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Views of the U.S. National Academies of Sciences, Engineering, and Medicine on Selected Preliminary Agenda Items for WRC-31

The following pages discuss the committee’s consensus opinions on the potential impact and relevance of a subset of the preliminary agenda items identified for the 2031 World Radiocommunication Conference (WRC-31). For quick reference, Figure 3-1 illustrates the bands under consideration for each preliminary agenda item, along with a brief (one- or two-word) summary of the topic and/or service the preliminary agenda item is considering.

Regulatory documents referred to below—Radio Regulations (RRs), WRC resolutions, and International Telecommunication Union (ITU) recommendations—are referenced in Appendix D.

The discussions of the committee’s views on these preliminary agenda items follows the structure of the discussions for the 2027 WRC (WRC-27) agenda items. The committee has opted to discuss Preliminary Agenda Items 2.1 and 2.6 together, given that they concern similar topics and spectral regions. Similarly, Preliminary Agenda Items 2.10 and 2.11 are discussed together, as are 2.12 and 2.13. The committee has no views on Preliminary Agenda Items 2.5, 2.7, or 2.8.

PRELIMINARY AGENDA ITEMS 2.1 AND 2.6: ALLOCATIONS FROM 102–275 GHz AND ABOVE

WRC-31 Preliminary Agenda Item 2.1 considers “potential new allocations to the fixed, mobile, radiolocation, amateur, amateur-satellite, radio astronomy, Earth exploration-satellite (passive and active) and space research (passive) services in the frequency range 275-325 GHz in the Table of Frequency Allocations of the Radio Regulations, with the consequential update of Nos. 5.149, 5.340, 5.564A and 5.565, in accordance with Resolution 721 (WRC-23).”

WRC-31 Preliminary Agenda Item 2.6 considers “the identification of the frequency bands [102-109.5 GHz, 151.5-164 GHz, 167-174.8 GHz, 209-226 GHz and 252-275 GHz] for International Mobile Telecommunications, in accordance with Resolution 255 (WRC-23).”

Figure 3-2 shows the bands under consideration in these preliminary agenda items, along with those allocated to, and/or afforded protections for, scientific use.

For Preliminary Agenda Item 2.1, Resolution 721 (WRC-23) considers that the spectrum above 275 GHz has the potential to enable

new high-capacity transmissions, and it points out that the radio astronomy service (RAS) and Earth exploration-satellite service (EESS, passive) sensors are already operating in this spectral range. The resolution discusses other identified uses and potential capabilities of this region and notes other prior studies. The resolution then invites studies on the spectrum needs for the fixed, mobile, radiolocation, amateur, amateur-satellite, radio astronomy, EESS (passive and active) and space research service (passive) in the frequency range 275–325 GHz. It also invites studies of the sharing and compatibility needs for the services identified in the preliminary agenda item as well as studies of potential new allocations to those services.

For Preliminary Agenda Item 2.6, Resolution 255 (WRC-23) considers that International Mobile Telecommunications (IMT) provides mobile communications on a worldwide scale and have contributed to global economic and social development. New ultra-low-latency and high-bit-rate applications demand contiguous blocks of spectrum for IMT use. The resolution references ITU reports detailing potential usage of the spectral region under consideration in Preliminary Agenda Item 2.6 for IMT but acknowledges that these bands are adjacent to bands allocated for passive use and afforded RR 5.340 “all emissions prohibited” protection. The resolution then invites studies of the spectrum needs for the terrestrial components of IMT in the bands under consideration in Preliminary Agenda Item 2.6, the technical and operational characteristics of systems that would or could be expected to operate in these bands, as well as future deployment scenarios. It further invites sharing and compatibility studies that take into account the protection needs of incumbent services potentially affected, including those in adjacent bands.

As shown in Figure 3-2, the bands under consideration in these combined agenda items encompass a large spectral range, and many of them overlap or are adjacent to bands allocated to the scientific services (RAS and EESS) on a primary basis, as well as some secondary allocations and bands identified for scientific use in various footnotes of the RR (notably RR 5.565, which identifies many bands above 275 GHz for passive scientific use). Thresholds for harmful interference into these bands is given in ITU-R RA.769-2 and ITU-R RS.2017-0 for RAS and EESS (passive), respectively.

