The radio frequency (RF) spectrum is a limited resource with ever-increasing demand from an expansive range of applications—all the way from commercial, such as mobile phones, to scientific, such as hurricane monitoring from space. The RF spectrum is conventionally defined as the part of the electromagnetic spectrum from 3 kHz up to 3000 GHz,1 but recent uses extend to frequencies approaching the edge of the infrared part of the spectrum. The selection of a frequency band for a particular use depends on many factors, including propagation characteristics, atmospheric attenuation, technological advances, sensitivity to physical parameters of interest, and cost. Changing the band of operation is not always viable, particularly for scientists, who in many cases must observe at specific frequency bands, which can be dictated in remote sensing by the electromagnetic properties of Earth parameters, such as sea surface temperature, sea salinity, surface winds, and soil moisture; or, in the case of radio astronomers, the transition frequencies of atoms and molecules, which are established by the laws of physics and chemistry.
NOTE: Portions of this text are taken from National Research Council, Views of the NAS and NAE on Agenda Items at the World Radiocommunication Conference 2015, The National Academies Press, Washington, D.C., 2013.
1 International Telecommunications Union (ITU) Radio Regulations, Article 2, Section I, 2016.
Since radio waves do not stop at national borders, international regulation is necessary to ensure effective use of the radio spectrum for all parties. The International Telecommunication Union (ITU) has as its mission to facilitate the efficient and interference-free use of the radio spectrum. One of the most important functions of the ITU is to maintain the International Table of Frequency Allocations. Every country is sovereign to allocate uses of the RF spectrum within its borders, but most choose to follow the International Table of Frequency Allocations out of convenience and to avoid potential interference to their neighbors.
In the radio regulations (RR), radio spectrum users are divided into different categories, which are usually referred to as radiocommunication services. Some examples are signal broadcasting, radionavigation, meteorology, space research, mobile, mobile-satellite, and the Earth Exploration-Satellite Service (EESS). In the regulations, the Radio Astronomy Service (RAS) is classified as a radio service but not a radiocommunication service; it is the only service so distinguished. Services can be active or passive, depending on whether they transmit and receive or receive only. Passive services use receivers to measure natural radio frequency emissions from ocean, land, and atmosphere phenomena such as hurricanes or, in the case of radio astronomy, cosmic sources such as solar system objects, stars, and the medium between stars, galaxies, and other celestial bodies. RAS, as defined in the radio regulations, is always a passive service. Active services, on the other hand, use both a transmitter, which radiates an electromagnetic signal within a given band of frequencies, and a receiver, which receives and detects a transmitted signal or, in some cases, receives a signal that is reflected back from a target, such as Earth’s surface. The EESS may be active or passive, and these two uses are distinguished in the regulations. While RAS is only a passive service, astronomers also use powerful radar to study the surface and other properties of asteroids, the planets, and their satellites (including radar observations to detect near Earth objects); radar astronomy is considered part of the Radio-location Services and follows its rules and regulations.
There are several major differences between RAS and EESS. RAS facilities are typically ground based, looking upward (to celestial objects), whereas most EESS sensors are on board satellites, usually looking downward to study Earth’s radio emission from atmosphere, land, and ocean. Active sensors that are airborne are potential radio frequency interference (RFI) sources for passive observations both on the ground and on satellites, depending on their power level, antenna radiation pattern, polarization, distance, and other parameters.
Radio telescopes and passive sensors operated by EESS measure the natural, noise-like emissions from targets under study. Passive RAS facilities with spectral power flux density (spfd) sensitivity on the order of −250 dBW/m2/Hz are common at gigahertz frequencies, and radio astronomers work to detect emissions even four to six orders of magnitude fainter in that frequency range. This high sensitivity makes them vulnerable to interference from in-band emissions, from spurious and out-of-band emissions from licensed and unlicensed users of neighboring bands, and from emissions that produce harmonic signals in the RAS bands.2 Weak signals that are unintended by-products of human activity can obstruct the scientific use of the spectrum. Remote sensing scientists observe the extremely weak natural emission from Earth’s surface and atmosphere. Their observations are similarly very vulnerable to interference from unintended human-made transmissions. Recommendations ITU-R RA.769 and ITU-R RS.2017 contain the threshold levels of interference that are deemed detrimental to the use of the radio spectrum by the passive scientific services.
