Date: Sun, 3 Sep 89 18:05:45 -0500
From: George Goble <ghg@en.ecn.purdue.edu>
Subject: Previous Experiences of Cellular & Trunk Radio in the Air

Up to a year or so ago, it was legal (FAA rules) to operate a cellular
phone (or trunk radio) onboard a private aircraft provided the "pilot
in command" determined no harmful interference occured.  When I was the
pilot in command this was no problem. In the last year or so, I believe,
the FCC has made it illegal to use cellular phones in flight, even
on private aircraft, for reasons which will follow. 

FCC rules (3 or 4 years ago) for 800 Mhz "trunk" radio systems allowed 
the use on aircraft "commonly flown below 5000 feet", but didn't mention an
altitude restriction.  The "trunk" system is a predecessor to the
cellular system (operates 855-870 Mhz base xmit, mobile xmit is 45 MHZ
lower, cellular is 870-890 Mhz base xmit, mobile 45 Mhz lower). Trunk
systems are widely used for police, emergency, and general business
"dispatching".  They use computer control which picks a "private"
(not from scanner listeners, but other users of the trunk system) channel
from a pool of channels and the user appears to have his own dedicated
channel for the duration of the conversation. Some systems even allocate
the channel everytime you key the mike. Portable trunk radios may not
be taken from city to city, and merely "turned on and used" as with a
portable cellular phone.  They are also push-to-talk (half duplex) and
are generally 3W of power vs 800 MW for portable cellular.  Trunk radios
can initiate/receive phone calls via a "telephone interconnect" in the
base end.  There are no "handoffs" possible, you have to stay in range
of your home system (20-35 mile radius)

5 or 6 years ago (before cellular days in Lafayette, IN), I tested
my GE MARC V  3W portable trunk radio in the air.  The ground range
was only 15-20 miles. I had no trouble keying up the repeater in Lafayette
from 6000 feet AGL (above ground level) over Richmond, IN (100 mi away).
Trunk systems are (or were) too sparse to have problems with adjacent
station interference like cellular.  Radio signal was fine, except for
a little "flutter", since I was talking through the prop. The worst
problem was cockpit noise, which made the other party have a hard time
hearing me (this happened on local flights also).

This trunk radio contributed to flight safety once.  A flight instructor
and I had to ferry a set of clothes of a co-worker of ours who had
passed away, from Lafayette to Muncie, IN (60 MI east) for his funeral.
A massive cold front was rapidly approaching from the NW, with a towering
line of heavy thunderstorms, tops above 45,000 feet.  We took off
and made it to Muncie with no problems.  The return flight was more trouble.
As expected, the line of storms was now over Lafayette, and closing in on
Muncie.  A call to Flight Service on the ground (in Terre Haute, IN)
returned constant "All circuits busy" when we tried to get weather and
file a return IFR flight plan.  LD calls to anywhere from Muncie ended
up with "All circuits busy" or "equipment failure" recordings.  Hoping to
get the weather by radio, we took off VFR (visual flight rules) and
headed SE, the only clear area left.  We tried to contact FSS (flight service
on radio), no response. We finally raised Indianapolis approach who said
there had been a massive lightning strike on the telephone system, and
all long distance, and many air traffic control landline ciruits were down.
Indy approach could not get any "weather" radar reports from anyplace.
They made a guess using their approach radar (which is not very good
at seeing "weather"), and gave us vectors back to Lafayette.  After
rolling out on the new heading it was very evident that we were
headed (at night) for a 45000 foot high wall of storms, lit up from
top to bottom with constant lightning.  Next plan was to set down
at Indy for the night (they were still in the clear).  I whipped out
the portable trunk radio, about 80 Mi SE of LAF, called a friend,
and had him tune his TV to the "weather channel" and give us the live
radar report.  This indicated a "hole" in the storms about 20 Mi
to the west, so we took that route and returned safely and even had
no turbulence!

