Current bugbears

[Jim Thompson]

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