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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
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Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
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Page 7
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
×
Page 8
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
×
Page 9
Suggested Citation:"1 Introduction." National Academies of Sciences, Engineering, and Medicine. 2021. Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023. Washington, DC: The National Academies Press. doi: 10.17226/26080.
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1 Introduction The radio frequency (RF) spectrum is a limited resource for which there is an 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 radio spectrum is conventionally defined as the part of the electromagnetic spectrum from 3 kHz up to 3000 GHz, 1 but use of radio technology now extends to frequencies approaching 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 possible, particularly for science applications. In many cases, scientists must observe at specific frequency bands. In remote sensing, these frequency bands are dictated by the electromagnetic properties of Earth parameters, including atmospheric temperature, humidity, cloud particles and precipitation, and composition; sea-surface temperature, heights, winds, and salinity; soil moisture and vegetation health; and cryosphere properties. In the case of radio astronomy, bands are dictated by the transition frequencies of atoms and molecules (as is also the case with Earth atmospheric composition and temperature sounding), which are established by the laws of nature. Use of the radio spectrum is regulated internationally by the Radio Regulations (RR), an international treaty. In the RR, spectrum users are divided into different categories, which are referred to as radiocommunication services. Examples of these services are Broadcasting, Radionavigation, Meteorology, Space Research, Mobile, Mobile-Satellite, and Earth Exploration-Satellite Service (EESS). In the RR, the Radio Astronomy Service (RAS) is classified as a radio service but not a radiocommunication service—the only service so distinguished. Radio services can be active or passive, depending on whether they transmit and receive, or receive only. The EESS may be active or passive, and these two uses are distinguished in the regulations. The EESS (passive) uses receivers to measure natural radio frequency emissions from the oceans, land, and atmosphere (including severe weather phenomena such as tropical storms). Active, transmitting, sensors in EESS are used for measuring winds, soil moisture, precipitation, cloud particles, and more. In the case of radio astronomy, cosmic sources, such as objects in the solar system, stars, the interstellar medium, galaxies, and other celestial bodies, are observed always in a receive only (passive) mode. Consequently, RAS is defined in the radio regulations exclusively as a passive service. While RAS is only a passive service, astronomers also use powerful radars to study the surface and other properties of asteroids, the planets, and their satellites, including radar observations to detect near Earth objects (NEOs). Radar astronomy is considered part of the Radiolocation Service and is subject to its regulations. Radio telescopes and passive EESS sensors measure the natural emissions from targets under study. Passive RAS facilities with spectral power flux density (spfd) sensitivity on the order of −250 dBW m−2 Hz−1 are common at gigahertz frequencies, and radio astronomers routinely detect emissions even 4 to 6 orders of magnitude fainter in that frequency range. This high sensitivity makes them vulnerable to NOTE: This Introduction is an updated version of National Academies of Sciences, Engineering, and Medicine, Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items of Interest to the Science Services at the World Radiocommunication Conference 2019, The National Academies Press, Washington, D.C., 2017. 1 ITU Radio Regulations, Article 2, Section I, 2020. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 1

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 uses of the spectrum. Remote-sensing satellite sensors observe the extremely weak natural emission from Earth’s surface and atmosphere. These observations are similarly very vulnerable to interference from unintended human transmissions. Recommendations ITU-R RA.769 and ITU-R RS.2017 contain the threshold levels of interference considered harmful to the use of the radio spectrum by the RAS and EESS, respectively. While EESS (passive) and RAS are both measuring the naturally occurring radio emissions from physical phenomena, the nature of these measurements leads to some differences in their vulnerability to radio frequency interference (RFI). For example, RAS facilities are typically ground-based, looking upward (to celestial objects), and thus may be protected with geographical restrictions and radio frequency coordination zones around the individual radio observatories. In contrast, EESS sensors are on- board satellites, looking downwards to study Earth’s radio emission from the atmosphere, land, cryosphere, and ocean. For EESS, temporal coordination based on satellite ephemerides may be an effective approach to sharing the radio spectrum. However, active transmitters that are airborne are potential sources of radio frequency interference for both ground-based and spaceborne receivers, depending on their power level, antenna radiation pattern, polarization, distance, and other parameters. Airborne and spaceborne transmitters are particularly challenging for radio astronomy observatories because they are not usually subject to geographic coordination or restrictions. 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 radio frequency monitoring is used to identify “unused” parts of the spectrum, may not be an appropriate method 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 geographical 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 “aggregate interference,” especially when dealing with spectrum users such as low-power radio frequency transmitting (active) devices that do not require an individual license 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. Whereas one such transmitter might comply with threshold levels of interference for a radio astronomy receiver, the sum of many such transmitters may not. This aggregation is essential to account for the actual power (spfd) from all transmitters reaching the protected passive EESS sensor or radio astronomy receiver. It is important to note that the threshold levels for harmful interference to EESS and RAS listed in Recommendations ITU- R RS.2017 and ITU-R RA.769, respectively, are considered in the aggregate, not for a single transmitter. Strong interference corrupts observations in clearly recognizable ways, and affected data can be excised, but only at the cost of reducing the number of available measurements for both the EESS and RAS. Weak levels of interference, on the other hand, result in EESS observations that, while not discernibly corrupted, are sufficiently impacted that they provide incorrect information to weather forecasting systems, undermining the reliability and value of their predictions. Similarly, because the power observed by radio astronomers is very faint, low levels of interference may be impossible to distinguish from the astronomical signal sought, leading to false results. 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 the facilitation of 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, a 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. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 2