When cellular arrived, I bought a Motorola 8000 ("F" series) portable
phone. Even though only .8W power, it far outperformed the GE trunk
system (3W), due to superior Motorola RF design.  The noisy cockpit
sounded no worse than a computer room and conversations were no problem.
(the cell phone has an excellent noise cancelling mike)

I never saw any instrument glitches caused by either radio (but this does
still does not make it legal for Airline use).  Transmitting with the aircraft
radios, would make all the navaids peg-out while the mike was keyed though. 
Not sure what power Aircraft radios use, but it must be 10-30W.  For example, 
VOR receiver was tuned to 115.6 Mhz and we were transmitting on 119.6 Mhz, and
the VOR wiped out when we keyed, this is to be expected with close freqs. To 
be safe, we didn't use portable radios during instrument critical phases of 
flight (ILS approach).

Portable FM receivers, have a "local oscillator", usually 10.7 Mhz above the 
station being tuned.  Tuning a portable FM RECEIVER from 97.3 thru 107.3 
causes the local oscillator to sweep the entire 108-118 Mhz "VOR" navaid band!
Local oscillators many times "leak" out and can be heard for tens of feet.  
Now imagine what happens when you are seated next to the VOR antenna, with  
your FM radio on, and the LO over powers the signal of a VOR which is 80 miles
away.  You know where the rules came from.

Somebody once called me on the cell phone while on final approach:
       "Hello, hang on for 30 sec... I have to land this plane first."

They thought it was a joke at first.

I once had radio problems while flying, and I called the control tower on the 
cell phone while airborne to report our position and get the next clearance.  
At first, they freaked, nobody had ever called them on the phone from an 
aircraft before!

The local air traffic controllers really thought this cell phone was neat 
after that, and were always ribbing me about "which shoe do you wear the phone
on?", when we would go up flying.  They even found out my number, and would 
call it and crack jokes while we were in the air in the local area.

We have no radar here, so the tower has to call out everybody's 
position for collision avoidance. I once heard something like:
"Centurion two-four-eight-five-uniform, traffic will be a Warrior
departing westbound off runway two eight", "Purdue 3, traffic is a
Cellular equipped Centurion northwest of the field, on the VOR alpha
approach and just reported Boiler inbound. Cleared for takeoff."

A couple of years back, while climbing out of 4000' SE of Lafayette, one
of the passengers noticed an overturned Semi on I-65.. Weather was
severe clear, and I could see cars backed up for 15-20 miles or so.
The previous night I received a brochure from GTE mobilnet, in 
cooperation with Indy radio station WIBC (1070) for car phone users
whom notice traffic problems to dial "1070" on their car phone
(free call) and report it to WIBC to suplement their chopper in
the sky traffic reporter ("Big John Gillis"?).  Perfect "traffic
report" time.  Dialed in 1070-SEND, WIBC answered, we gave them the
traffic report of the wreck, they almost freaked, and the 20 mile
backlog. They said they knew of a problem there but had no idea
of the backlog. Hung up, dialed in "1070" on the ADF receiver
and listened to WIBC. About 30 seconds later they had "a special report
from a Cessna leaving Lafayette.." gave the report, then noted that
their helicopter traffic reporter "Big John Gillis", was stuck in
that traffic jam and could not get to his copter to go to work.

My cell phone can be put into "maint mode", where it shows the
channel in use and the received signal strength. When the phone
is not inuse, it listens to the strongest "paging" channel, and this
can be used to determine which cell site you are talking to.
Signal strength reads from 13 (nothin') to 50 (at the cell site).
Previous postings to the group about "mushroom" cell coverage
areas seem pretty accurate.  It seems more like a "donut".  They
do not want to waste power transmitting "up", but put most of it
out in a "disc" toward the horizon.  When flying, it is important
to "lock down" the cell phone to a single system. You can force it
to only work with one system, by entering the system ID.  If you
don't lock it down, it would sometimes lock up to Chicago while
far south as Indy!  Cells that you are close to are weaker then
those 60-80 mi away!  It seemed that no matter where in the
midwest I was, I was always seeing SOME cell site with a signal
strength of 40-45! This seems be similar to the "silent pileups"
reported by the Astronauts working 2 meter Ham radio, when 
the whole country tried to contact them at once.

