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The Telecom Digest for January 18, 2012
Volume 31 : Issue 18 : "text" Format
====== 30 years of TELECOM Digest -- Founded August 21, 1981 ======
|
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Date: Mon, 16 Jan 2012 14:40:12 -0500
From: Bill Horne <bill@horneQRM.net>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: LightSquared seeks probe of GPS advisory board member
Message-ID: <jf1uet$iu$1@dont-email.me>
LightSquared says the vice chairman of an official body, also on the
board of a company fighting LightSquared, has a conflict of interest
By Stephen Lawson, Computerworld
IDG News Service - LightSquared is seeking an investigation of a federal
official involved in deciding whether the company can deploy its hybrid
satellite-LTE network, saying he simultaneously serves on the board of a
GPS company opposing the network.
On Thursday, the mobile broadband startup petitioned the Inspector
General of NASA to investigate Bradford Parkinson, the vice chairman of
a board that advises the government on GPS. Parkinson should be removed
from discussions about potential interference between GPS and
LightSquared's proposed LTE (Long Term Evolution) network because he is
also a director of GPS vendor Trimble Navigation, LightSquared said in
its petition.
http://goo.gl/2yiBZ
--
Bill Horne
(Remove QRM from my address to write to me directly)
Date: Mon, 16 Jan 2012 17:25:49 +0000 (UTC)
From: wollman@bimajority.org (Garrett Wollman)
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: Lightsquared/GPS
Message-ID: <jf1mit$2hqp$1@grapevine.csail.mit.edu>
In article <20120116161242.GA7041@telecom.csail.mit.edu>,
Bill Horne <bill@horneQRM.net> wrote:
>On Mon, Jan 16, 2012 at 02:16:30AM +0000, Garrett Wollman wrote:
>> In article <20120115192027.GA1666@telecom.csail.mit.edu>,
>> Bill Horne <bill@horneQRM.net> wrote:
>>
>> >What LightSquared has is a license for an "Ancilliary Terrestrial
>> >Component", which is supposed to allow a company that has satellite(s)
>> >in orbit to provide coverage in the very limited places that
>> >satellites can't reach: tunnels under cities, etc.
>>
>> Not that limited. Sirius XM has more than a thousand (maybe more than
>> two thousand) such licenses.
>
>I meant "limited" as in "a place where a receiver is unable to get
>signals from a satellite", not limited in number. I'm sure there are
>thousands of highway tunnels,
Sirius XM doesn't transmit from tunnels, they transmit from broadcast
towers and skyscrapers. LightSquared, as we know, would be
transmitting from cell sites. The ancillary terrestrial component
license is not about tunnels, it's about serving broad areas of cities
and suburbs where the satellite signals would otherwise get clobbered
by multipath. (They still get clobbered by multipath, of course, but
the terrestrial signal is strong enough that even in the areas of
destructive interference there's enough signal left to decode.) Here
in Boston they have a site on the Prudential Tower and another on one
of the Newton/Needham towers (I believe 350 Cedar, if I'm remembering
the RFR study correctly).
One relevant difference is that Sirius XM is required by the terms of
its licenses to transmit a uniform signal nationwide, both from their
satellites and from their terrestrial repeaters. Apparently this
doesn't apply to LightSquared.
-GAWollman
--
Garrett A. Wollman | What intellectual phenomenon can be older, or more oft
wollman@bimajority.org| repeated, than the story of a large research program
Opinions not shared by| that impaled itself upon a false central assumption
my employers. | accepted by all practitioners? - S.J. Gould, 1993
Date: Tue, 17 Jan 2012 11:29:11 -0500
From: Bill Horne <bill@horneQRM.net>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: Lightsquared/GPS
Message-ID: <20120117162911.GA18461@telecom.csail.mit.edu>
On Mon, Jan 16, 2012 at 05:25:49PM +0000, Garrett Wollman wrote:
> In article <20120116161242.GA7041@telecom.csail.mit.edu>,
> Bill Horne <bill@horneQRM.net> wrote:
>> I meant "limited" as in "a place where a receiver is unable to get
>> signals from a satellite", not limited in number. I'm sure there
>> are thousands of highway tunnels ...
