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

Sun, 14 Apr 2002 22:43:14 -0400

On Sun, Apr 14, 2002 at 07:37:32AM -0400, Denis Trapp wrote:
> 
> 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.

	My understanding is that they have proven to be quite a bit
of a bust for the carriers who invested in providing the system.  I 
don't think it has been anywhere nearly as heavily utilized as the
carriers hoped it would be.

	If one looks at the relevant spectrum with a spectrum analyzer
at least here in the Boston area - a pretty high traffic area for
airline travel considering that some traffic over the NYC area is above
the radio horizon here if it is high enough, and Europe bound traffic
passes over off the coast - you typically can see maybe 3 to 7 or 8 
calls active at any one moment on the entire visible system (all 20 200
khz chunks) during most of the active travel periods.    This (in my
case) is airborne traffic either at flight altitudes (up to 230 miles
from me) or airborne traffic into or out of Logan - traffic while on the
ground does not reach me here.   Looking near Logan with a spectrum
analyzer one sees perhaps 1 or 2 to maybe 9 or 10 calls max up on the
Logan sites (there are two quite near each other - one for each
carrier), most of which seem to be to aircraft on the ground at Logan.

	Of course rates are pretty high and audio quality is not
all that great, and the mechanics of making a call not quite as simple
as a cellphone either.   And receiving incoming calls, while possible,
is quite awkward and limited.

	As I understand it, there has been some thought given to 
providing micro cell sites on board planes that would allow use of
regular passenger cellphones or PCS phones via some kind of gateway that
would link to the ground over either the existing air to gound system,
Ku or Ka band satellites, Iridium/Globalstar or some new allocation.  
By having the cell site on the plane command the phones to very low
transmit power levels it might be possible to ensure they would not
interfere with ground cell sites, which is the primary reason use of
cellphones on planes is banned (by the FCC rules, not just FAA).
I don't know if anyone has actually done the various engineering and
regulatory work to establish that such a service would work and would
not interfere - but I have read that some people are exploring it...

	

> 
> 
> 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,  [email protected]  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:[email protected]
>                       BC780XLT/BC245XLT
>                     ICQ:  5625443
>                    http://www.qsl.net/n8wsh
> 
>                  GOD BLESS AMERICA!!!
> ======================================
> 
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-- 
	Dave Emery N1PRE,  [email protected]  DIE Consulting, Weston, Mass. 
PGP fingerprint = 2047/4D7B08D1 DE 6E E1 CC 1F 1D 96 E2  5D 27 BD B0 24 88 C3 18