chapter of the RR. Every country is sovereign to allocate uses of the radio frequency 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. Every 2 to 5 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 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”) participate in the WRC, but only the more than 190 Member States (Administrations) of the ITU are entitled to introduce proposals and to vote. In the period between 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. To ensure their continued ability to access the radio spectrum for scientific purposes, scientists must participate in the discussions leading up to each WRC (the next of which is WRC-23, currently scheduled to be held in 2023). Indeed, the fact that more than 95 percent of the spectrum allocations below 3 GHz are for active users underscores how important it is that the vulnerable passive services participate in the process and express their concerns about potential adverse effects on their operations. 3 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 policymakers as they prepare for WRC-23 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-23 agenda items of relevance to U.S. radio astronomers and Earth remote sensing researchers, along with proposed agenda items for WRC-27. The agenda items are discussed in numerical order to facilitate locating a specific one, for easy perusal. The committee has determined that some outcomes of the agenda items shown in Table 1.1 and Table 1.2 may impact RAS and EESS operations, and has provided the reasons for its view, as well as the scientific application of the bands that may be impacted. Agenda items not discussed in this report are not expected to have a direct impact on protected RAS or EESS observations. Potential impacts are assessed based on criteria related to in-band, out-of-band, and spurious emissions, as appropriate, as noted in each discussion. 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 in the radio spectrum can be found in Handbook of Frequency Allocations and Spectrum Protection for Scientific Uses: Second Edition. 4 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. 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. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 3

