[Scan-DC] Airphones (Re:BC895XLT Question)

[Denis Trapp]

A question regarding those Airphones in the back of the seats...How much do 
they really get used? Has anyone seen frequent use of them?

My personal experience flying both domestically and internationally, is 
that I have never seen anybody use one. Most people are firing up the cells 
phones as soon as the plane comes to a stop.


At 11:51 PM 4/13/2002 -0400, you wrote:
>On Wed, Apr 10, 2002 at 02:16:57PM -0400, William Rossiter III wrote:
> > I decided to listen to the 800 MHz band today on my BC895XLT.  My scanner
> > does not cover between 868 and 894 MHz because that is a cell phone band.
> > But when I listened between 894 and 895, I heard telephone conversations.
> > How is this possible if my scanner blocks the cell phone band?  What was it
> > that I was listening to?
>
>         In the USA, the 894-896 band is used for the air to ground
>downlink for those seat back phones in airliners.   It is paired
>with 849-851 mhz for the ground stations, most of which are located
>at or within a couple of miles of big airports (so they provide coverage
>on the ground as well as while flying).
>
>         The modulation used here is digital SCPC (4800 baud QAM) in 6 khz
>wide channels - each ground station is allocated a 200 khz wide block in
>which all its uplinks are located.   There is usually a continuous
>control channel on the high  end of the 200 khz wide block - sometimes two.
>
>         This digital system replaced an old analog system which used
>SSB AM with a partially suppressed carrier, also in 6 khz spaced
>channels.   A few years back there were still a very few aircraft around
>with the old equipment, but I think they have been entirely phased out
>by now and all traffic is digital.
>
>         What all this means is that almost certainly did not overhear
>the setback phones (AirFone (Verizon/GTE) and AT&T being the two big
>carriers), but some kind of spurious response from your scanner to
>nearby analog cell traffic.  This would be especially true if you heard
>the conversations in nbfm mode of the scanner as the old air to ground
>traffic before the digital conversion would appear to be in AM or SSB
>mode and not intelligable in fm mode (though it might sound vaguely
>speechlike but very very distorted).  The digital signals are a distinctive
>hissy buzzing noise - don't sound at all like speech.
>
>         Needless to say, depending on how near the nearest 800 mhz cell
>site carrying analog traffic is and how old and cheap your scanner is
>there may well be images or other more complex spurious responses that
>make cell calls seem to appear between 894-895.  Very new scanners are
>required to be designed so these spurious responses do not appear except
>when signal levels are very very high (eg the antenna is just outside
>the window), but some less expensive older scanners have image responses
>to cell phone traffic that aren't all that many db less sensitive than
>if they really were allowed to tune the cell band directly.
>
>
>--
>         Dave Emery N1PRE,  die@die.com  DIE Consulting, Weston, Mass.
>PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 
>C3 18
>
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======================================
                     Denis A.Trapp/N8WSH
            mailto:SCAN@MEDIEVALBEAR.COM
                      BC780XLT/BC245XLT
                    ICQ:  5625443
                   http://www.qsl.net/n8wsh

                 GOD BLESS AMERICA!!!
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