Scientific users understand the need to share the spectrum between active and passive users, but it is important to note that some sharing techniques that work for active services do not necessarily work for passive uses. For example, dynamic spectrum management (DSM), where RF monitoring is used to identify “unused” parts of the spectrum, may not be appropriate to share the spectrum with the passive services that have no detectable radio signature within the sensitivities and integration times of the DSM systems. On the other hand, in specific cases, either geographic or time separation may be used to share the spectrum effectively between active and passive services.
A factor that needs to be considered in some cases is the “aggregate interference,” especially when dealing with spectrum users such as low-power RF transmitting (active) devices that do not require individual licenses and are designed to be used by thousands or even millions of transmitters at a given moment. For example, wireless mobile handsets and radars mounted on vehicles are two increasingly numerous potential sources of radio frequency interference. The aggregate factor takes into account the power (spfd) at the passive “victim” facility. Whereas one such transmitter might
2 Allocations and protection for scientific use is discussed in National Academies of Sciences, Engineering, and Medicine, Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses: Second Edition, The National Academies Press, Washington, D.C., 2015.
comply with threshold levels of interference for a radio astronomy receiver, the sum of many such transmitters may not.
Every 3 to 6 years, the ITU convenes a World Radiocommunication Conference (WRC) to review and revise the international RR. Changes to the RR are formulated through proposals to the conference according to the agenda items, which are agreed on at the previous WRC. Governments or Member States (referred to within the ITU as “Administrations”) and academic, industrial, and scientific organizations (referred to within the ITU as members, with lower case “m”) can participate in the WRC, but only the more than 190 Member States (Administrations) of the ITU are entitled to formulate proposals and to vote.
In the period between two WRCs, Administrations work internally and with their regional counterparts to develop a consensus position on each agenda item, to the extent possible, given varying national priorities and interests. The national delegations then submit proposals to the WRC and negotiate with other delegations before adoption of each proposal. Much of this negotiation takes place between regional groups—for example, the European Post and Telecommunications Conference (CEPT), with more than 40 members, or the Inter-American Telecommunications Commission (CITEL), which has more than 30 members. The outcome of a WRC, a revision of the RR, is an international treaty.
Agenda items are typically very specific and propose substantial changes to the use of the spectrum that can have a significant impact on services. Because more than 95 percent of spectrum allocations below 3 GHz are for active uses of the spectrum, it is critical for vulnerable passive services to participate in the process and express their concerns about potential adverse effects on their operations.3
To ensure their continued ability to access the radio spectrum for scientific purposes, scientists must participate in the discussions leading up to WRC-19, scheduled to be held in November 2019 in Geneva, Switzerland. By request of the National Science Foundation (NSF) and the National Aeronautics and Space Administration (NASA), a committee was convened by the National Academies of Sciences, Engineering, and Medicine to provide guidance to U.S. spectrum managers and policy makers as they prepare for
3 In the United States, the Radio Astronomy Service (RAS) and the Earth Exploration-Satellite Service (EESS) are allocated 2.07 percent of the spectrum on a primary basis and 4.08 percent of the spectrum on a secondary basis below 3 GHz. Allocations for RAS and EESS are comparable in the ITU’s international allocation tables. From National Research Council, Spectrum Management for Science in the 21st Century, The National Academies Press, Washington, D.C., 2010, pp.137-138.
the WRC-19, to protect the scientific exploration of Earth and the universe using the radio spectrum (see Appendix A for the committee’s statement of task). While the resulting document is targeted primarily at U.S. agencies dealing with radio spectrum issues, other Administrations and foreign scientific users may find its recommendations useful in their own WRC planning.
