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The Telecom Digest for May 4, 2010
Volume 29 : Issue 123 : "text" Format
Messages in this Issue:
Re: IEEE article on GSM interference affecting GPS landing systems (Thad Floryan)
Re: IEEE article on GSM interference affecting GPS landing systems (David Clayton)
Re: IEEE article on GSM interference affecting GPS landing systems (Rob Warnock)
Re: IEEE article on GSM interference affecting GPS landing systems (Sam Spade)
Re: IEEE article on GSM interference affecting GPS landing systems (Scott Dorsey)
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Date: Sun, 02 May 2010 22:56:07 -0700
From: Thad Floryan <thad@thadlabs.com>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: IEEE article on GSM interference affecting GPS landing systems
Message-ID: <4BDE6577.5060904@thadlabs.com>
On 5/2/2010 8:08 AM, Michael D. Sullivan wrote:
> [...]
> AT&T uses GSM, which in turn employs TDMA transmissions. These switch
> your phone's transmitter on and off about 1000 times per second when
> the handset is communicating with a base station (yes, even when you
> aren't using it, the handset periodically checks in). [...]
Technical nit:
" [...]
" 1. ... General R/F pollution. Any system that switches its R/F
" transmitter on and off rapidly (GSM does it 217 times a second, TDMA
" does it 50 times) will scatter EMI throughout the adjacent radio
" spectrum.
" [...]
The above is from the comp.dcom.telecom archives, 11-March-1994. A copy
of it was emailed to me last year during the "GSM Interference" thread
and I posted that to the group. I also placed a copy on my web site so
it'd be easier to find. :-) That copy can be found here:
http://thadlabs.com/FILES/GSM_and_TDMA_Problems_1994.txt
Date: Mon, 03 May 2010 18:10:19 +1000
From: David Clayton <dcstar@myrealbox.com>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: IEEE article on GSM interference affecting GPS landing systems
Message-ID: <pan.2010.05.01.22.43.07.795441@myrealbox.com>
On Sat, 01 May 2010 09:54:51 -0700, Sam Spade wrote:
> David Clayton wrote:
>
>> On Thu, 29 Apr 2010 18:15:27 -0700, Thad Floryan wrote:
>>
>>
>>>L-o-n-g article here (with pictures, graphs, etc.):
>>>
>>>http://spectrum.ieee.org/aerospace/aviation/unsafe-at-any-airspeed/0
>>>
>>>Quick summary: "Our [IEEE] data and the NASA studies suggest to us that
>>>there is a clear and present danger: cellphones can render GPS
>>>instruments useless for landings."
>>
>> .........
>>
>> So when the terrorists eventually figure out that taking a doctored
>> phone/DVD player/Laptop on board a flight (something designed to blast
>> out interference at the press of a button) is a potentially effective
>> way of bring down an airplane, then perhaps all that stuff at the
>> airports scanning for explosives will be effectively obsolete?
>>
>> As the article says, people won't accept a total ban on electronic
>> devices so how are the authorities going to stop anything like this?
>
> It just won't happen. First, most of the instrument approaches made by
> airliners today are Instrument Landing System (ILS) approaches, which are
> ground-based and have nothing to do with GPS.
>
> Second, where GPS is used the integrity and alerting modes would provide a
> warning in no uncertain terms to not rely on the GPS as the primary source
> for navigation.
>
> Finally, the RF from a cell phone is highly unlikely to affect the GPS
> receivers, which are isolated from the cabin. The incidents referred to
> in the report were extremely isolated and the cause and effect were never
> determined with certainty. As I recall, the most likely suspect was a
> laptop in use by a passenger seated directly over the electronics
> compartment in an aircraft where the electronics compartment extended
> back past the flight deck to the first class cabin.
I'm not talking about normal electronic items and their incidental
effects, I'm talking about items that pass inspection for normal but
actually are devices designed to interfere with the self-same critical
systems that planes in flight rely on.