Radio Astronomy Service

Radio astronomical observations of millimeter-wave spectral lines associated with a wealth of important molecules provide access to information about the structure and dynamics of celes-

tial objects—and about the astrochemical processes accounting for the synthesis and abundance of chemical species—that cannot be obtained by other means. This motivates the many primary and secondary allocations, and footnote protections, afforded to RAS in the millimeter-wave spectral region, including those within or adjacent to the spectral bands under consideration in these two preliminary agenda items. Just a few of the important species covered by these and adjacent bands include methyl acetate (CH₃CCH), methanol (CH₃OH), carbon monoxide (CO) and its important isotopologues, the hydronium ion (H₃O⁺), and deuterated water vapor (HDO). Besides molecular line observations, these bands are used for continuum imaging of cool matter throughout the universe, providing a view into regions that cannot be seen at visible or infrared wavelengths, and revealing objects enshrouded by dust.

The resolutions underpinning these two preliminary agenda items acknowledge these protections and invite the sharing and compatibility studies required to maintain them. For the bands below 275 GHz considered under Preliminary Agenda Item 2.6, the appropriate interference thresholds are covered by Recommendation ITU-R RA.769-2. Because of the importance of millimeter-wave continuum observations, the more stringent thresholds applicable to continuum in RA.769 Table 1 need to be applied in these studies. All the bands in Preliminary Agenda Item 2.6 are adjacent to RR 5.340 “all emissions prohibited” bands, and footnote 1 of resolves 2 of Resolution 255 specifically notes that these studies need to extend to adjacent band protection. For the 275–325 GHz band covered by Preliminary Agenda Item 2.1, Resolution 721 (WRC-23) notes that RR 5.565 applies to this band and urges “all practicable steps” protection to portions thereof. Interference thresholds above 275 GHz are not defined in RA.769, but use of the thresholds given that at 270 GHz is appropriate, given the typical RAS receiver sensitivity does not vary greatly across this band, and that atmospheric emission and attenuation is likewise similar until the increase associated with the terrestrial 325 GHz H₂O (water) line becomes significant a few gigahertz away from the line center.

Earth Exploration-Satellite Service

As described in the discussion of WRC-27 Agenda Item 1.18, the region under consideration in these agenda items encompass many bands that are central to atmospheric observations for weather forecasting and research into climate and atmospheric chemistry. Notable among these are those bands around the 118 GHz oxygen and 183 GHz water vapor lines, which convey information on

atmospheric temperature and humidity, respectively. These lines are very broad (multiple gigahertz) at Earth’s surface, narrowing with increasing altitude, thus decreasing pressure (e.g., hundreds of megahertz in the ~15 km region). This pressure-dependent line width enables information on different (broadly overlapping) altitude regions to be obtained by measuring closer to or further from the line center. However, this demands interference-free access to broad, contiguous bands for accurate results. Other bands in the regions under consideration are in less opaque “window regions” of the spectrum. These have multiple uses including measurement of surface properties (e.g., sea ice and snow). The window bands also convey information on clouds and precipitation that have a two-fold importance. Firstly, they provide valuable information on clouds and precipitation in their own right. Secondly, as clouds and precipitation impact signals observed across the spectrum, notably including in the oxygen and water vapor bands, the window measurements are essential for correcting (or in some cases excising) observations in other bands. That said, in contrast to observations at shorter wavelengths (e.g., in the infrared or visible), at these frequencies, the EESS (passive) measurements are responsive to the size of the cloud and precipitation particles (as well as the particle number density), making them a unique resource for operational weather forecasting and Earth system research.

A wide range of satellites from multiple agencies, countries, and regions routinely make daily global EESS (passive) observations across all these bands. As for WRC-27 Agenda Item 1.18, an increasing number of such missions is planned going forward, including those under development by commercial entities. Notably, the 118 GHz oxygen band is receiving increased attention as it conveys much the same information as the “workhorse” 50–60 GHz band while requiring a smaller antenna for a given measurement footprint size. The limited number of sensors focused on atmospheric chemistry employ limb sounding (looking at the atmosphere edge-on) in the higher frequencies (e.g., 118 GHz and above), as opposed to the majority of other EESS (passive) observations which view in a nadir, cross-track, or conical-scanning geometry.