This report identifies the WRC-19 agenda items of relevance to, and with potential impact on, U.S. radio astronomers and Earth remote sensing researchers. The agenda items are discussed in numerical order to facilitate locating a specific one. The committee has determined that some outcomes of the agenda items shown in Table 1.1 may impact RAS and EESS operations and provides the reasons for its view as well as the passive application of the bands that may be impacted. Agenda items not discussed in this report are not expected to have an impact on RAS or EESS operations. It is noted that potential impact is assessed based on criteria related to in-band, out-of-band, and spurious emissions, as appropriate.
To provide context for the potential scientific impact of these agenda items, a brief overview of some of the scientific results derived from the passive use of the radio spectrum by EESS and RAS is described below. A more complete view of both the scientific uses and the frequency allocations can be found in the Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses: Second Edition.4
EARTH EXPLORATION-SATELLITE SERVICE
Satellite remote sensing is a uniquely valuable resource for monitoring the global atmosphere, land, and oceans. Microwave remote sensing from space presents a global view, vital for obtaining atmospheric and surface data for the entire planet. Instruments operating in the EESS bands provide data that are important to human welfare and security and include support for scientific research, commercial endeavor, and military operations in areas such as meteorology, atmospheric chemistry, climate studies, and oceanography. For example, measurements of ocean temperature and salinity are needed to understand ocean circulation and the associated global distribution of heat and hurricane genesis. Measurements of soil moisture are needed for agriculture and drought assessment, for weather
4 National Academies of Sciences, Engineering, and Medicine, Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses: Second Edition, The National Academies Press, Washington, D.C., 2015.
TABLE 1.1 World Radiocommunication Conference (WRC)-19 and WRC-23 Agenda Items of Relevance to and Potential Impact on the Radio Astronomy Service and the Earth Exploration-Satellite Service
|1||On the basis of proposals from administrations, taking account of the results of WRC-15 and the Report of the Conference Preparatory Meeting, and with due regard to the requirements of existing and future services in the frequency bands under consideration, to consider and take appropriate action in respect of the following items:|
|1.2||To consider in-band power limits for Earth stations operating in the mobile-satellite service, meteorological-satellite service, and Earth exploration-satellite service in the frequency bands 401-403 MHz and 399.9-400.05 MHz, in accordance with Resolution 765 (WRC-15)|
|1.5||To consider the use of the frequency bands 17.7-19.7 GHz (space-to-Earth) and 27.5-29.5 GHz (Earth-to-space) by Earth stations in motion communicating with geostationary space stations in the fixed-satellite service and take appropriate action, in accordance with Resolution 158 (WRC-15)|
|1.6||To consider the development of a regulatory framework for non-GSO FSS satellite systems that may operate in the frequency bands 37.5-39.5 GHz (space-to-Earth), 39.5-42.5 GHz (space-to-Earth), 47.2-50.2 GHz (Earth-to-space), and 50.4-51.4 GHz (Earth-to-space), in accordance with Resolution 159 (WRC-15)|
|1.7||To study the spectrum needs for telemetry, tracking, and command in the space operation service for non-GSO satellites with short-duration missions, to assess the suitability of existing allocations to the space operation service, and, if necessary, to consider new allocations, in accordance with Resolution 659 (WRC-15)|
|1.8||To consider possible regulatory actions to support Global Maritime Distress Safety Systems (GMDSS) modernization and to support the introduction of additional satellite systems into the GMDSS, in accordance with Resolution 359 (Rev. WRC-15)|
|1.9||To consider, based on the results of ITU-R studies:|
|1.9.1||regulatory actions within the frequency band 156-162.05 MHz for autonomous maritime radio devices to protect the GMDSS and automatic identification systems (AIS), in accordance with Resolution 362 (WRC-15)|
|1.9.2||modifications of the Radio Regulations, including new spectrum allocations to the maritime mobile-satellite service (Earth-to-space and space-to-Earth), preferably within the frequency bands 156.0125-157.4375 MHz and 160.6125-162.0375 MHz, to enable a new VHF data exchange system (VDES) satellite component, while ensuring that this component will not degrade the current terrestrial VDES components, applications specific messages (ASM) and Automatic identification systems (AIS) operations and not impose any additional constraints on existing services in these and adjacent frequency bands as stated in recognizing d) and e) of Resolution 360 (Rev. WRC-15)|