>
> Sirius XM doesn't transmit from tunnels, they transmit from broadcast
> towers and skyscrapers.
I don't care if they transmit from old CB sets strapped to the backs
of migratory birds! It doesn't matter! Insert <in limited
circumstances where the satellite signal won't do the job> in place of
<tunnels>, and then you'll see the point I'm making!
> Garrett A. Wollman | What intellectual phenomenon can be older, or more oft
> wollman@bimajority.org| repeated, than the story of a large research program
> Opinions not shared by| that impaled itself upon a false central assumption
> my employers. | accepted by all practitioners? - S.J. Gould, 1993
You are working from a false central assumption that an entertainment
service which which was planned, constructed, marketed, and is working
under the terms of its license can be compared with LightSquared's
attempt to construct a terrestrial cellular data network by hiring
lobbyists to convince Congress that it can revoke the laws of physics.
--
Bill Horne
(Remove QRM from my email address to write to me directly)
Date: Mon, 16 Jan 2012 12:51:45 -0800
From: John David Galt <jdg@diogenes.sacramento.ca.us>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: Ringing Finally Ended, but There's No Button to
Message-ID: <jf22l1$g72$1@blue-new.rahul.net>
Bill Horne wrote:
> Cellphones are inherently anti-social devices. Carrying one around and
> answering it in public places is an announcement to the world that you
> think your time and convenience is more important than anyone
> else's.
Answering it (or even letting it ring out loud) in public places is indeed
selfishness -- though if it wasn't your choice to carry the device, it's the
boss's selfishness and not your own. But carrying it? It's unreasonable to
expect anyone to refrain from doing that, because it would have the effect
of depriving them of the device for some time before and after the event.
It would be nice if all such devices had an "absolute silence" setting that
cannot be overridden by the boss, or any app, or anything at all. I wouldn't
even mind if the law required such a capability and that it be easy to find.
At the very least, I would have the law mandate a warning on any device which
does not have such a setting.
As far as the concert, though, the venue could and should have solved the
problem themselves in any of several ways. (1) Require all electronics to
be turned in to the hat-check person before going inside. [Simply banning
them without a 'check' option would be excessive and unacceptable.] (2) The
same, but applied only to devices known not to have "absolute silence"
settings, such as the iPhone. (3) Make the building opaque to radio by
putting a Faraday cage in the walls, or by running a jammer in countries
that allow them (of course, this would not stop "alarm clock" functions if
they are internal to the phone and don't require an outside signal).
***** Moderator's Note *****
This was not a problem that "the venue could and should have solved":
it was a lapse of common sense by a person who took a device that he
hadn't learned how to use into a public gathering where silence is not
only expected, but essential. The person who did that could have, and
should have, obviated the problem by leaving it in his car, or
checking it with his coat, or entrusting it to the concierge.
As to your other suggestions, I stopped being interested in them the
moment you said that the managers of a concert hall should take the
place of individual responsibility.
Bill Horne
Moderator
Date: Tue, 17 Jan 2012 12:14:00 -0500
From: Barry Margolin <barmar@alum.mit.edu>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: Ringing Finally Ended, but There's No Button to
Message-ID: <barmar-0FB467.12140017012012@news.eternal-september.org>
In article <jf22l1$g72$1@blue-new.rahul.net>,
Telecom Digest Moderator wrote:
> This was not a problem that "the venue could and should have solved":
> it was a lapse of common sense by a person who took a device that he
> hadn't learned how to use into a public gathering where silence is not
> only expected, but essential. The person who did that could have, and
> should have, obviated the problem by leaving it in his car, or
> checking it with his coat, or entrusting it to the concierge.
I think you're being way too judgemental. The guy honestly thought that
he'd silenced the device. Was he really supposed to say to himself, "I
know I just put it into silent mode, but just in case it's not going to
do what I said, I'd better leave it in the car."?