Finding and Recommendation

Recommendation: Groups undertaking studies under the 2031 World Radiocommunication Conference Preliminary Agenda Items 2.1 and 2.6 should ensure that future deployment of International Mobile Telecommunications or other technolo

gies in the bands under consideration in these two preliminary agenda items does not cause harmful interference to with radio astronomy service (RAS) or Earth exploration-satellite service (EESS, passive) observations. Specifically, the studies should

• Use the thresholds given in Recommendations ITU-R RA.769-2 and ITU-R RS.2017-0 for RAS and EESS (passive), respectively, as the starting point for any consideration of interference. In the case of the RAS (ITU-R RA.769-2) recommendations, the “continuum” thresholds should be applied.
• Consider aggregate interference from realistic levels of deployment of future services.
• Include studies of the extent to which signals in these bands, even when largely directed horizontally, can reflect off Earth’s surface and be directed into an EESS (passive) sensor’s field of view.

Finding: The 270 GHz continuum interference thresholds defined in Recommendation ITU-R RA.769-2 are appropriate for sharing and compatibility studies at 275–325 GHz under Resolution 721 (WRC-23).

PRELIMINARY AGENDA ITEM 2.2: WIRELESS POWER TRANSMISSION

WRC-31 Preliminary Agenda Item 2.2 considers “the possible frequency bands for non-beam and beam wireless power transmission to avoid harmful interference to the radiocommunication services caused by wireless power transmission, in accordance with Resolution 910 (WRC-23).”

The associated Resolution 910 (WRC-23) invites the ITU Radiocommunication Sector (ITU-R) to study possible frequency bands for wireless power transmission (WPT) “on the basis of avoiding harmful interference to the radiocommunication services caused by WPT.” Currently, WPT “is not a defined radio service in the Radio Regulations” and has no consistent international regulations in place. In particular, some administrations treat WPT systems as industrial, scientific, or medical services, and others treat them as short-range radiocommunication devices (although this is not precisely defined as a service). The resolution also notes that some administrations “classify certain applications of WPT as a radio service that is not defined in the Radio Regulations.”

WPT systems are sorted into two broad categories: (1) non-beam wireless power transmission for electric vehicles, mobile, and portable devices and (2) beamed methods using radio frequencies as the power delivery means for mobile/portable devices and sensor networks. Recommendations ITU-R SM.2110-1 and ITU-R SM.2129-1 list potential frequencies for non-beamed WPT in two ranges: very low frequency (magnetic resonant/inductive: 19–21 kHz, 55–57 kHz, 63–65 kHz, 79–90 kHz, 100–148.5 kHz), and high frequency (HF, magnetic resonant: 6765–6795 kHz). For beamed WPT wireless charging of mobile/portable devices and charging/powering of sensor networks, Recommendation ITU-R SM.2151-0 lists potential frequencies at ultra-high frequency (915–921 MHz), S band (2410–2483.5/2486 MHz), C band (5725–5875 MHz), and millimeter-wave (61–61.5 GHz).

Relevant to the avoidance of interference by WPT systems, both Recommendations ITU-R SM.2110-1 and ITU-R SM.2129-1 indicate that radio astronomy should be “treated as [a] radiocommunication service.”

Radio Astronomy Service

Although none of the proposed WPT bands in the recommendations above occur in-band for identified radio astronomy applica-

tions, out-of-band emissions (OOBE) are of concern given the potentially high-power transmissions of these systems, and OOBE need to be regulated to avoid interference hazards for the radio astronomy service (RAS). This is particularly relevant for beamed systems due to the nature of these WPT systems which aim to propagate substantial energy to target devices. Recommendation ITU-R RA.769-2 defines continuum RAS protections in the 611 MHz and 1413.5 MHz bands surrounding the 915–921 MHz beam WPT proposed band at −253 to −255 dB(W/(m² Hz)) for 2000 second integrations. Similarly, RA.769-2 defines spectral line radio astronomy protections for 48 GHz and 88.6 GHz, surrounding the 61–61.5 GHz proposed WPT band, as −209 to −208 dB(W/(m² Hz)) for 2000 second integrations. WPT systems in space are of particular concern due to the absence of terrain-shielding attenuation, and the RA.769-2 levels are quoted for terrestrial sources of interference confined to the neighborhood of the horizon. Therefore, the committee recommends quantitative study of boresight-to-boresight avoidance and 0 dBi sidelobe levels for OOBE from orbiting WPT operations within the fields of view of major radio telescope facilities. Also recommended are studies of aggregate emission power flux densities for multiple simultaneously operating WPT systems as compared to Recommendation ITU-R RA.769-2 protection levels.