TABLE 1.1 WRC-23 Agenda Items of Relevance to and Having Potential Impacts on RAS and EESSa 1 On the basis of proposals from administrations, taking account of the results of WRC-19 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 identification of the frequency bands 3 300-3 400 MHz, 3 600-3 800 MHz, 6 425-7 025 MHz, 7 025-7 125 MHz and 10.0-10.5 GHz for International Mobile Telecommunications (IMT), including possible additional allocations to the mobile service on a primary basis, in accordance with Resolution 245 (WRC-19); 1.4 To consider, in accordance with Resolution 247 (WRC-19), the use of high altitude platform stations as IMT base stations (HIBS) in the mobile service in certain frequency bands below 2.7 GHz already identified for IMT, on a global or regional level; 1.5 To review the spectrum use and spectrum needs of existing services in the frequency band 470-960 MHz in Region 1 and consider possible regulatory actions in the frequency band 470- 694 MHz in Region 1 on the basis of the review in accordance with Resolution 235 (WRC-15); 1.8 To consider, on the basis of ITU-R studies in accordance with Resolution 171 (WRC-19), appropriate regulatory actions, with a view to reviewing and, if necessary, revising Resolution 155 (Rev.WRC-19) and No. 5.484B to accommodate the use of fixed-satellite service (FSS) networks by control and non-payload communications of unmanned aircraft systems; 1.9 To review Appendix 27 of the Radio Regulations and consider appropriate regulatory actions and updates based on ITU-R studies, in order to accommodate digital technologies for commercial aviation safety-of-life applications in existing HF bands allocated to the aeronautical mobile (route) service and ensure coexistence of current HF systems alongside modernized HF systems, in accordance with Resolution 429 (WRC-19); 1.10 To conduct studies on spectrum needs, coexistence with radiocommunication services and regulatory measures for possible new allocations for the aeronautical mobile service for the use of non-safety aeronautical mobile applications, in accordance with Resolution 430 (WRC-19); 1.11 To consider possible regulatory actions to support the modernization of the Global Maritime Distress and Safety System and the implementation of e-navigation, in accordance with Resolution 361 (Rev.WRC-19); 1.12 To conduct, and complete in time for WRC-23, studies for a possible new secondary allocation to the Earth exploration-satellite (active) service for spaceborne radar sounders within the range of frequencies around 45 MHz, taking into account the protection of incumbent services, including in adjacent bands, in accordance with Resolution 656 (Rev.WRC-19); 1.13 To consider a possible upgrade of the allocation of the frequency band 14.8-15.35 GHz to the Space Research Service, in accordance with Resolution 661 (WRC-19); 1.14 To review and consider possible adjustments of the existing or possible new primary frequency allocations to EESS (passive) in the frequency range 231.5-252 GHz, to ensure alignment with more up-to-date remote-sensing observation requirements, in accordance with Resolution 662 (WRC-19); 1.15 To harmonize the use of the frequency band 12.75-13.25 GHz (Earth-to-space) by earth stations on aircraft and vessels communicating with geostationary space stations in the fixed satellite service globally, in accordance with Resolution 172 (WRC-19); 1.16 To study and develop technical, operational and regulatory measures, as appropriate, to facilitate the use of the frequency bands 17.7-18.6 GHz and 18.8-19.3 GHz and 19.7-20.2 GHz (space-to-Earth) and 27.5-29.1 GHz and 29.5-30 GHz (Earth-to-space) by non-GSO FSS earth stations in motion, while ensuring due protection of existing services in those frequency bands, in accordance with Resolution 173 (WRC-19); PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 4

1.17 To determine and carry out, on the basis of the ITU-R studies in accordance with Resolution 773 (WRC-19), the appropriate regulatory actions for the provision of intersatellite links in specific frequency bands, or portions thereof, by adding an inter-satellite service allocation where appropriate; 1.19 To consider a new primary allocation to the fixed-satellite service in the space-to-Earth direction in the frequency band 17.3-17.7 GHz in Region 2, while protecting existing primary services in the band, in accordance with Resolution 174 (WRC-19); 9 To consider and approve the Report of the Director of the Radiocommunication Bureau, in accordance with Article 7 of the Convention; 9.1 On the activities of the Radiocommunication Sector since WRC-19: a) In accordance with Resolution 657 (Rev.WRC-19), review the results of studies relating to the technical and operational characteristics, spectrum requirements and appropriate radio service designations for space weather sensors with a view to describing appropriate recognition and protection in the Radio Regulations without placing additional constraints on incumbent services; b) Review of the amateur service and the amateur-satellite service allocations in the frequency band 1 240-1 300 MHz to determine if additional measures are required to ensure protection of the radionavigation-satellite (space-to-Earth) service operating in the same band in accordance with Resolution 774 (WRC-19); c) Study the use of International Mobile Telecommunication system for fixed wireless broadband in the frequency bands allocated to the fixed services on primary basis, in accordance with Resolution 175 (WRC-19); d) Protection of EESS (passive) in the frequency band 36-37 GHz from non-GSO FSS space stations. NOTE: Acronyms are defined in Appendix B. a A complete list of WRC-23 Agenda Items can be found, for example, at the ITU-R Preparatory studies for WRC- 23 website, https://www.itu.int/en/ITU-R/study-groups/rcpm/Pages/wrc-23-studies.aspx. This website also provides a link to the complete list of WRC-27 Preliminary Agenda Items and all of the relevant WRC resolutions. TABLE 1.2 Preliminary WRC-27 Agenda Items Relevant to RAS and EESS WRC-27 a 2 On the basis of proposals from administrations and the Report of the Conference Preparatory Meeting, and taking account of the results of WRC-23, to consider and take appropriate action in respect of the following items: 2.1 To consider, in accordance with Resolution 663 (WRC-19), additional spectrum allocations to the radiolocation service on a co-primary basis in the frequency band 231.5-275 GHz and identification for radiolocation applications in frequency bands in the range 275-700 GHz for millimetre and sub-millimetre wave imaging systems; 2.2 To study and develop technical, operational and regulatory measures, as appropriate, to facilitate the use of the frequency bands 37.5-39.5 GHz (space-to-Earth), 40.5-42.5 GHz (space-to-Earth), 47.2-50.2 GHz (Earth-to-space) and 50.4-51.4 GHz (Earth-to-space) by aeronautical and maritime earth stations in motion communicating with geostationary space stations in the fixed satellite service, in accordance with Resolution 176 (WRC-19); 2.3 To consider the allocation of all or part of the frequency band [43.5-45.5 GHz] to the fixed- satellite service, in accordance with Resolution 177 (WRC-19); 2.4 The introduction of pfd and e.i.r.p. limits in Article 21 for the frequency bands 71-76 GHz and 81-86 GHz in accordance with Resolution 775 (WRC-19); PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 5

