC requires that a working group identify and reconcile conflicts between 4G and GPS, but the onus appears to be on the GPS community, not LightSquared:

Because the GPS interference concerns stem from LightSquared’s transmissions in its authorized spectrum rather than transmissions in the GPS band, the Commission expects full participation by the GPS industry in the working group and expects the GPS industry to work expeditiously and in good faith with LightSquared to ameliorate the interference concerns.

The FAA has gone to considerable lengths over the past several years to create RNAV approaches with vertical guidance and to expand the WAAS service volume. With the number of LPV approaches outnumbering the number of ILS approaches, the potential conflict between GPS users and companies that want to provide satellite-based broadband is very, very serious. For my money, it's more important to have accurate RNAV than to be able to update my Facebook page while hiking the John Muir Trail.

LightSquared reportedly wants to install up to 40,000 high-power transmitters operating at up to 15,000 watts (42 dB). So the latest threat to the integrity of GPS isn't solar flares or aging satellites, it's wireless broadband. Until this gets resolved, be sure to check those NOTAMs. And if you haven't practiced navigating with VORs lately, you might want to dust off those skills.


Dave Starr said...

I've written about the Lightsquared issue several times on my GPS blog. Surprisingly very little comment/feedback, especially from pilots, whom this issue affects significantly.

Initial testing has shown significant issues with ground-based GPS receiver systems, I have yet to see anything authoritative on the airborne side, but the very size of the areas NOTAMed for the testing should give any thinking pilot pause.

As a non-engineer, but an operational user of the GPS since it was a one satellite system (delivering precision time signals in classified locations), I can state with some degree of certainty that the entire Lightsquared "frequency grab" is a significantly sub-optimal idea.

On the cell phone in flight issue. First, the FCC restricts the use in flight because use of any cell phone in flight can, under certain circumstances, significantly degrade performance of the cellular network for others ... including potential emergency services use. The entire cell network is engineered for line of sight to ground-based transceivers (LOS), the increased LOS while airborne can play havoc with the cellular grid frequency re-use algorithm.

The cellular system only works because a relatively small number of frequencies are re-used thousands and thousands of times across the country based on a complex, distance-based formula.

My rule would be, no use of cells in flight at all, even VFR, except as a last-ditch emergency comm backup ... it's impossible to tell as the user with whom you may be interfering.

Secondly, each generation of cell phones is increasingly made cheaper and much less sophisticated in terms of RF design (by the lowest common denominator contractor, typically in China ... even the fabulous iPad is made by $100 USD per month workers in Shenzen, PR of China (well known in China as the capital city of fake "replica" knock-off products)).

So what doesn't cause interference in your airplane today may not be as safe next month or next year when you replace today's phone with the next bright, shiny object that comes on the market.

Jake Brodsky said...

John, allow me to delurk and introduce myself. In addition to my private pilot and instrument rating, I am also an electrical and control systems engineer.

I have been burned by the FCC's incompetence before. Back in 1988, we installed our first SCADA system for a large water utility, licensed on 928 MHz. Only a few months after we went online the FCC introduced 929 MHz paging less than 500 kHz away. Our radios could not cope with the nearby noise. We had so much energy coming off the antenna that you could make a neon bulb glow.

Our signals had 5 Watts of power, Theirs had 3000 Watts of power. It's not hard to imagine what happened. Fortunately, we were able to apply some very creative engineering to the problem and we skirted what would have been a disaster to most utilities. However, the option of trying that kind of creative engineering may not present itself in the highly regulated aviation environment.

The people running the FCC are lawyers and politicians, not engineers. They don't know much about radio except that they have a spectrum and that they think they can license and allocate it however they please, with no regard to what the technical state of the art is. Engineers like me have to jump through horrible bureaucratic hoops to get anything done with this bunch and yet they feel free to crap all over us whenever they feel like it. Why the hell should we bother with licenses if these idiots are going to trash us anyway?

Make your opinions known. The FCC has screwed up things in the past and they will screw them up again unless we make noise. Too many morons think their phone can not possibly create a problem on the airliner. I've seen too many nitwits diddling around with an MP3 player (many of which have FM radios) on approach to an airport. We already have problems.

We need to insure the FCC understands that and doesn't crap all over Aviation the way they have with other critical infrastructure.


(Sorry, this is a sore subject with me)

HIPAR said...

Pilots all should be made aware of what was found when GPS avionics was subjected to Lightsquared signals. An interference test was sponsored by FAA. It was conducted at White Sands Missle Range. A test report is being prepared by RTCA:



Based on tests of four FAA-certified aviation receivers — out of approximately 50 models now being flown on board aircraft in the national air space — and, the effects of a five-megahertz-wide transmission from a single-city base station deployment at the upper end of the LightSquared allocation “is expected to be complete loss of GPS receiver function.”

There are 150,000 (Plus) affected avionics receivers in everyday service. So what will become of the NextGen airspace initiative? Will FAA withdraw thousands of GPS based landing approaches? Can the airborne equipment be redesigned for the forthcoming electromagnetic environment and retain certification?

It seems nobody in government wanted to hear about the ramifications of the Lightsquared license modification. Now the only solutions are political.

--- Charlie