|1.13||To consider identification of frequency bands for the future development of International Mobile Telecommunications (IMT), including possible additional allocations to the mobile service on a primary basis, in accordance with Resolution 238 (WRC-15)|
|1.14||To consider, on the basis of ITU-R studies in accordance with Resolution 160 (WRC-15), appropriate regulatory actions for high-altitude platform stations (HAPS), within existing fixed-service allocations.|
|1.15||To consider identification of frequency bands for use by administrations for the land-mobile and fixed services applications operating in the frequency range 275-450 GHz, in accordance with Resolution 767 (WRC-15)|
|1.16||To consider issues related to wireless access systems, including radio local area networks (WAS/RLAN), in the frequency bands between 5150 MHz and 5925 MHz, and take the appropriate regulatory actions, including additional spectrum allocations to the mobile service, in accordance with Resolution 239 (WRC-15)|
|2||On the basis of proposals from administrations and the Report of the Conference Preparatory Meeting, and taking account of the results of WRC-19, to consider and take appropriate action in respect of the following items:|
|2.2||To conduct, and complete in time for WRC-23, studies for a possible new allocation to the Earth Exploration-Satellite Service (active) for spaceborne radar sounders with the range of frequencies around 45 MHz, taking into account the protection of incumbent services, in accordance with Resolution 656 (WRC-15)|
NOTE: Acronyms defined in Appendix B.
prediction (heat exchange with the atmosphere), and for defense (planning military deployment). Passive sensors also provide temperature and humidity profiles of the atmosphere, information to monitor changes in the polar ice cover, and information needed in assessing hazards such as hurricanes, wildfires, and drought. For many applications, satellite-based RF remote sensing represents the only available method of obtaining atmospheric and surface data for the entire planet. Major U.S. governmental users of EESS data include the National Oceanic and Atmospheric Administration (NOAA), the National Science Foundation, NASA, the Department of Defense (DoD), the Department of Agriculture, the U.S. Geological Survey, the Agency for International Development, the Federal Emergency Management Agency (FEMA), and the U.S. Forest Service. Much of these data are also available free to anyone anywhere in the world.
However, passive instruments in space are particularly vulnerable to anthropogenic emissions because they rely on very faint signals emitted naturally from Earth’s surface and atmosphere. This is especially a concern for EESS because sensors view large swaths of the surface at one time. Measurement accuracy is already limited by the available bandwidth, and some of these valuable measurements are being blocked by RFI, even within protected bands. For instance, it is now impossible to retrieve soil moisture by the Advanced Microwave Scanning Radiometer 2 (AMSR2) at 10.7 GHz in some areas of the globe due to RFI. Similarly, the Soil Moisture Active Passive (SMAP) and Soil Moisture and Ocean Salinity (SMOS) missions, which operate at 1.413 GHz, are adversely impacted by RFI even though they operate in a band protected for passive use only. Figure 1.1 shows the impact of RFI as observed by the SMAP radiometer. In addition to the RFI effects on passive instruments, recent measurements from active instruments are also found to be affected by RFI, as is the case for the L-band scatterometer on board the Aquarius/SAC-D satellite.
RADIO ASTRONOMY SERVICE
Radio astronomy is a vital tool to study our universe. Radio astronomy provides valuable data for the benefit of society such as the monitoring of solar flares and sunspots. Such monitoring allows for 1- to 4-day forecasts of geomagnetic disturbances that can affect the operation of satellite communications, Global Positioning System (GPS) navigation systems, and terrestrial power grids. The first planets outside the solar system, circling a distant pulsar, were
discovered through the use of radio astronomy. Pulsars are stars rotating so precisely that their electromagnetic (primarily radio) pulses can be tracked with precision rivaling atomic clocks on Earth. Subsequent observations of pulsars have revolutionized our understanding of the physics of neutron stars and general relativity and resulted in the first experimental evidence for gravitational radiation.
Radio astronomy has also enabled the discovery of organic matter and prebiotic molecules outside our solar system, leading to new insights into the potential existence of life elsewhere in our Milky Way galaxy. Measurements of radio spectral line emission have identified and characterized the birth sites of stars in the Milky Way, the processes by which stars slowly die, and the complex distribution and evolution of galaxies in the universe.