2.5 The conditions for the use of the 71-76 GHz and 81-86 GHz frequency bands by stations in the satellite services to ensure compatibility with passive services in accordance with Resolution 776 (WRC-19); 2.6 To consider regulatory provisions for appropriate recognition of space weather sensors and their protection in the Radio Regulations, taking into account the results of ITU-R studies reported to WRC-23 under agenda item 9.1 and its corresponding Resolution 657 (Rev. WRC- 19); 2.7 To consider the development of regulatory provisions for non-geostationary fixed satellite system feeder links in the frequency bands 71-76 GHz (space-to-Earth and proposed new Earth-to-space) and 81-86 GHz (Earth-to-space), in accordance with Resolution 178 (WRC- 19); 2.8 To study the technical and operational matters, and regulatory provisions, for space-to-space links in the frequency bands [1 525-1 544 MHz], [1 545-1 559 MHz], [1 610-1 645.5 MHz], [1 646.5-1 660.5 MHz] and [2 483.5-2 500 MHz] among non-geostationary and geostationary satellites operating in the mobile-satellite service, in accordance with Resolution 249 (WRC- 19); 2.9 To consider possible additional spectrum allocations to the mobile service in the frequency band 1 300-1 350 MHz to facilitate the future development of mobile-service applications, in accordance with Resolution 250 (WRC-19). NOTE: Acronyms are defined in Appendix B. a Note attached to the title of the Resolution: The appearance of square brackets around certain frequency bands in this Resolution is understood to mean that WRC-23 will consider and review the inclusion of these frequency bands with square brackets and decide, as appropriate. 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, particularly when optical remote sensing is blocked by clouds or attenuated by water vapor. Instruments operating in the EESS bands provide data that are important to human welfare and security and provide critical information for scientific research, commercial endeavors, and government operations in areas such as defense, security, meteorology, hydrology, agriculture, 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, weather prediction (sensible and latent heat exchange with the atmosphere), and for defense (planning military deployments). Passive sensors also provide atmospheric temperature and humidity, information to monitor changes in the polar ice cover, and measurements needed for assessing hazards such as hurricanes, wildfires, and drought. For many of these applications, satellite-based radio frequency remote sensing represents the only available method of obtaining atmospheric and surface data for the entire planet. This monitoring becomes even more important as the impacts of global climate change result in more extreme weather conditions. Major U.S. governmental users of EESS data include the National Oceanic and Atmospheric Administration (NOAA), NSF, NASA, the Department of Defense (DoD), the U.S. Department of Agriculture (USDA), the U.S. Geological Survey (USGS), the U.S. Agency for International Development (USAID), the Federal Emergency Management Agency (FEMA), and the Forest Service of the U.S. Department of Agriculture. Many of these data sets are also available free to anyone anywhere in the world and, as a result, are used for Earth science research and operational forecasting worldwide. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 6

