Study Calls for Improved Spectrum Management to Protect Earth Observations

EarthzineArticles, Earth Observation, OpEd, Original, Politics, Sections, Technology

Map of portugal showing burned areas

Figure 1. Ocean surface wind field observed during landfall of Hurricane Katrina, August 2005. This data is representative of new products being developed using passive microwave observations. Several quiet radio bands are required to provide this data (image courtesy NRL)

Figure 1. Ocean surface wind field observed during landfall of Hurricane Katrina, August 2005. This data is representative of new products being developed using passive microwave observations. Several quiet radio bands are required to provide this data (image courtesy NRL)

During the past two years a committee of the U.S. National Research Council has quietly and painstakingly assessed the use of the radio spectrum for scientific purposes and associated societal benefits. The foci of the study were the two broad scientific applications of radio waves: passive environmental measurements critical to weather forecasting, climate studies, and related Earth science applications, and radioastronomy measurements necessary for understanding the origins of the universe, nature of matter, and processes of formation of stars and galaxies. Although the study was confined to passive uses of the spectrum below ~1 THz, the conclusions of the committee on the value of the spectrum to the public through weather forecasting and environmental monitoring, as well as basic physical science via radio astronomy observations was clear: these services enhance public safety, environmental security, and extend human knowledge of the Earth and the larger universe.

Of great concern, however, was the related conclusion that the capability to use the spectrum for passive scientific measurements is increasingly under threat due to the widespread development of active communication and other radio devices. Passive remote sensing of the Earth relies on precise measurements of the faint natural thermal noise emitted by all matter, including the oceans, atmosphere, soil, land cover, snow, and ice. Similarly, radio astronomy relies on measurements of noise signals radiated by planets, stars (including our sun), galaxies, and radiation otherwise leftover from the earliest moments of the universe. Measurement of such faint signals requires the utmost in receiver sensitivity, and is easily confounded by the smallest levels of radio interference from transmitters of a variety of types. The rapid and widespread development of the wireless industry along with proliferation of other active services such as radars and radio location devices is making passive measurements increasingly difficult.

The study was thus commissioned to both assess the growing demand for the use of the spectrum and to ascertain the need to preserve part of the spectrum for scientific applications. The levels of interference being experienced at frequencies critical to these applications is increasing, and in one case one of the products (measurements of soil moisture) of a NASA Earth science mission launched in 2002 was seriously compromised due to radio interference. Missions currently being planned that use frequencies below 10 GHz are highly subject to interference. It is anticipated by the committee that unless measures are taken to protect and regulate use of a small percentage of the radio spectrum, similar interference will render future planned missions unable to make several key environmental measurements. Due to similar radio interference effects, radioastronomy facilities costing billions of dollars are often unable to observe during events such as the periods of passage of transmitting satellites, and some parts of the radio spectrum below 1 GHz are unusable for radioastronomy except at the most remote locations on the globe.

Figure 2. A radio image of the nucleus, jets, and outer lobes of the radio galaxy Cygnus A. The plasma jets are approximately 500,000 light years across, or over 5 times the size of our Milky Way galaxy (image courtesy NRAO).

Figure 2. A radio image of the nucleus, jets, and outer lobes of the radio galaxy Cygnus A. The plasma jets are approximately 500,000 light years across, or over 5 times the size of our Milky Way galaxy (image courtesy NRAO).

In the area of Earth observation, the future NASA Aquarius and current SMAP missions will both use the international radio quiet band from 1400 to 1427 MHz, but they will be subject to interference from tracking radars especially near populated areas of the U.S. The current U.S. rules for emissions from tracking radars do not preclude out-of-band emissions in this band, even though international radio regulations state that “all emissions in this band are prohibited.” In another case, a band about 300 MHz wide, in the range from 6 to 7 GHz, is required for observation of soil moisture from future U.S. polar orbiting weather satellites, but no empty spectrum is available. Increasing use of spread spectrum communication systems is likely to further reduce the possibility that any bands will be available for Earth remote sensing of soil moisture in this range. Other devices that threaten the use of bands for scientific purposes include cell phones, radiolocation systems, automobile collision avoidance radars, wideband 60 GHz communication systems, and new software-defined radios.