The problem doesn't seem to be going "out of range" of a cell site
but the FM capture of a stronger site overpowering it. A couple
of years ago, I was "roaming" over southern Ohio, at 10,000',
and punched off 5 or 6 outgoing calls in rapid succession, and kept 
careful records.  The roaming charges came in from "Columbus,
Cincy, Lexington KY, Philadelphia, and Louisville KY in a period
of 4 mins.  It would be a real pain to receive incoming calls
in this manner, "follow me roaming" would get real confused I bet!

When flying over Indy once, one could see the channel number
(during a live call) flip every 14 sec, indicating constant
handoffs were taking place, bet the techs had a fit when they
saw somebody handoff from the NW corner to the SE corner of the
city!

When one takes the time and thinks about things, it is clear
why the FCC recently (outlawed?)  cellphones in private
aircraft recently. The last I heard was this was in a "proposed"
rulemaking about a year ago.  

In normal ground use, a cellular mobile is talking to his strongest
cellsite (cell A) .  Surrounding cellsites are constantly monitoring 
the mobile's signal strength, and the switch decides to hand 
off to another cellsite (cell B) based on this information. At 
some still farther distance past the neighboring cell (cell B)
sites, the original cellsite's (cell A) frequencies (and paging channels)
be reused at still other cellsites (cell C).  In the rural midwest,
GTE's cellsites have approx 30 mi radius. In 100-120 miles
a frequency may be reused (in cities this shrinks down somewhat, and
the power and tower height is reduced)

CELL                      CELL                     CELL
A--- 30 mi ---><--- 30 mi --B--- 30 mi--><--- 30 mi--C ...
LAFAYETTE              LEBANON			GREENFIELD
CH 339		         CH 352			  CH 339

The cellular system is based on the premise that CELL "C" (which is
using the same freq set as CELL A) CANNOT hear anybody who is using
CELL A.  If you are flying, CELLs A, B, C are illuminated at approximately
equal strength.  If your call setup and is on CELL A, CELL C may be
used for another conversation (to a ground mobile).  Suddenly, you
tilt your radio a little, and CELL C starts hearing you stronger
(100 mi away), than the poor guy on his car phone. The FM capture 
effect means you just "took over" his conversation.  However, FM capture
will probably prevent you from hearing his conversation on cell C,
since you are closer to CELL A.  With movement and turbulance, you 
may briefly hear CELL C stronger than A for a short time and possibly
"butt in" on this call.  A "butt in" is pretty unlikely since the system
uses a small number of different "SAT" or "pilot" tones (around 6Khz)
on the audio.  What probably will happen, will be that you are heard
on CELL C stronger than the car phone, but your SAT tones will not match,
and the cellular switch will detect a "fatal error" and drop the connection
and log a trouble on the poor guys car phone.  All this takes a few seconds
to happen, so you may get bits and pieces of other conversations.

As you approach a larger city, such as Chicago, you now illuminate, say
120 or more cellsites, with almost equal intensity.  One phone in the
air may cause hundreds of calls to drop on the ground for the above
reasons.  Also when you hit "SEND" and initiate a call, you have 
a bunch of cellsites all hearing you on the same control channel (impossible
on the ground), and all feeding into the same cellular switch, causing
confusion or maybe even crashing the switch. I never used a cell phone in the 
air even close to Chicago for the above reason.  My "testing" over Indianapolis
was done before they were big enough to reuse frequencies (I checked first). 

No wonder the FCC wants cell phones to stay out of the air, especially near 
big cities.

Anyway, this is all probably illegal now, as it should be, but it was fun 
while it lasted.

--ghg
Geo. Goble