With support from the FAA and assistance from three major airlines, we first tested our equipment on parked airliners. We next ran a trial on a maintenance flight. Thus prepared, we then measured the RF environment on 37 passenger flights in the eastern United States from September 2003 through November 2003. We collected more than 50 hours' worth of data. (We did not listen in on or record the cellphone conversations themselves.)

On our tests, the airlines—which by agreement remain unnamed—imposed two simple requirements: that the test equipment fit easily in a carry-on bag and that it not be opened while passengers were on board. Because of these size limitations and cost considerations, our equipment could not simultaneously monitor multiple frequency bands, although the computer could switch the spectrum analyzer between bands and change the resolution of the observations according to a program set prior to each flight.

While mobile phones are obvious emission sources, wireless devices in computers and spurious emissions from a variety of other electronic products are also of concern. The airline industry refers to all portable electronic devices collectively as PEDs. We logged PED emissions in nine different frequency bands of interest. However, we focused much of our attention on the bands used by cellphones and by navigation systems, including GPS [see chart, "Cellular Stands Out"].

Several different mobile phone technologies are used in the United States. The two principal frequency bands are the cellular band, 824 to 849 megahertz, which uses a combination of analog and digital technologies, and the PCS (Personal Communications Services) band, 1850 to 1910 MHz, which is all digital. The more dominant cellular technologies are code-division multiple access (CDMA), used by carriers like Verizon and Sprint; Global System for Mobile Communication (GSM), used by Cingular and T-Mobile; and time-division multiple access (TDMA), used by older Cingular base stations and several other carriers.

Other frequency ranges are increasingly being used for cellular service, such as Integrated Digital Enhanced Network (iDEN), in the 806 to 821 MHz frequency range, a technology made popular by the push-to-talk service of Nextel, formerly an independent network but now part of Sprint. Sprint is also the best-known PCS operator. The technologies that transmit in the cellular and PCS bands accounted for more than 75 percent of the mobile phone service in the United States at the time of the study, so our in-flight monitoring concentrated on these frequency bands.

For the cellular and PCS frequency bands, given the monitoring parameters we selected, we couldn't conclusively identify the technologies underlying the signals we detected. However, the FCC permits only cellular telephones to operate in these frequency bands and restricts emissions from unintentional radiators. The recorded power levels are also evidence that the signals are due to cellphone use: an unintentional PED radiator operating at the maximum allowable emission level would show up as being at least 70 decibels below that of an onboard cellular signal.

We could easily identify CDMA cellphone signals in the frequency spectrum analysis by their correlation to prescribed CDMA channels, their relatively wide bandwidth (1.23 MHz), and a distinctive flat top. In other words, it is almost impossible to miss the "Bart Simpson hairdo" profile of a CDMA call. It was harder to identify other cellphone signals unambiguously, such as TDMA or those of older analog phones. While the particular technology associated with these signals could not be identified, there is little doubt that they were cellular in nature, given the high emission level typically observed.

We were able to clearly identify some cellphone signals that originated from on board the aircraft [again, see chart, "Cellular Stands Out"]. Ours was a conservative estimate, since a call made at the other end of the cabin from the instrumentation would be below the threshold we could observe. Our measurements also found emissions from other onboard sources—devices used by passengers—in the frequency used by GPS.

Our research shows clearly that, in violation of FCC and FAA rules, calls are regularly made from commercial aircraft. Results from our analysis imply that calls from on board scheduled commercial aircraft in the eastern United States occur at a rate of one to four per flight. In addition, we saw other signals that suggest that at least one passenger neglects to turn off his or her cellphone on most flights.