Important Characteristics of Radio Frequency Interference
Following is a list of important characteristics of radio frequency interference (RFI):
Licensed transmitters, such as television, taxi radios, and cellular telephones occupy fixed spectral bands. RFI from these sources can in some cases be eliminated by avoiding those frequency bands. However, vigilance in keeping spurious and out-of-band emissions down to acceptable levels is always necessary.
Strong spurious and out-of-band signals are in fact seen in Radio Astronomy Service (RAS) and Earth Exploration-Satellite Service (EESS) experiments. For example, Figure 3.10 in Chapter 3 shows an example of interference in the band 1610.6-1613.8 MHz, a band allocated to the RAS on a shared primary basis. Figures 2.15 through 2.18 in Chapter 2 show inadvertent RFI to the NASA-Japan Aerospace Exploration Agency Advanced Microwave Scanning Radiometer-Earth (AMSR-E) sensors at 10.6-10.7 GHz.
Low-power, unlicensed transmitting devices are rapidly proliferating. They range from cordless telephones to local area computer networks to digital cameras to automotive anticollision radars, to name only a few of many examples. Since these are personal devices, the total emission level is generally proportional to the population and level of development in any given area.
Radio telescopes gain a great deal of protection from RFI by locating in remote areas—for example, in the National Radio Quiet Zone in West Virginia, behind high mountains, or in remote desert areas. However, the RAS cannot hide from RFI caused by airplanes or satellites flying overhead. The locating of observatories far from commonly used flight paths is considered, when possible.
The EESS, operating mainly from low-Earth-orbit satellites, cannot escape the RFI caused by multitudes of low-power-radiating devices as it passes over populated areas. Over parts of Europe and North America, some EESS data products are now ruined by RFI.
Many active communications systems, including television, are moving to more efficient use of spectrum, especially in filling up their assigned bands uniformly. This results in less white space where scientists might be able to operate with passive equipment. It also means that the signals more closely resemble the random noise of natural signals and are thus less recognizable as RFI.
ENABLING SCIENTIFIC USES OF THE RADIO SPECTRUM
The goal of this report is to highlight the importance of the passive uses of the radio spectrum, to identify issues that threaten the ability of the science services to provide benefits to society, and to recommend steps for the mitigation or elimination of these threats while recognizing the importance of the other services. Chapters 2 and 3 discuss the knowledge gained from and benefits to society produced by the EESS and RAS, respectively, as well as current and future spectrum requirements for maintaining progress. Chapter 4 discusses current trends in spectrum use and technology that shape the environment in which the EESS and RAS operate, as