FIGURE 1.1 Percent of the time that the 1.413 GHz passive microwave sensor on NASA’s SMAP mission detects an RFI level of 5 K or more in horizontal polarization for data from April 2015 to March 2016. Despite the “all emission prohibited” designation on this spectral region, particularly strong radio frequency interference (RFI) events occur over Europe, the Middle East, and Asia. Soil moisture retrievals from some regions are severely contaminated and hence inaccurate or altogether unusable due to RFI events corrupting the entire footprint. SOURCE: Reprinted, with permission, from Mohammed et al., IEEE TGARS, 54, 2016, doi: 10.1109/TGRS.2016.2580459, © 2016 IEEE. However, passive instruments in space are particularly vulnerable to anthropogenic emissions because they rely on very weak signals emitted naturally from Earth’s surface and atmosphere. These weak signals require the sensors to integrate over space and time, which makes the measurements even more sensitive to competing human-made signals present in these averages. This is especially a concern for EESS because sensors in space monitor globally and view large swaths of the surface at one time and are thus subject to aggregate interference from all emitters in the area scanned. Measurement precision and accuracy is already limited by the available bandwidth, and some of these valuable measurements are being blocked by radio frequency interference, even within protected bands. For instance, it is now impossible to retrieve soil moisture from measurements made by the Advanced Microwave Scanning Radiometer (AMSR2) at 6.9 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. Thresholds for harmful interference for EESS observations are listed in Recommendation ITU-R RS.2017. These thresholds should form the basis of all considerations for acceptable levels of emissions, either in-band or out-of-band, direct-beam or sidelobe for active services that might affect EESS observations. Particular attention should be paid to the highly variable nature of atmospheric attenuation, driven by a combination of pressure (i.e., density), humidity (i.e., season and/or location), and path (i.e., transmitter location and angle). Strong atmospheric absorption in some spectral regions enables ground- based transmission to proceed in certain key EESS bands without risk of interference. However, airborne transmissions in some of these same regions would be far less attenuated and render EESS observations of a volume containing such a transmitter unusable. Similarly, ground-based transmissions oriented vertically will be far less attenuated than those aimed horizontally. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 7

Radio Astronomy Service Since radio waves were first detected from an astronomical object in 1932, radio astronomy has been an indispensable tool for studying the universe. It is important to realize that the universe (outside the environment of Earth) is essentially transparent at radio frequencies, so radio waves propagate unimpeded. This is not true at optical frequencies, where many objects and phenomena are unobservable due to the absorption of pervasive interstellar dust. For example, the deep interiors of molecular clouds, places where new stars are forming, are completely cloaked by interstellar dust, yet transparent to radio emission. Thus, radio astronomy observations provide a unique and unfettered view of the universe. Notable radio astronomy discoveries include the identification of the first planets outside the solar system, which were found circling a distant pulsar. Further, the Nobel Prize–winning discovery of pulsars by radio astronomers has led to the recognition of a widespread population of rapidly spinning neutron stars with gravitational fields at their surface up to 100 billion times stronger than on Earth’s surface. Subsequent radio observations of pulsars have revolutionized understanding of the physics of neutron stars and have resulted in the first experimental evidence for gravitational radiation, which was recognized with the awarding of another Nobel Prize. Recently, radio astronomy was used to detect the first electromagnetic signal from merging compact objects identified by their gravitational waves and to provide the first ever image of the accretion disk around a supermassive black hole. 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 the Milky Way galaxy. Radio spectroscopy and broadband continuum observations 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 observations uncovered the first evidence of the existence of a black hole in our galactic center, a phenomenon that may be crucial to the formation and dynamics of many other galaxies. Radio 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. Radio astronomy measurements led to the Nobel Prize–winning discovery of the cosmic microwave background (CMB), the radiation left over from the earliest period in our universe (the Big Bang). Later, exquisitely precise radio observations discovered the weak fluctuations in the CMB of only one- thousandth of a percent, generated in the early universe. Such fluctuations are responsible for the eventual formation of all stars and galaxies we know today, and detailed measurements of them allow cosmologists to constrain the history and eventual fate of our expanding universe. Radio observations were also used to measure the rotation of external galaxies and provide some of the first evidence of dark matter, which makes up 95 percent of the gravitating matter in the universe. Radio astronomy also provides valuable information that produces practical benefits to society—for example, by monitoring solar flares and coronal ejections (so called “space weather”). Such monitoring allows for 1- to 4-day forecasts of geomagnetic disturbances that can affect the operation of satellite communications, the Global Navigation Satellite Services (GNSS), including the U.S. Global Positioning System (GPS) and other navigation systems, and terrestrial power grids. Techniques developed for high spatial resolution imaging of celestial radio sources are used to determine Earth Orientation Parameters (EOP), which describe anomalies in the rotation of Earth due to the changing of distribution of Earth’s mass over time. In addition, the extreme sensitivity required for observing the very faint signals from celestial sources poses extraordinary challenges that have demanded highly innovative technology and engineering solutions to overcome. Many of these solutions have proven to be extremely useful in other applications, including 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 detection; time calibration for GPS; and wireless technologies, including fast Fourier transform (FFT) chips, solid-state oscillators, frequency multipliers, and cryogenics. Other practical applications include data correlation and recording technology and image restoration techniques, among many others. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 8

