[RFI] cellphones on airplanes

Jim Jarvis jimjarvis at verizon.net
Thu Mar 2 14:51:37 EST 2006

Quoth the popularized story about the Carnegie Mellon study:
Researchers noted that there is no definitive instance of an electronic
device used by a passenger causing an accident. However, they said their
data support the conclusion that use of devices like cell phones "will, in
all likelihood, someday cause an accident by interfering with critical
cockpit instruments such as GPS receivers."

My March IEEE Spectrum hasn't arrived, yet.
Given that it was published in a peer-reviewed journal, it
merits reading in the original, rather than popularized report.

My first inclination is to say, "horse-hockey".
But the reality is that the GPS bands and cdma/tdma phone bands
aren't all that far apart...there could be a question of
fundamental overload, if enough cellphones were transmitting at
high power.

I know I can use my cellphone right next to my handheld Garmin,
without impact.  I know the aircraft GPS antennas are topside
mounted on the fuselage...and there's a shield between them and
the passenger compartment.  So...I'm gonna have to see field strength
data before I believe this one.  Looking forward to the March Spectrum.

Moving back to entertainment devices...it turns out that walkmen and
iPods ARE a problem.  A 40MHz oversampled DAC is often used...and so
is a 1 meter headset cord.  3rd harmonic of 40 is 120...right smack in
the air to ground comm's band.  And, pilots have reported hearing
heterodynes in their comm's gear.


Back on the cellphone topic...the real problem is that commercial aircraft
moving at mach .75 are jumping between cells too fast for network switching
to follow.  In addition, it would be easy for a handset at 30,000' to bring
up two  cells, causing network port arbitration problems.

Interesting dilemma.  But frankly, I relish my peace and quiet on a flight,
and having some yahoo yelling into his cell will be cause for serious
Surveys of frequent travellers have shown the overwhelming majority do NOT
favor in-flight cell use, for that reason.

jimjarvis at ieee.org
jimjarvis at verizon.net

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