He made a mistake.
--
Barry Margolin, barmar@alum.mit.edu
Arlington, MA
*** PLEASE post questions in newsgroups, not directly to me ***
Date: Mon, 16 Jan 2012 12:15:57 -0600
From: bonomi@host122.r-bonomi.com (Robert Bonomi)
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: More on the Captured U.S. Drone
Message-ID: <5fCdneo0aunA9InSnZ2dnUVZ_sydnZ2d@posted.nuvoxcommunications>
In article <4F1310EE.8020106@horne.net>, Bill Horne <bill@horneQRM.net> wrote:
>Another story from Bruce Schneier: this is from his "Schneier on
>Security" blog.
>
>There's a report that Iran hacked the drones' GPS systems:
>
> "The GPS navigation is the weakest point," the Iranian engineer told
> the Monitor, giving the most detailed description yet published of
> Iran's "electronic ambush" of the highly classified US drone. "By
> putting noise [jamming] on the communications, you force the bird
> into autopilot. This is where the bird loses its brain."
>
> The "spoofing" technique that the Iranians used -- which took into
> account precise landing altitudes, as well as latitudinal and
> longitudinal data -- made the drone "land on its own where we wanted
> it to, without having to crack the remote-control signals and
> communications" from the US control center, says the engineer.
>
>Now, here's the part that confuses me: if Iran was able to spoof GPS
>signals to misguide a drone to land outside it's home field,
There is no indication of successful 'spoofing' in the above quote,
nor in the other articles.
The published reports indicate that GPS was simply 'jammed' -- such that the
drone could not get a GPS fix -- and had to 'fall back' to purely 'on-board'
navigation smarts (i.e. "autopilot", as explicitly mentioned in the quote
above).
>to indicate that the drone wasn't using the encrypted military version
>of the GPS signal, which raises lots of questions about just who is in
>charge of designing the guidance systems for these devices, and how much
>Uncle Sam is paying for them.
You're assuming a LOT of things that are not supported by, and are, in
fact, decidedly contrary_to, the published information.
> If all the Iranians did was jam the GPS
>frequencies, which is a much more believable attack, then the questions
>get more pointed and less polite. After all, interfering with military
>communications is as old as the telegraph, so I would have thought that
>a military airborne vehicle would have some internal logic and inertial
>navigation adequate to return it to friendly territory if it lost the
>GPS signals.
Pure "inertial" navigation for anything other than ground-based vehicles
is very problematic. there is no reliable 'self-contained' means for
measuring speed -- the best you can do is estimate, based on integration
from acceleration. And this means that 'position' -- integrated from
speed -- has order-of-magnitude higher uncertainty.
In addition, while you know what direction the aircraft is pointed in,
and what 'airspeed' it is moving at, you simply don't know what
direction it is moving in, or what the TRUE 'speed over ground' is.
One can derive this data, IF one has a reference-point -- at a known
location and distance -- or multiple reference-points at known locations,
without need of knowing distance. (This is basically what GPS does, albeit
with somewhat more 'smarts', to accommodate the the fact that both the
reference and the target are moving, and that one cannot use 'directional'
bearings on the GPS transmitter.)
Unfortunately the 'reference point(s)' can be subject to disinformation
attacks as well. RF emitters can be duplicated, 'terrain recognition'
can be spoofed, if you know (or can figure out) what is being used.
Jam a number of drone flights, and observe where they go -- 'going home'
on autopilot, and you learn the terrain you need to simulate. set it
up, and wait for a drone to show in the right place, jam -it-, and it
follows your faked terrain waypoints.
There are a multitude of design trade-offs in such a vehicle. You have
to consider;
1) compromise of command-and-control links -- where the bad guy can
issue commands to the vehicle.
2) blocking of command-and-control links -- a DOS attack, you can't tell
it what to do, but neither can anybody else.
3) loss of navigation reference data -- how does it know 'where to go from
here'?
