Re: Headset & FCC


Mike Perry <dmperry1012@...>
 

Several comments re cell phones:

1) My first flight instructor always carried a cell phone and said she had
used it exactly once to land in controlled airspace with radios out. I
don't remember the details.

2) "cell phones" include both analog and digital; digital includes at
least a couple of protocols. The protocol Verizon uses is called CDMA; in
this system every tower sends out a brief time signal several times per
second; the cell phone decides which signal is best and sends out a signal
each second "this is who I want to talk to." The system routers then
decide which signals to ignore. The problem with signals hitting too many
towers is system overload: the system knows which signals to pay attention
to and which to ignore but you still have to filter out the extraneous
signals. However, this is far easier with digital than analog signals.

3) One problem with digital cell phones is that they will revert to analog
under some conditions. This "feature" allows you to get some signal in
areas that only have analog service (eg: some resort areas here in
California have a single analog cell tower just for the town or
resort). See Craig Steffen's comments below re analog cell phones in the
cockpit.

My son once worked on a software project for a cell phone system; I picked
up bits and pieces of info on the protocols.

Mike Perry

At 08:55 AM 11/2/2006 -0600, you wrote:

Quoting FR Jones <<mailto:seabeevet%40gmail.com>seabeevet@...>:

Shooting from the hip on this, two issues come to mind. First, why would
using a cell phone in a private plane make a difference?
As has already been pointed out here:

The core prohibition is from the FCC, because it would cause your phone
to claim
an active channel on dozens of towers at once, rather than one or two
as is the
design.

The Mythbusters show on the discovery channel tried this Myth out (on the
ground; they weren't allowed to in the air). They built a device that
broadcast broad-band signal on cell frequencies. They couldn't get a
certified
aircraft to react at all.

However, they also built a mock-up of a cockpit with just some surplus
instruments. The digital cell phone signal (1.8 GHz or so) didn't produce any
results. However, analog cell signals (900 MHz range) made the VOR go
haywire.

SO...I think that depending on shielding, in a homebuilt particularly,
an active
analog cell phone on board could very easily cause the VORs to lose
their lock. This would be particularly true in a fiberglass airplane,
which doesn't have the
natural conductive shielding of a metal airplane.

I've always assumed that the cell phone jack on aviation headsets was to call
flight service to activate or close your flight plan while taxiing, or to call
other people while you're on the ground and the engine is running.

Craig Steffen

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