On Sep 4, 2007, at 7:27 AM, John Seghers wrote:
Alexey Feldgendler wrote:
Except GPS, I guess. Because GPS is just a receiver, it cannot add
any
interference and hence doesn't need to be disabled. Actually, GPS
could
provide some fun in a flight.
NO. Including GPS. Every flight I've been on recently has state that
electronic devices which send *or* receive signals are not allowed
(or those
capabilities switched off.)
its easier to turn them all off.
Part of the process of receiving signals involves heterodyning--
basically
mixing a received signal with lower intermediate frequencies (IFs) to
amplify the desired actual signal, while making the carrier signal
something
easier to work with. See http://en.wikipedia.org/wiki/Heterodyne
for a very
basic description.
The IF can be higher or lower, depending on your design's needs.
The receiver section in your radio is likely superhet.
Any oscillator will radiate some electromagnetic interference.
Well, electromagnetic engergy, anyway... interference is an artifact
of receiver deisgn.
Furthermore, because the intermediate frequencies are often similar
(or in the case of
GPS identical) to other such devices, those stages can interfere
with each
other.
AM/FM radios are banned (by the FAA) because there are documented
cases of harmful interference with flight control systems.
http://www.airweb.faa.gov/Regulatory_and_Guidance_Library/
rgAdvisoryCircular.nsf/0/b5c85ae8e38df676862571da00529da4/$FILE/AC%
2091.21-1B.pdf
http://www.fcc.gov/Bureaus/Engineering_Technology/Documents/cfr/
1997/47cfr15.pdf
http://gpsinformation.net/airgps/gsm_intf1.pdf
This may be interesting: http://gpsinformation.net/airgps/g3noise.htm
For another example, in states where radar detectors are illegal,
the police
can use sensors tuned to the first stage IF (around 900MHz IIRC) of
the
radar receiver to find cars that have them onboard.
(http://radar.757.org/VG2.htm)
Only if their "detector" is tuned to the IF of the intended target.
Some radar detectors employ a shielded oscillator and other 'counter
measures'.
As the onboard electronics in aircraft are upgraded to those
designed in an
age of digital electronics, Bluetooth, GPS, cell phones, etc, these
restrictions will probably fall by the wayside.
Unlikely you're dealing with an industry that is afraid of falling
out of the sky while on fire. Note that the (US) law is very broad:
http://ecfr.gpoaccess.gov/cgi/t/text/text-idx?
c=ecfr&sid=8e7fdab3dcebd30df90ea9cd49cf777b&rgn=div8&view=text&node=14:2
.0.1.3.10.1.4.11&idno=14
essentially, "everything that is not allowed is prohibited".
However, today the regulatory agencies have to look at the least
capable
machinery flying out there when they make the rules. Most airlines
also
tend to over-generalize things simply because it makes it easier to
communicate what is allowed and what isn't.
Of course. They're worried about future equipment, too.
Jim
