[time-nuts] GPS from a window seat

Lux, Jim (337C) james.p.lux at jpl.nasa.gov
Sun Oct 4 18:56:29 UTC 2009

On 10/4/09 11:08 AM, "David I. Emery" <die at dieconsulting.com> wrote:

> On Sun, Oct 04, 2009 at 09:14:29AM +0000, Robert Atkinson wrote:
>> Hi,
>> This is correct. There was also an issue with harmonics from the local
>> oscillator in the aircraft's own VHF nav/comm receivers blocking the
>> GPS. The answer is a 1575MHz notch filter, e.g.
>> http://www.edmo.com/index.php?module=products&func=display&prod_id=18006
>         I have not (not being involved in avionics professionally) heard
> of any problems with GPS receivers causing interference to other GPS
> receivers or other avionics ... I would be interested in your comments
> on whether you know of any such issues and what the mechanisms are.

Most of the receiver designs I'm familiar with use some sort of single bit
sampler running at some tens of MHz.  If the sample clock happens to be say,
1/3 of a "bad" frequency (e.g. Around 40 MHz, so the third harmonic is 120
MHz, in the middle of the aviation VHF band), you might have an issue with
the clock coupling back out the antenna port.  Fortunately, most receivers
ALSO have some sort of filtering (because the receiver would be sensing the
self same 120 MHz aviation signals, etc.) which should suppress the
harmonics.  The L band bandpass filter would let a very high harmonic out,
but I'll bet it wouldn't interfere with other GPS receivers.

Then, there's all the usual "radiation through the case" problems, but I
suspect that GPS receivers are no where near as bad an offender as a laptop
or PDA or iPod or or or or..
>         I have read that there have been studies with a spectrum
> analyzer system on planes that have shown that compliance with the no
> radiating device rules and electronics off during takeoff and landing is
> far less than 100% though I certainly would not personally deliberately
> violate the law whether or not the probability of it causing a problem
> is significant.  Apparently one or two cellphones can be seen registering
> with cell systems during takeoff and landing on many flights - probably
> most of them unintentionally left on.

There was an article in IEEE Spectrum about this. A Agilent portable
spectrum analyzer with battery power and a data logger in the overhead bin,
as I recall.

>         Publicly discussed and documented cases of interference causing
> serious problems are fairly rare... it is unclear how many actual cases
> there have ever been.

There's a web page at the FAA website that discusses it, and the events
where interference is suspected.  It's not common, but it's also not unheard
of, although usually, it's just inferred that this is what happened.

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