Obviously, under 'irrecoverable' circumstances, you want a self-destruct
mechanism. BUT, that is over-reaction to a 'temporary', recoverable,
outage. Deciding 'how' and 'where' to draw that line is NOT easy.
It's also a lot easier to 'armchair quarterback' the decisions after the
fact, than it is to make them in the first place.
Especially, when what you 'think you know' about the other side's capabilities
turns out, AFTER THE FACT, to have been incorrect.
One other possibility to consider -- this just might be a 'disinformation'
event. That the drone had 'special' equipment -meant- to be captured.
Date: Tue, 17 Jan 2012 15:12:29 -0500
From: Bill Horne <bill@horneQRM.net>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: More on the Captured U.S. Drone
Message-ID: <jf4kne$h0g$1@dont-email.me>
On 1/16/2012 1:15 PM, Robert Bonomi wrote:
> In article<4F1310EE.8020106@horne.net>, Bill Horne<bill@horneQRM.net> wrote:
>> Another story from Bruce Schneier: this is from his "Schneier on
>> Security" blog.
>>
>> There's a report that Iran hacked the drones' GPS systems:
>>
>> "The GPS navigation is the weakest point," the Iranian engineer told
>> the Monitor, giving the most detailed description yet published of
>> Iran's "electronic ambush" of the highly classified US drone. "By
>> putting noise [jamming] on the communications, you force the bird
>> into autopilot. This is where the bird loses its brain."
>>
>> The "spoofing" technique that the Iranians used -- which took into
>> account precise landing altitudes, as well as latitudinal and
>> longitudinal data -- made the drone "land on its own where we wanted
>> it to, without having to crack the remote-control signals and
>> communications" from the US control center, says the engineer.
>>
>> Now, here's the part that confuses me: if Iran was able to spoof GPS
>> signals to misguide a drone to land outside it's home field ...
>
> There is no indication of successful 'spoofing' in the above quote,
> nor in the other articles.
Huh? You're kidding, right? Since when does jamming a radio signal
require "precise landing altitudes, as well as latitudinal and
longitudinal data"?
As for the "other articles", the one Bruce Schneier quoted was from the
Christian Science Monitor, a publication not given to exaggeration.
Here's a paragraph from the original article:
Using knowledge gleaned from previous downed American drones and
a technique proudly claimed by Iranian commanders in September,
the Iranian specialists then reconfigured the drone's GPS coordinates
to make it land in Iran at what the drone thought was its actual
home base in Afghanistan.
Which is a straightforward claim by the Iranians that they did exactly
what you say wasn't indicated.
> The published reports indicate that GPS was simply 'jammed' -- such that the
> drone could not get a GPS fix -- and had to 'fall back' to purely 'on-board'
> navigation smarts (i.e. "autopilot", as explicitly mentioned in the quote
> above).
The report that Schneier quoted indicated to me that either GPS spoofing
or jamming might have occurred. YMMV.
>> ... to indicate that the drone wasn't using the encrypted military version
>> of the GPS signal, which raises lots of questions about just who is in
>> charge of designing the guidance systems for these devices, and how much
>> Uncle Sam is paying for them.
>
> You're assuming a LOT of things that are not supported by, and are, in
> fact, decidedly contrary_to, the published information.
Huh? Are you joking? I assumed that the Christian Science Monitor
reported the facts, and that Bruce Schneier, who coined the term
"Security Theater" to describe the TSA, thought that the Monitor's
article was worth lending his weight to.
My comments covered both the possibilities: jamming and spoofing.
>> ... If all the Iranians did was jam the GPS
>> frequencies, which is a much more believable attack, then the questions
>> get more pointed and less polite. After all, interfering with military
>> communications is as old as the telegraph, so I would have thought that
>> a military airborne vehicle would have some internal logic and inertial
>> navigation adequate to return it to friendly territory if it lost the
>> GPS signals.
