Astronomers Concerned About Cellphones in Planes

The USA's National Academy of Sciences has said that it does not support the use of mobile phones on planes, on the grounds that they would interfere with radio astronomers. The National Research Council's Committee on Radio Frequencies (CORF) say that supports proposals in the FCC proposals that could have the effect of reducing the likelihood and severity of interference from airborne cellular telephone transmissions to sensitive radio astronomy observations.

The frequencies at which radio astronomers observe are dictated by the laws of nature. Furthermore, the emissions that radio astronomers receive at these frequencies are extremely weak - a typical radio telescope receives only about one-trillionth of a watt from even the strongest cosmic source. Because radio astronomy receivers are designed to pick up such remarkably weak signals, such facilities are particularly vulnerable to interference from spurious and out-of-band emissions from licensed and unlicensed users of neighboring bands, and those that produce harmonic emissions that fall into the RAS bands.

CORF's primary concern in this proceeding is that spurious emissions of second harmonics of cellular telephone transmissions at 830-835 MHz could create damaging interference to radio astronomy observations in the 1660-1670 MHz band. Accordingly, CORF supports proposals from the FCC that require the use of picocells and that modify the required emission mask for cellular telephone hand units, which could reduce the possibility and severity of such interference.

Harmonics occur at integer multiples of the transmitted frequency. For example, the second harmonic of 830 MHz is 1660 MHz. Hence, any transmission in the band at 830-835 MHz has the potential to radiate a second harmonic in the RAS band at 1660-1670 MHz. The signal level of such harmonic transmission at an RAS site is significantly increased when the propagation path is directly in the line of sight, as occurs in the case of airborne transmissions from aircraft above the observatory's horizon. Such transient interference requires, at a minimum, that the affected data be identified and removed from the observation data set. These added steps reduce the effective observing time for such observations. Even more serious is the case in which the interference is too weak to be identified and removed, potentially affecting the accuracy of a scientific result.

CORF takes no position as to whether the Commission should authorize the airborne use of cellular telephones, but CORF strongly supports the proposal to permit airborne use only if the handsets are controlled by an airborne picocell so that the likelihood and severity of interference to RAS facilities is minimized. Control by picocells would limit transmissions on the primary cellular telephone frequencies to communications within the aircraft, obviating the need for transmissions to the ground, and would accordingly limit the power of cellular handset transmissions within the aircraft - reducing the potential for interference to RAS observations. In addition, CORF supports the proposal to adjust the limits on out-of-band and spurious transmissions to account for airborne transmissions. Specifically, adjustment of the Commission's permissible out-of-band and spurious emission limits on cellular handsets should include tighter constraints on permissible second harmonic levels.

A calculation presented to the committee by Dr. Andrew Clegg of the National Science Foundation's Electromagnetic Spectrum Management Office showed that harmonic and out-of-band emissions (-13 dBm/MHz) can exceed ITU-R RA.769 limits by 44 dB when as few as seven cellular telephones are used. The calculation reflected the following assumptions: the devices are of the lowest power class defined for GSM Base Transceiver Station (BTS)/mobile stations (0 dB per European Telecommunications Standards Institute GSM 05.05 specifications for radio transmission and reception for mobile stations), a 20 dB average attenuation of signals propagated outside of the aircraft, an average slant-range path of 5 miles, an average of 10 visible aircraft, an average of 7 users per GSM carrier aboard the aircraft (fully loaded GSM Broadcast Control Channel), and a noise-equivalent bandwidth of ~200 kHz. The expected spectral power flux density on the ground is -182 dB(W/m2/Hz). A single aircraft at 100 mile slant range exceeds ITU-R RA.769 limits by 19 dB."

Posted to the site on 10th June 2005