9/11 Airborne Cell Phone Performance

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9/11 Airborne Cell Phone Performance

Installed 01 Feb 2006. Latest update 18 Jul 2008.
Additions or changes are shown in bold.

On page 12 of The 9/11 Commission Report it is stated, that aboard United Flight 93 (shortly after 9:32) "the passengers and flight crew began a series of calls from GTE airphones (sic) and cellular phones." [Emphasis added.]

To check on the veracity of the reported inflight use of cell phones, Physics 911 Project Achilles researchers performed low altitude cell phone tests in fixed wing private aircraft (in the London, Ontario area of Canada). They found limited communications above 2,000 feet AGL (above ground level) and essentially no communications at 8000 feet AGL and above.

This page was created to provide additional technical background information pertaining to these tests.

The Airfone system(1) used a widely spaced grid of specially configured cellular ground stations. These used exclusive air-to-ground frequencies and air-to-ground antennas. Airplanes equipped with with Airfones had exterior antennas and could transmit up to a total of 30 watts effective radiated power.

Approximate Locations of Airfone Ground Sites, In and Near Pennsylvania in 1991
Flight 93's approximate reported flight path is shown in red. The 60 mile radius circle encloses the airspace near the Pittsburgh area Airfone ground station where a hand held cell phone at altitude, . . . , and with the appropriate frequencies and control codes programmed in to it, could most likely maintain radio contact with the ground station.

(Verizon acquired Airfone from GTE in 2000.)

The following qualitative schematic diagram illustrates (in part) why ordinary cell phones don't work at altitudes a half mile or more above ground.

Cell phone received
signal strength versus location

Idealized Cell Phone Coverage for a Typical Cellular Tower
(With no multipath fading)

Antenna patterns for cellular system towers are usually designed to operate in a flattened pancake-like pattern that serves surface customers. (Many cellular tower antenna patterns have a 1-2 degree (in some cases, maybe more) tiltdown to better serve mobile users near the edge of their service areas.) Highway related cellular towers, for a given provider, are typically spaced five to six miles apart.

Earlier I had proposed some addtional tests to add to our knowledge about cell phone performane at altitude, but according to current U.S. Federal Aviation Regulations, the use of cell phones on any kind of airborne platform, including private aircraft and even balloons (once the platform is airborne) is forbidden.

On June 23, 2006, Verizon Communications, Airfone's parent company, announced that they would be discontinuing their Airfone service on all commercial flights by the end of 2006. Verizon will continue to provide service on 3,400 private and government aircraft. Source: Wikipedia. (Use search term Airfone.)

Doppler Induced Communications Blocking

Doppler induced communications blocking effects should be noticeable at lower altitudes, say below 2,000 feet AGL, and will be most pronounced for aircraft headed nearly directly toward or away from a given cellular tower.

Based on two take-off-roll tests (aircraft still on the ground) done by the author, it appears that for CDMA cell phones (1900 MHz), Doppler induced loss-of-service (no bars) occurs at relative speeds (aircraft to cell tower) of 150 kts (nautical miles per hour) and greater. At a speed of 150 kts, toward or away from a given cell tower, the Doppler shift for 1900 MHz signals is roughly 500 Hertz (cycles per second). The author notes that in cell phone system simulations, a Doppler offset of 480 Hertz is typically employed. Robust systems can apparently handle that amount of Doppler shift.

Since Airfone frequencies (849-851 MHz and 894-895 MHz) are about one half of that for 1900 MHz CDMA cell phones, the Airfone equipped aircraft speed, relative to a given Airfone ground station, could be on the order of 300 kts before Doppler blocking sets in.

The following diagram illustrates Doppler shift blockage of cellular signals for a mobile cell phone in an aircraft which is flying at a speed of 500 kts at an altitude of 1,000 feet. The grey area represents the region of no service (slant range speeds equal to or greater than 150 kts). Cell tower "usability" times are shown as arrows in the usable signal footprint. The shorter the times, the more problematic the chances are for actual communications to occur. [Added 12 Feb 2006. Modified 14 Feb 2006.]

Airborne CDMA Cell phone
foot print, 500 kts, 1000 feet AGL

Ground motion added 02 Aug 2007. (If the shaded squares
move in surges, then your internet pipe is probably clogged.)

The boundaries of the Doppler blocking zone, in the above and the following diagram, were extracted from printouts produced by a QuickBASIC program, called DOPPLER.BAS. The program calculates slant ranges as functions of time (and from them, slant-range-speeds) for a given airplane's path with respect to a ground radio facility. [Added 14 Feb 2006.]

The next diagram shows the cell phone footprint for an aircraft at 1000 feet AGL, as before, but traveling at a groundspeed of 250 kts. This situation is fairly representative of airliners making their final approaches for landings. It can be seen that there should be ample opportunity for cell phone communications using cellular towers located two to five miles to either side of the aircraft. [Added 13 Feb 2006.]

Airborne CDMA Cell phone
foot print, 250 kts, 1000 feet AGL

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The Author of this article worked from 1994 to 2001 as a Land Mobile Radio (including cell phones) telecommunications specialist.

References

(1) Methods and means for providing improved air/ground radio telephone communications United States Patent 4419766