Re: American Airlines response

From: Anonymous Poster (N_at_where.com)
Date: 01/19/05 

Date: Wed, 19 Jan 2005 01:09:15 -0500

"Searching_ut" <Searching_ut-nospam-@sprynet.com> wrote in message
news:bteGd.8267$Ii4.4777@newsread3.news.pas.earthlink.net...
>I get somewhat of a kick out of some of the responses to this issue. Having
>worked on aircraft for almost 30 years, it's an issue I've followed to a
>small extent over the years. To begin with, virtually all electronic
>devices to include receivers can and do emit signals of various
>frequencies. Engineers have considered this, and built protections into the
>assorted avionics systems, but there is always the possibility of
>"unexpected", or un foreseen interactions. For instance, on one military
>aircraft I used to work on, moisture working it's way into the tail light
>assemble would cause the magnetic flux compass to have significant errors.
>Since the wiring to the light was in the area of the flux sensors for the
>compass, it was all designed as twisted pairs so the magnetic fields would
>cancel out. Should the floating ground of the light assembly conduct part
>of that current to the aircraft structure however....... The simple wiring
>to a 25 watt light bulb now has part of the navigation system rendered
>worthless.
>
> As for electronic devices causing problems on commercial aircraft, there
> have been reports of it happening over the years, mostly anecdotal, but
> worth looking into. See for examples:
>
> http://www.house.gov/transportation/aviation/hearing/07-20-00/07-20-00memo.html
>
> http://techreports.larc.nasa.gov/ltrs/PDF/2001/cr/NASA-2001-cr210866.pdf
>
> I personally doubt a GPS unit would cause any issues, but can understand
> the airlines not feeling it's worth looking into unless there is
> considerable customer pressure to allow it to be used. My guess is that
> there has already been a fair bit of pressure on the airlines from
> customers regarding Laptops, game boys etc but that they haven't received
> much input on GPS use yet. They aren't going to assume what I believe they
> probably do see as a risk unless the economic incentive exceeds the risk.
> That might not be the way it should be, but.........
>
> For what it's worth
>
> Jeff
>

 IMO there is some reason to be conservative with commercial handheld GPS
receivers on aircraft, although not necessarily for the reasons AA gave in
their response to the original poster.

GPS receivers differ from any other piece of electronics that passengers
might bring on board, in that they are equipped with reasonably
well-designed GPS-frequency antennas. These antennas are as effective at
transmitting as they are receiving (this comes from the reciprocity theorem
of antenna engineering).

In a normal receiver a low level signal will be radiated from the antenna,
as long as the electronics have passed CFR47 part 15 and similar
regulations. The receiver might not be normal: it might have manufacturing
defects, a solder joint might have broken, part of the antenna might come
unstuck, the receiver might have been modified by the owner and so on.

In these cases it's possible that the amplifiers connected to the receive
antenna might oscillate. For example if a solder joint broke and changed the
output match on the input low-noise amplifier, it might put as much as a
milliwatt out the antenna. Or, the receiver might be used with an outboard
active antenna with a defective leaky cable which re-radiates to the
antenna, again causing oscillation and radiation.

Some back-of-the-envelope calculation: Given a faulty receiver radiating a
milliwatt (0 dBm), typical receive thermal noise power spectral density of
about -170 dBm/Hz and the 60 dB spreading ratio for a C/A receiver, this
means that receiver positioning performance will start degrading
significantly with a signal power link loss from handheld to avionics
receivers of 110 dB or less, which doesn't seem improbable. With smaller
link losses the results would range from significant positioning performance
degradation, to loss of ability to (re)acquire satellites, to total loss of
functioning GPS navigation at around a link loss of 90 dB or less.

Relevant Pages

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