>
> Pure "inertial" navigation for anything other than ground-based vehicles
> is very problematic. there is no reliable 'self-contained' means for
> measuring speed -- the best you can do is estimate, based on integration
> from acceleration. And this means that 'position' -- integrated from
> speed -- has order-of-magnitude higher uncertainty.
"The best you can do" is one hell of a lot better than an estimate, and
commercial airliners like the 747 were doing it in the /1970's/! The
systems available then, to commercial aviation, were so accurate
that the mid-ocean navigation ships that provided radio guidance to
aircraft started asking the 747 pilots to tell them where they were!
THAT WAS WHAT AIRLINES HAD ACCESS TO, IN THE 70'S!
> In addition, while you know what direction the aircraft is pointed in,
> and what 'airspeed' it is moving at, you simply don't know what
> direction it is moving in, or what the TRUE 'speed over ground' is.
>
> One can derive this data, IF one has a reference-point -- at a known
> location and distance -- or multiple reference-points at known locations,
> without need of knowing distance. (This is basically what GPS does, albeit
> with somewhat more 'smarts', to accommodate the the fact that both the
> reference and the target are moving, and that one cannot use 'directional'
> bearings on the GPS transmitter.)
>
> Unfortunately the 'reference point(s)' can be subject to disinformation
> attacks as well. RF emitters can be duplicated, 'terrain recognition'
> can be spoofed, if you know (or can figure out) what is being used.
Spoken like a true theoretician: overcomplicated, assuming systemic
perfection is required, and facts not in evidence. <grin>
Look, this isn't that complicated: all that is required is some simple
logic to cover GPS failures. Before takeoff, the drone can be programmed
to turn around and run away if its GPS fails.
Here are the failure modes that come to mind, just off the cuff:
1. Drone shot down
2. Drone hits obstacle
3. Drone runs out of fuel
4. Controller doesn't regain control
5. Enemy exploits recovery logic to force aircraft to
remain in area until it runs out of fuel
... and all of the possible failures are better than having an
intact drone fall into enemy hands.
> Jam a number of drone flights, and observe where they go -- 'going home'
> on autopilot, and you learn the terrain you need to simulate. set it
> up, and wait for a drone to show in the right place, jam -it-, and it
> follows your faked terrain waypoints.
I'm sure that t'rain-following guidance systems are important in cruise
missiles. I'm equally sure that they're not called for in drone
aircraft, but for the sake of argument, let's assume that these drones
have terrain-following capability - after all, how else could one of
them have landed without its GPS and while out of touch with its
controller? I'm also sure that not even jihadists can move a mountain.
> There are a multitude of design trade-offs in such a vehicle. You have
> to consider;
>
> 1) compromise of command-and-control links -- where the bad guy can
> issue commands to the vehicle.
That's what encryption is for.
> 2) blocking of command-and-control links -- a DOS attack, you can't tell
> it what to do, but neither can anybody else.
See above: the drone can simply be programmed to back up until the
blocking doesn't work anymore.
> 3) loss of navigation reference data -- how does it know 'where to go from
> here'?
Again, see above. Where it needs to go is "away from the jammer".
> Obviously, under 'irrecoverable' circumstances, you want a self-destruct
> mechanism. BUT, that is over-reaction to a 'temporary', recoverable,
> outage. Deciding 'how' and 'where' to draw that line is NOT easy.
Actually it is: if the bird's maximum flight time is over and it's not
back, then it's a good idea to blow it up.
> It's also a lot easier to 'armchair quarterback' the decisions after the
> fact, than it is to make them in the first place.
Quarterbacks play a game, and even though football is certainly a
rehearsal for war, the outcome of any game is as unimportant as the
comments made by armchair quarterbacks.
I however, am paying half my income in taxes, and I reserve the right to
criticize the decisions made by supposedly competent designers,
generals, and/or politicians. If they have so much difficulty making a
decision to install what I feel are obvious, cheap, easily implemented
safeguards in remotely-operated drone aircraft, then I can only suggest
that they're in the wrong line of work, and should pursue other careers.