Earth-Exploration-Satellite Service

Similar to RAS, Earth exploration-satellite service (EESS, passive) allocations need to be protected from OOBE by WPT systems. Recommendation ITU-R RS.2017-0 defines performance and interference criteria for satellite-based passive remote sensing systems. For the 61–61.5 GHz millimeter-wave potential WPT band from Recommendation ITU-R SM.2151-0, Table 2 in ITU-R RS.2017-0 lists a maximum interference level of −169 dBW in a 100 MHz band at 0.01 percent duty cycle that needs to be maintained in the bracketing bands of 52.6–59.3 GHz and 86–92 GHz.

Recommendation

Recommendation: Groups undertaking studies under Preliminary Agenda Item 2.2 for the 2031 World Radiocommunication Conference should ensure the continued protection of radio astronomy service (RAS) and Earth exploration-satellite service (EESS, passive) observations in bands allocated to those services that are adjacent to planned beamed wireless power transmission (WPT) transmissions. Specifically, studies should

• Quantify the limits on beamed WPT out-of-band emissions (OOBE) that will be needed to ensure that the interference thresholds identified in Recommendations ITU-R RA.769-2 and ITU-R RS.2017-0, for RAS and EESS (passive), respectively, are not exceeded.
• Include consideration of aggregate emissions from multiple uncoordinated WPT devices and systems in devising those OOBE limits.

PRELIMINARY AGENDA ITEM 2.3: AERONAUTICAL AND MARINE GROUND STATIONS IN 12.75–13.25 GHz

WRC-31 Preliminary Agenda Item 2.3 considers “the use of aeronautical and maritime earth stations in motion communicating with non-geostationary space stations in the fixed-satellite service (Earth-to-space) in the frequency band 12.75-13.25 GHz, in accordance with Resolution 133 (WRC-23).”

Figure 3-3 shows the bands affected by this preliminary agenda item.

Aeronautical and maritime Earth stations in motion are respectively known as A-ESIMs and M-ESIMs. The associated Resolution 133 (WRC-23) for this preliminary agenda item invites the ITU Radiocommunication Sector (ITU-R) to study technical and operational characteristics of A-ESIMs and M-ESIMs planning to communicate with space stations in non-geostationary orbits (non-

GSO) in the frequency band 12.75–13.25 GHz (Earth-to-space) and to study compatibility between this proposed new application and incumbent active satellite services in this band. In addition, recognizing that the systems that use this band for uplink typically use the 10.7–10.95 GHz band (space-to-Earth) for downlink, and that passive sensors within the Earth exploration-satellite service (EESS) operate in the adjacent 10.6–10.7 GHz band, the resolution states that potential for interference from unwanted emissions into the passive band “should be studied to ensure protection of existing and future use of the frequency band by the EESS (passive).”

Both EESS (passive) and the radio astronomy service (RAS) have a primary allocation in the 10.6–10.7 GHz band, and Resolution 133 (WRC-23) acknowledges that all emissions are prohibited in the 10.68–10.7 GHz band under RR 5.340.

Radio Astronomy Service

Although RAS is not explicitly acknowledged in Resolution 133 (WRC-23), the 10.6–10.7 GHz band is a preferred band for RAS continuum observations. Modern radio astronomical receivers and signal-processing systems normally observe a wider bandwidth on an unprotected basis—for example, the entire X-band from 8–12 GHz, with the allocated band affording an interference-free sub-band with protected data quality. X-band observations target energetic non-thermal sources such as synchrotron emission from supernova remnants and active galactic nuclei (AGN). Observing the continuum spectrum of these objects across multiple widely spaced bands can provide insight into their otherwise unresolved structure, for instance by locating the spectral break associated with synchrotron absorption in a surrounding envelope. X-band observations are also suitable for detecting free-free thermal emission from clouds of ionized atomic hydrogen in the interstellar medium known as HII regions.