Moreover, remote sensing of the Earth and radioastronomy are global activities. Atmospheric, oceanic, and land surface conditions far away from any one country often influence the local weather and climate. Global change can only be understood on the basis of observing such conditions around the Earth. The global penetration of portable wireless devices – even on ships far out at sea ‰ÛÒ leaves no location radio quiet. New radioastronomy facilities are being set up in some of the most remote places on Earth, for example, Western Australia and the Karoo desert of South Africa in order to operate in radio quiet areas. In spite of their remoteness (which comes at considerable cost) these facilities will still be subject to interference due to, for example, reflections from airplanes and the global penetration of wireless devices.

Figure 3. Artist‰Ûªs rendering of the NASA Aqua spacecraft showing the AMSR-E sensor at the top (image courtesy NASA)

Figure 3. Artist‰Ûªs rendering of the NASA Aqua spacecraft showing the AMSR-E sensor at the top (image courtesy NASA)

Better coordination of the communications and science communities through appropriate national and international policy could prevent interference and allow both active and passive services appropriate use of the spectrum. Foremost is the need to for radio regulators to recognize that the passive spectral bands ‰ÛÒ in spite of their being empty of manmade emissions ‰ÛÒ are productively used for the above scientific purposes and societal benefits. Moreover, since Earth remote sensing and radioastronomy are global activities, recognition of the important uses of passive bands must occur on a worldwide basis. In addition, regulations need to be adopted to ensure that out-of-band emissions from active devices are effectively unable to cause interference in the primary allocated science bands.

While some degree of radio frequency interference mitigation can be realized by new detection technologies, the committee assessed this potential and concluded that such ‰ÛÏunilateral‰Û RFI mitigation is at best a short term and partially effective solution. Rather, new ways of managing spectrum are needed to ensure the availability of quiet spectrum to the passive services. Opportunistic coordinated use of spectrum by the active and passive services is a potentially valuable new means of spectrum sharing made conceptually possible by new internet-based communications protocols such as IPv6. However, implementation of coordinated sharing techniques requires standards development and the educational forums needed for consensus between active and passive users, and this requires financial support. As a precursor to standards development the committee urged that a thorough assessment of actual spectrum usage in the U.S. from licensed, unlicensed, and inadvertent radiators be performed. Such an assessment would provide a strong basis for the development of coordinated sharing techniques.

Figure 4. RFI interference map for AMSR-E showing areas where obviously noticeable amounts of interference exists (image courtesy NASA).

Figure 4. RFI interference map for AMSR-E showing areas where obviously noticeable amounts of interference exists (image courtesy NASA).

When considered from an economic perspective the committee concluded that passive spectrum, much like public parkland, defies monetization. Currently, 3.6% of the U.S. is in the National Park system, while only 2% of spectrum below 3 GHz is allocated for passive services on a worldwide basis. The preservation of such small amounts of spectrum for public passive use is well justified given that the vast majority of spectrum is available for the active services, but currently is inefficiently used. Coordination on the remaining unused high-frequency portion of spectrum is also feasible to support new passive scientific uses that currently have no allocations. The study recommends that such appropriate coordination and support become a primary objective of all radio regulatory agencies.

The NRC spectrum study report, entitled “Spectrum Management for Science in the 21st Century”, is available at http://www.nap.edu/catalog.php?record_id=12800. A GEO task (AR-06-11) on the topic of radio spectrum management has been identified and can be downloaded at http://www.earthobservations.org/documents/tasksheets/latest/ar-06-11.pdf.

A.J. Gasiewski is Professor of Electrical and Computer Engineering at the University of Colorado, Boulder, Colorado, and co-chair of the NRC spectrum study. He is past President of the IEEE Geoscience and Remote Sensing Society, and a member of the executive committee of the IEEE‰Ûªs Committee on Earth Observations. M.H. Cohen is Emeritus Professor of Astronomy at the California Institute of Technology and co-chair of the study. He is a former member of the IEEE and the IRE.