The bare fact is that you are currently allowed to take RF sources onto
flights on the assumption that they are what they appear to be, where in
fact they may well not be. One doesn't have to look back too far to see
the chemical equivalent to this paradigm and the threat it posed.
--
Regards, David.
David Clayton
Melbourne, Victoria, Australia.
Knowledge is a measure of how many answers you have, intelligence is a
measure of how many questions you have.
Date: Mon, 03 May 2010 05:37:53 -0500
From: rpw3@rpw3.org (Rob Warnock)
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: IEEE article on GSM interference affecting GPS landing systems
Message-ID: <oaydnVhusb0cOkPWnZ2dnUVZ_s2dnZ2d@speakeasy.net>
Michael D. Sullivan <mds@camsul.com> wrote:
+---------------
| AT&T uses GSM, which in turn employs TDMA transmissions. These switch
| your phone's transmitter on and off about 1000 times per second when
| the handset is communicating with a base station (yes, even when you
| aren't using it, the handset periodically checks in). The result is
| that a 1 kHz square wave is emitted. This can be induced into nearby
| unshielded electronics and heard as an audio buzzing sound. This
| often happens when a TDMA phone is next to a speakerphone, amplified
| computer speaker, etc. This emission is very low level, so the phone
| needs to be very close to the audio device.
+---------------
The basic GSM TMDA frame rate[1] is ~217 Hz, not 1000 Hz. A "full-rate"
voice call is allocated only one slot per frame, so the duty cycle of
the R.F. transmitter is only 1/8; a "half-rate" call [common with recent
handsets with better codecs] uses only one slot every other frame, so it
will appear to sound at ~108 Hz, rather than at 217 Hz. In either case,
this makes the interference sound very rough -- more a "burp" (like a
chain gun) than a "whistle".
Other than that, most of what you say is correct. All it takes is just
a bit of nonlinearity in nearby electronics to demodulate the R.F. and
produce the 217 Hz (or 108 Hz) buzz.
-Rob
[1] 8 slots of 577 us or 4.615 ms per frame, thus 216.7 frame/s.
Rob Warnock <rpw3@rpw3.org>
627 26th Avenue <URL:http://rpw3.org/>
San Mateo, CA 94403 (650)572-2607
Date: Mon, 03 May 2010 08:22:09 -0700
From: Sam Spade <sam@coldmail.com>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: IEEE article on GSM interference affecting GPS landing systems
Message-ID: <JpedncVQl-a8d0PWnZ2dnUVZ_qSdnZ2d@giganews.com>
Rob Warnock wrote:
>
> Other than that, most of what you say is correct. All it takes is just
> a bit of nonlinearity in nearby electronics to demodulate the R.F. and
> produce the 217 Hz (or 108 Hz) buzz.
>
I get that from my iPhone with two devices in my house. It seems I have
to be within about 3 feet of them to get it to happen.
Date: 3 May 2010 15:26:20 -0400
From: kludge@panix.com (Scott Dorsey)
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: IEEE article on GSM interference affecting GPS landing systems
Message-ID: <hrn80s$3e9$1@panix2.panix.com>
In article <4BDA2F2F.4070608@thadlabs.com>,
Thad Floryan <thad@thadlabs.com> wrote:
>L-o-n-g article here (with pictures, graphs, etc.):
>
>http://spectrum.ieee.org/aerospace/aviation/unsafe-at-any-airspeed/0
>
>Quick summary: "Our [IEEE] data and the NASA studies suggest to
>us that there is a clear and present danger: cellphones can render
>GPS instruments useless for landings."
>
>I believe the NASA study alluded-to in the article is this one:
>
>http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/20010066904_2001108092.pdf
Ross' paper is very well-known and is a good starting point. However, since
2001, GPS system front ends have improved substantially, while on the other
side of the coin the current cellphone technologies cause more severe
interference issues because of their waveforms.
--scott
--
"C'est un Nagra. C'est suisse, et tres, tres precis."
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