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The common follow-up argument is that the FCC ban is out-of-date and that there's absolutely no problem with leaving your cellphone on or using it in flight.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.