My relevant experience
I have worked for several communications companies, and have been in the business over 15 years.
First as a bench technician, repairing cell phones, 2 way radios, setting up microwave, 2-way, and paging systems,
troubleshooting base stations (cell sites).
Base station troubleshooting is very complex, and needs a electronic background,
as well, as an understanding of RF (radio Frequency)
The last few years i worked as a design engineer, and senior design engineer
(although i have been out of the business for about 2-3 years now)
also doing a LOT of "in building testing" and RF design, for places like the New Orleans Superdome,
Convention Center and a couple of airports.
The in-bulding testing gave me a lot of insight as to what effects a barrier,
such as a wll, metal, concrete, or other had on RF frequencies.
(ever try to use a cell phone in an elevator ?)
Some technical notes:
CDMA, GSM, and ANALOG are different "technologies"
but, there are also different frequency ranges associated with different carriers.
(SPRINT, NEXTEL, etc...)
So, if Sprint owned the airspace frequency range for 1900mhz in Pittsburg,
then they had a choice still of which technology they wanted to use (CDMA, GSM etc...)
USUALLY the 1900mhz span was used with CDMA (digital) and 800mhz was used by analog phones.
Now, by sept 2001 MOST
cell phones were working on CDMA at
I am sure there were some GSM used (and still are), and they could have been at 800mhz OR 1900mhz
The reason all this is
important is because the RF signal works a
bit differently with different technologies,
and on different frequencies, working differently on barriers, distance etc....
Different systems -
Since different carriers may use different frequencies AND technologies,
when you cross a border, the phone drops. PERIOD.
(border handoffs work in many places NOW, but not in 2001)
the different technologies, phones and frequencies could NOT cross carriers boundaries.
(if bought a phone in New Orleans, and flew to Baton Rouge,
the New Orleans system could NOT handoff to the baton rouge system)
now, you could re-originate the call and be ROAMING, but as soon as you go over the border it drops.
i don't know exactly what the borders for different carriers were in 2001, in Pa
but going 500mph, you are sure to cross one in 1/2 hour at the MOST i would guess.
I have HEARD of
people using cell phones on planes, but i never got a
clear answer as to the altitude,
except one guy who flew a cesna at 2,000ft (or under ?)
There are a few different
types of phones,
GSM, CDMA, and Analog.
analog is the OLD type, CDMA is digital.
CDMA uses 1 frequency, for everyone to talk on, but each conversation is CODED differently.
Here is another article i wrote with some technical details.
I am a senior design RF cell phone engineer ( cellular/CDMA engineer) and have designed systems for Sprint, Verizon and Alltel. I have 2 FCC licenses and have worked in the industry for 13 yrs. That out of the way...
OK all of the new handsets are .6 to 1 watt max.
From what i read several calls were made on flight 93 some were reported to last as long as 20 minutes. NO WAY !
We design these systems to work on the ground, usually providing "downtilt" to the antenna, so the signal gets pointed downward.
There are calculations to do this. The range for a call site is 5 to 20 miles max, but that is on the ground. The handshaking is now referred to in digital as soft handoff, or hard handoff if you are going between systems, or in an analog mode.
most phones today are now digital, though some can do both. for a handoff to occur( going from one cell to another) the system needs to have a neighbor list setup, telling the phone which cell is next on the list( i have made around 100 of these). at 500mph, the phone would have passed several sites and it would try to handoff to a site already passed. The phone can be in handoff with up to 3 sites at one time, but generally this can make things worse as it will degrade the signal.
Also inside of a metal plane, the signal degrades
( it does in a car, or building by 3 to 6 db or more)
I have done in building testing for years. each 3 db means 1/2 power. inside a metal plane i would guess it would be about the same, if you stick the antenna next to a window, MAYBE only 1 to 2 bd loss, but still a loss. i haven't tested it, but i have a lot of experience in this area. Note there is a lot of messaging between the phone and the site, and at that height the signal would be quite low and the phone would get confused as to which site it is on.
I dont know the height the plane was at at that time, but the higher it is, the more signals come in, which degrades the quality and confuses the phone more, not to mention lowering the signal level.
One more thing, in remote areas, where these calls were made i believe, these companies do not build many sites. The sites cost 1/2 million a piece or so.
Just try driving from 1 city to another through a very rural area and see if your phone works, we point the antennas down the highways to get the most out of each site. Planes do not fly along highways.
--------------------Passenger jet makes emergency landing in L.A.
More technical discussions here...
original links (expired)
one call was said to have
been recieved by a 911 (emergency
in Chicago, hundereds of miles from where the plane was...