Radio astronomy measurements discovered the cosmic microwave background (CMB), the radiation left over from the original
Big Bang. Later observations discovered the weak fluctuations in the CMB of only one-thousandth of a percent, generated in the early universe, which later formed the stars and galaxies we know today. Radio observations uncovered the first evidence for the existence of a black hole in our galactic center, a phenomenon that may be crucial to the creation of many other galaxies. Observations of supernovas have allowed astronomers to witness the distribution of heavy elements essential to the formation of planets like Earth, and of life itself.
In addition, since radio astronomy poses extra challenges due to the extreme sensitivity required for observing very faint signals, engineers have to come up with solutions to these challenges to enable cosmic radio astronomy observations. Many of these solutions have proven to be extremely useful to other applications. These include the following: optical mapping technology adapted for laser eye surgery; wireless networking technology; sensitive microwave receiving systems, including high-gain antennas and low-noise receivers; cancer therapy using knowledge obtained from observing black hole environments; time calibration for GPS; and wireless technology, including fast Fourier transform chips, solid-state oscillators, frequency multipliers, and cryogenics. Other examples include data correlation and recording technology and image restoration techniques, among many others.
For context, it is important to understand the exceedingly weak nature of the typical signals detected by radio telescopes. They can be a million times smaller than the internal receiver noise, and their measurement, or even just their detection, can require bandwidths of many gigahertz and integration times of a day or more. This requirement puts a premium on operating in a very low noise environment. It should be emphasized that serious interference can result from weak transmitters even when they are situated in the sidelobes of a radio astronomy antenna. In addition, radio telescopes are particularly vulnerable to interference from airborne and satellite transmitters, since terrain shielding cannot block the signal from high-altitude emitters.
As mentioned above, radio spectroscopic observations require measurements at frequencies determined by the physical and chemical properties of individual atoms and molecules. In particular, knowledge of the chemical makeup of the universe comes through measurement of spectral lines arising from quantum mechanical transitions, so it is important to protect the frequencies characteristic of the most important atomic and molecular cosmic constituents. However, the necessary parameters are not yet known for all species
of interest. In addition, due to the expansion of the universe, even known spectral lines may be Doppler-shifted by up to an order of magnitude for distant objects. Therefore, detection of molecules in distant sources may require observations at frequencies well below the characteristic frequency measured in the laboratory. Thus, observations at spectral frequencies well outside the bands allocated to RAS on a primary or secondary basis are often conducted in order to search for new molecular species and to detect Doppler-shifted spectroscopic lines from both nearby and distant sources, including the very early universe.
The situation with continuum observations of radio emission from cosmic thermal and nonthermal sources, however, is different from that of spectral lines. There are no preferred frequencies, but observations at multiple frequencies and potentially wide bandwidths are required to define the properties of stars, galaxies, quasars, pulsars, and other cosmic radio sources. Historically, narrow bands spaced throughout the spectrum have been given various levels of protection to enable these important continuum spectral studies. However, improvements in antenna and receiver design now permit instantaneous fractional bandwidths of 50 percent or more to be used in the latest generation of radio telescopes. This results in an improvement in sensitivity over earlier narrow-band systems by up to an order of magnitude; furthermore, broad bandwidths are also used to study many spectral lines simultaneously. Unfortunately, receivers can become nonlinear as a result of RFI at neighboring frequencies, and intrinsically weak emissions can be easily overwhelmed by RFI. Thus, the advent of routine observations over broad bandwidths by radio telescopes requires even more vigilance in RFI mitigation to enable further advances in radio astronomy. In particular, while improved RFI mitigation and excision techniques have expanded the scientific return of many facilities, they are an inferior option relative to a clean, interference-free spectrum. Indeed, it is clear that all users, both passive and active, benefit from a clean spectrum. Thus, while radio astronomy facilities rely in part on geographic shielding and local designations of radio quiet zones to reduce sources of RFI, it is the shared responsibility of all users to assure effective use of the radio spectrum and to enable both active and passive services to coexist.