> Especially, when what you 'think you know' about the other side's capabilities
> turns out, AFTER THE FACT, to have been incorrect.
I think I know a blatant exercise in spin-control and demagogy when I
see it (not you, Robert, them). I think I know that any guidance
system that can be used as a way to capture a valuable drone aircraft is
incompetently designed. I think I know that taxpayers have been taking
it on the chin and that defense contractors laugh themselves to sleep
every night. I didn't know these things before the fact, but I did
suspect them, and now I'm sure.
> One other possibility to consider -- this just might be a 'disinformation'
> event. That the drone had 'special' equipment -meant- to be captured.
Yeah, right. Tell me another one. No, really, please, tell me another
one: the very idea that delivering a military asset to people who would
put it up for auction on the international military marketplace, during
an election year, could be anything but a massive cock-up of the lowest
order is funny.
Go ahead, tell me another one. I'm just a taxpayer, and therefore
extremely gullible and stupid.
Bill
--
Bill Horne
(Remove QRM from my address to write to me directly)
Date: Mon, 16 Jan 2012 21:09:03 -0600
From: Neal McLain <nmclain@annsgarden.com>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: English Wikipedia anti-SOPA blackout
Message-ID: <4F14E64F.9000800@annsgarden.com>
From Wikipedia:
| To: English Wikipedia Readers and Community
| From: Sue Gardner, Wikimedia Foundation Executive Director
| Date: January 16, 2011
|
| Today, the Wikipedia community announced its decision to black out
| the English-language Wikipedia for 24 hours, worldwide, beginning at
| 05:00 UTC on Wed, January 18. The blackout is a protest against
| proposed legislation in the United States -- the Stop Online Piracy
| Act (SOPA) in the US House of Representatives, and the PROTECTIP Act
| (PIPA) in the U.S. Senate -- that, if passed, would seriously damage
| the free and open Internet, including Wikipedia. This will be the
| first time the English Wikipedia has ever staged a public protest of
| this nature, and it's a decision that wasn't lightly made.
Continued at
http://bit.ly/x6qWHC
***** Moderator's Note *****
The Wikipedia blackout with start at Midnight Wednesday, in the
Eastern Time Zone of the United States.
I understand why Wikipedia is taking this step: SOPA is a bad bill,
and is being backed by very rich people who want to stifle creativity
at the same time they arrogate the ability to censor dissent.
I support Wikipedia's decision to oppose SOPA. I hope you will, too.
Bill Horne
Moderator
Date: Tue, 17 Jan 2012 14:32:46 +0000 (UTC)
From: "Adam H. Kerman" <ahk@chinet.com>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: Ringing Finally Ended, but There's No Button to
Message-ID: <jf40qe$tm2$2@news.albasani.net>
>Ringing Finally Ended, but There's No Button to Stop Shame
>By DANIEL J. WAKIN
>January 12, 2012
>They were baying for blood in the usually polite precincts of Avery
>Fisher Hall.
>The unmistakably jarring sound of an iPhone marimba ring interrupted
>the soft and spiritual final measures of Mahler's Symphony No. 9 at
>the New York Philharmonic on Tuesday night. The conductor, Alan
>Gilbert, did something almost unheard-of in a concert hall: He
>stopped the performance. [Moderator snip]
>
>
>http://tinyurl.com/7hazd7s
>
>***** Moderator's Note *****
[Moderator snip]
>The "Patron X" mentioned in the NY Times story claimed that his
>employer had just switched him from a blackberry to an iphone, and
>that the sound was actually an alarm signal, not the phone
>ringing. Sounds like revisionist history ex post factotum to me, but
>even if it's true, that only proves that the person in question felt
>entitled to take an active electronic device into a concert hall
>without bothering to RTFM. The [gall] of some people!
I carry a cell phone that just makes phone calls. It has an alarm function.
If the phone is turned off but the alarm is set, the phone wakes up and
sounds the alarm at the time set.
It's still unbelievable that the alarm could get set without it being
the user's responsibility.
