[time-nuts] GPS from a window seat

David I. Emery die at dieconsulting.com
Sun Oct 4 18:08:58 UTC 2009

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.

	Obviously I - and I think most anyone on this list - would never
consider doing anything or using any equipment that might endanger an
aircraft (especially one I am flying on, needless to say) - so I have
tried to keep aware of any issues that might remotely exist with
handheld GPSes on planes.

> Another problem was bias oscillators in tape players, these could
> interfere with Omega/VLF receivers. Not a problem now of course. While
> on the subject of Omega/VLF If you come across a Global GNS-500A OEU
> box, it has a Efratom FRK Rb in it. There is one on ebay at the moment,
> item 150257671674, but the price is way to high.

	I had heard of that, I know many cheap tape recorders radiate
enough to be detected at some distance - this has been used in the past
by TSCM specialists to find covert planted recorders.

> It's very had to predict interference on aircraft. While the
> probability is low the consequences during take-off and landing are
> severe. Hence the total ban on electronics during these flight phases.
> The "illegality" is endangering an aircraft, however you do it. Avionics
> design is my day job.

	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.

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

  Dave Emery N1PRE/AE, die at dieconsulting.com  DIE Consulting, Weston, Mass 02493
"An empty zombie mind with a forlorn barely readable weatherbeaten
'For Rent' sign still vainly flapping outside on the weed encrusted pole - in 
celebration of what could have been, but wasn't and is not to be now either."

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