Archived posting to the Leica Users Group, 2001/10/04
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Re cellular phones and aircraft interference, let me clear up a few
misconceptions.
GSM phones perform regular location updates and send measurement reports
back to the system using a crazy burst of signals at about 217Hz (modulated
at the carrier freq). Any electrical apparatus with non-linear components
(e.g. a diode) will demodulate this, and here's your interferer. Also known
as the 'galloping horses' effect you might hear from time to time if you sit
your GSM phone near a radio or amplifier. Hospitals with fancy electronic
equipment are frightened by this (with reason), and so to are gas stations
(with good reason). And I guess this can potentially interfere with ANY
sensitive instrumentation on an aircraft. I'm not sure about CDMA though. I
don't think it is an issue with TDMA phones (eg digital AMPS). Anyway, air
lines don't care what system your phone uses.
And regarding the phones chewing up capacity when in the plane at altitude
... it just doesnt work like that. A phone in a high rise can be 'heard' by
dozens of base stations, but it will only ever use a carrier from one of
them when it makes a call. The system chooses the most appropriate base
station, based on signal strength, quality and other parameters. Handing
over between cells is not an issue either. In GSM and TDMA systems,the phone
only ever uses on base station and one pair of frequencies at a time,
regardless of how fast you move ('hard handover'). And it is possible to
move so fast that the system cant hand you over to the next base station
fast enough - and the call will drop out. The limit depends on your speed
and the density of base stations on the ground. With CDMA phones however,
handovers are different, as the system is designed to support the phone
communicating via two base stations at the same time ('soft handover'). So
using the phone in flight is not likely to impact system capacity.
Also, the radiation pattern from base stations is aimed towards the ground,
not up in the air. If you are lucky enough to snag a base station from the
air, you usually have to be flying low (less than 110 storeys?) or the
network operator has installed his antennas upside down. And there is also
the possiblity that you will pick up the back lobe of an antenna - front
lobe points down, so back lobe (at much reduced gain) points into the air.
I hope this confuses things.
Regards,
Mark Wrigley
AUSTRALIA
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