It is important to understand the exceedingly weak nature of the typical signals detected by radio telescopes. They can be a million times fainter 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. As mentioned above, the threshold levels of interference harmful to radio astronomy observations are listed in Recommendation ITU-R RA.769, and these are the levels that have been used to develop these views with regard to the RAS. It should be emphasized that serious interference can result from weak transmitters even when they are situated in the side-lobes of a radio astronomy antenna (i.e., the far-field sensitivity spots that may be at large angles from the desired detection direction). Further, radio telescopes are particularly vulnerable to interference from airborne and satellite transmitters, since terrain shielding cannot block the signal from high-altitude emitters. The new mega-constellations of satellites currently being launched or under design will pose a particularly difficult challenge for radio astronomy (as well as for optical astronomy). It is worth mentioning in this regard that some of the assumptions used to derive the harmful interference levels in Recommendation ITU-R RA.769 break down when dealing with such mega-constellations because of the large number of satellites expected to be in beams at any one time. More restrictive levels may be necessary in the future for the protection of radio astronomy observations in the presence of such constellations. As mentioned above, radio observations of spectral lines 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 measurements of spectral lines arising from atomic and molecular transitions, so it is important to protect radio astronomers’ access to the characteristic frequencies of the most important atomic and molecular transitions, including sufficient bandwidth to measure spectral lines broadened by both thermal and kinematic processes. However, important spectral lines continue to be discovered at new frequencies, and, in addition, the expansion of the universe causes spectral lines of distant galaxies to be red-shifted to lower frequency, sometimes by orders of magnitude from their rest frequency. Thus, detection of atoms or molecules in distant sources may require observing at frequencies well below the rest frequency of a line measured in the laboratory. Consequently, 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 red- shifted spectroscopic lines from distant sources, including the very early universe. The requirements for continuum observations of cosmic thermal and nonthermal sources are different from those of spectral lines. They are not dependent on fixed frequencies that are determined by atomic or molecular transitions. Rather, observations at multiple frequencies and potentially wide bandwidths are needed to define the continuum spectrum of stars, galaxies, quasars, pulsars, and other cosmic radio sources. Historically, only narrow bands spaced at approximately one-octave separation throughout the spectrum have been given protection at various levels to enable these important continuum spectral studies. 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 considerable improvement in sensitivity over earlier, narrowband systems. Broad bandwidths are also employed 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. 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, RFI mitigation and excision adds additional costs, whether in receiver design, increased data storage due to more frequent time sampling, or increased computational speeds required to process the high volume of recorded data. Thus, it is clear that all users, both passive and active, benefit from a clean spectrum. In other words, while radio astronomy facilities rely in part on geographic shielding and local designations of radio quiet zones to reduce sources PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 9

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 co-exist. PREPUBLICATION COPY – SUBJECT TO FURTHER EDITORIAL CORRECTION 10

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Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Agenda Items at Issue at the World Radiocommunication Conference 2023 Get This Book
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The radio frequency spectrum is a limited resource for which there is an 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. Since radio waves do not stop at national borders, international regulation is necessary to ensure effective use of the radio spectrum for all parties. Use of the radio spectrum is regulated internationally by the Radio Regulations (RR), an international treaty. The International Telecommunication Union (ITU) has as its mission the facilitation of the efficient and interference-free use of the radio spectrum. Every 2 to 5 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 Agenda Items, which are agreed on at the previous WRC.

At the request of the National Science Foundation and the National Aeronautics and Space Administration, this report provides guidance to U.S. spectrum managers and policymakers as they prepare for the 2023 WRC to protect the scientific exploration of Earth and the universe using the radio spectrum. This report identifies the 2023 agenda items of relevance to U.S. radio astronomers and Earth remote sensing researchers, along with proposed agenda items for the 2027 WRC.

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