Date: Mon, 16 Jan 2012 17:49:37 +0000 (UTC)
From: wollman@bimajority.org (Garrett Wollman)
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: Lightsquared/GPS
Message-ID: <jf1nvh$2i6b$1@grapevine.csail.mit.edu>
In article <YLudneJVI4C5zYnSnZ2dnUVZ_vadnZ2d@posted.nuvoxcommunications>,
Robert Bonomi <bonomi@host122.r-bonomi.com> wrote:
>Unfortunately, what constitutes 'interference' is not sufficiently precisely
>defined in the FCC regulations, with regard to causing problems with other
>licensed operations.
Actually, it is very precisely defined in the FCC regulations: when
one transmitter's specified electric-field "interfering" contour
overlaps with another transmitter's specified "protected" contour,
interference is deemed to result.
>E.g. who is responsible for 'intermodulation interference' caused by
>the mixing of two unrelated RF signals (both from licensed
>transmitters0, occurring in a passive device (like the 'structural
>steel' of a building) and being re-radiated on the frequency of a
>third licensed transmitter?
The permittee of the third element to be built, when it is being
constructed, is responsible for resolving such interference. If the
building is the most recent, then the building owner must resolve the
issue. If the second transmitter is the most recent, then the
permittee of that transmitter must resolve it.
>Suppose the interference is not 'exactly' on the frequency of the third
>transmitter, just 'close' to it?
>
>_Maybe_ one can, by using sufficiently 'fancy' (read as "large", "bulky",
>and 'expensive') filtering devices, siphon off enough of the 'interference'
>to be able to recover the 'licensed' signal, and maybe NOT.
Broadcasters do this all the time; there is no magic, and the primary
difficulty is usually in getting the people who built the new structure
-- often a cell tower! -- to admit their responsibility. Installing a
wire skirt to detune a cell tower is pretty simple. (Broadcasters
have to deal with this even without intermodulation, because
structures like cell towers can reradiate broadcast signals and thus
distort the station's directional pattern beyond tolerance.)
>Taken to extremes, I wouldn't want to guess as to the cost, or size,
>of a GPS receiver that required a separate antenna, each with it's own
>'cavity resonator' narrow-band filter for each GPS transmitter frequency.
There's no reason you would want to do that. Maybe you might want
to put a GPS passband before the front end, just to reduce the risk of
overload. Since consumer GPS receivers seem to be single-chip
devices, there may not be any way to improve their IF filtering
without redesigning the whole thing. (What IF frequency do they use,
anyway? There are good mechanical filters available for the usual
choices of communications receivers, but none of them will likely fit
in a smartphone, which is the important application here.)
-GAWollman
--
Garrett A. Wollman | What intellectual phenomenon can be older, or more oft
wollman@bimajority.org| repeated, than the story of a large research program
Opinions not shared by| that impaled itself upon a false central assumption
my employers. | accepted by all practitioners? - S.J. Gould, 1993
Date: Mon, 16 Jan 2012 19:55:23 +0000 (UTC)
From: Colin <colin_sutton@ieee.org>
To: telecomdigestmoderator.remove-this@and-this-too.telecom-digest.org.
Subject: Re: Ringing Finally Ended, but There's No Button to
Message-ID: <jf1vbb$6mr$1@adenine.netfront.net>
David Clayton <dcstarbox-usenet@yahoo.com.au> wrote:
> On Sun, 15 Jan 2012 08:58:55 -0500, Barry Margolin wrote:
> ..........
> I have some sympathy for this person, I use an Android phone and while it
> has various "Silent" and "Airplane" modes that seem to work for "normal"
> things like incoming calls I have also installed various apps to signal me
> on things like incoming e-mails and I am not sure that all of these use
> the aforementioned settings.
>
> All it may need is one lazy app developer to bypass the bits that control
> to alarm modes and then you have a device that can only be shut off with a
> hammer (or the power button....)
>
The iphone, blackberry and nokia all ignore the off switch to sound an
alarm that you have set.
--
Colin
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