Archived posting to the Leica Users Group, 2001/10/04

[Author Prev] [Author Next] [Thread Prev] [Thread Next] [Author Index] [Topic Index] [Home] [Search]

Subject: RE: [Leica] Re: New Aviation Rules
From: Peter Klein <pklein@2alpha.net>
Date: Thu, 04 Oct 2001 14:50:53 -0700

I'm a long-time ham radio operator, so I've got some experience in this 
area, The "range" of any radio device is meaningless unless we talk about 
the terrain, and the height above it that the transmitter and receiver are 
located.  Put even a very weak transmitter high up, and it can get much 
farther than its specs or actual performance over "average terrain" would 
indicate.

Case in point:  Several ham radio operators in Latin America and Hawaii 
(where there was no competition from higher powered stations) routinely 
talked to the Mir space station while standing in a parking lot, using a 
handheld transceiver with a couple of watts of power.  Over flat land, the 
range of the hanheld would probably be 10-15 miles, tops.  I don't remember 
the exact height of Mir, but it was at least 125 miles and probably more.

I myself have had several conversations with Shannon Lucid and John Blaha 
aboard Mir from my home using power and antenna gain that could not travel 
nearly that far over normal terrain.  I have also stood on a 5300 foot 
mountaintop and spoken with another ham over 100 miles away using 2 watts 
of power.

Given a clear line-of-sight, VHF and UHF transmissions can go a lot farther 
than experience on the ground would indicate.  Cellular phones are weaker 
than the examples cited above, but from a plane you're talking about clear 
line-of-sight once the signal gets out of the plane.  A plane at 30,000 
feet has a radio horizon of over 200 miles.  Trust me, a cell phone on a 
plane could conceivably trigger every cell tower within several tens of 
miles.  In rural areas, this probably wouldn't matter.  But near an urban 
area, with the position of the plane changing rapidly, it could confuse 
hell out of the cellular system.

One other point--if a cell phone shouldn't work from a plane, how do we 
explain the numerous cell phone conversations from the doomed airliners on 
Sept. 11?  I don't think all of them were using the plane's own 
radiophones, and several of the people who talked to the survivors said 
that so-and-so called on "his" or "her" cell phone.

As far as interference goes, the issues become very complex.  In the 
presence of any two imperfectly connected conductors, which act as a 
rectifier, any radio signal can mix with any other radio signal to create 
sum and difference frequencies, or radiate multiples (harmonics) of the 
initial frequency.  Cell phones don't operate on the same frequencies as 
navigation equipment.  But mixing effects within equipment or nearby 
objects can create other signals which could conceivably interfere with 
navigation.  On a plane, where one "oops" can get you killed, it's probably 
best to avoid random and uncontrolled radio signals as much as possible.

As for computers, CD players, etc:  Any digital device has clocks which 
create square waves--waves that are turning on and off instantaneously 
rather than oscillating smoothly.  Square waves are by their nature loaded 
with harmonics.  And there are many such square waves in digital devices, 
because clock frequencies are routinely multiplied and divided to get 
various timing pulses.

Shielding and bypassing can eliminate the radiation of such radio "hash" 
for all practical purposes.  But given the marketplace, devices that have 
been made cheaper by not including such refinements are going to be 
around.  For a real eye-opener, try putting a portable shortwave radio next 
to your computer and tune around.

Laptops are more tightly controlled and tested than the cheap consumer CD 
player you picked up at K-Mart for $19.95, which is why the former may be 
allowed and the latter not.

Finally, U.S. FCC Amateur regulations state that no amateur radio 
transmitter may be operated on board a ship or plane without the permission 
of the captain or master.  I believe there are similar restrictions in most 
other radio services.  So whether there's a good reason or not, the pilot 
has discretion, and he's bound by the policies of his company.  As stated 
above, there are good reasons.

- --Peter Klein
Seattle, WA

> > My understanding--I'm sure I've read this in more than one place--is
> > that at an airliner's altitude, cells over a large fraction of the
> > country would pick up transmissions from the cell phone.  This would
> > really screw up the system.  I also read that the prohibition is an
> > FCC, not an FAA, regulation.

Austin says:
>I don't believe that's right.  Cell phones typically only have a 1 mile
>range.  I use one on my boat quite frequently, but more than a mile or so
>off shore...it doesn't work.



- --
To unsubscribe, see http://mejac.palo-alto.ca.us/leica-users/unsub.html