Expanding use of the fixed-satellite service (FSS) 12.75–13.25 GHz (Earth-to-space) uplink band to A-ESIMs could expose RAS observatories operating in the 10.6–10.7 GHz RAS primary allocation to unwanted emissions from associated downlink emissions in the adjacent 10.7–10.95 GHz band. For this reason, the sharing and compatibility studies relating to EESS (passive) called for by this resolution need to be expanded to include impacts on RAS. It should be emphasized that in the most directly adjacent portion of the passive allocation, from 10.68–10.7 GHz, all emissions are prohibited

under RR 5.340 (with a minor exception to allow non-airborne fixed and mobile transmissions in some countries, but only from equipment installed before 1985). Recommendation ITU-R RA.769-2 provides the practical interference thresholds needed to ensure that this requirement is satisfied.

Earth Exploration-Satellite Service

The 10.6–10.7 GHz EESS (passive) allocation protects one among a suite of channels used by multichannel passive microwave radiometers to sense Earth properties, including water vapor, cloud liquid water, soil moisture, sea surface salinity and temperature, land surface roughness and wind-driven sea-surface roughness, and vegetation. The use of multiple channels enables the contributions of these different parameters to be disentangled through their overlapping but diverse spectral emission signatures. Thus, interference in a single band hampers interpretation of signals in other bands and reduces the information yield from the complete multi-band observing system. Table 3-1 lists current and planned EESS (passive) sensors that employ the 10.6–10.7 GHz passive band.

As recognized by Resolution 133 (WRC-23), expanding use of the FSS 12.75–13.25 GHz (Earth-to-space) uplink band to A-ESIMs and M-SIMs, with attendant expanded use of the associated 10.7–10.95 GHz downlink band, may result in increased unwanted emissions into the 10.6–10.7 GHz EESS (passive) band. This could occur either by reflection of space-to-Earth transmissions from Earth’s surface into the field of view of an EESS (passive) sensor, or by line-of-sight sidelobe-coupled reception into the calibration or scene view of the EESS (passive) sensor. Moreover, the diverse and dynamic locations of A-ESIMs and M-ESIMs compared with earth stations of the FSS would increase the fraction of Earth potentially impacted by this interference.

The sharing and compatibility studies invited in Resolution 133 need to produce realistic transmission loss factors for the different coupling modes noted above, and realistic models for the number of simultaneously active downlink transmissions to multiple A-ESIMs and M-ESIMs likely to fall within the field of view of an EESS (passive) sensor. These data, together with unwanted emission masks for downlink transmissions in the 10.7–10.95 GHz (space-to-Earth) downlink band are needed to ensure that the interference thresholds for EESS (passive) sensors defined in Recommendation ITU-R RS.2017-0 can be met under rules for new frequency use by ESIMs.

TABLE 3-1 Current and Planned Satellite Missions Utilizing the 10.6–10.7 GHz Passive Microwave Frequency Band

* While the Aqua mission continues to operate at time of writing, the AMSR-E instrument ceased observations in 2016.

NOTE: Acronyms are defined in Appendix C.

SOURCE: Courtesy of World Meteorological Organization OSCAR (Observing System Capability Analysis and Review Tool), n.d., “Satellite Frequencies for Earth Observation, Data Transfer and Platform Communications and Control,” https://space.oscar.wmo.int/satellitefrequencies, accessed on December 12, 2024.

Recommendations

Recommendation: Groups undertaking studies under Preliminary Agenda Item 2.3 of the 2031 World Radiocommunication Conference should ensure protection of Earth exploration-satellite service (EESS, passive) observations in the 10.6–10.7 GHz band from unwanted out-of-band emissions (OOBE) from the adjacent 10.7–10.95 GHz (space-to-Earth) downlink band serving aeronautical Earth stations in motion (A-ESIMs) and maritime Earth stations in motion (M-ESIMs) operating in the 12.75–13.25 GHz (Earth-to-space) uplink band. Specifically, studies should

• Produce realistic estimates of transmission loss factors for direct and Earth-reflected power coupled from downlink transmissions into an EESS (passive) sensor, considering interference into both Earth-observing beams and observations of cold space used for calibration.
• Derive thresholds for unwanted OOBE into the 10.6–10.7 EESS (passive) band from 10.7–10.95 GHz downlink transmission links serving A-ESIMs and M-ESIMs, sufficient to satisfy the interference thresholds in Recommendation ITUR RS.20170, and taking into account realistic models for the aggregate number of simultaneously active links.

Recommendation: The International Telecommunication Union Radiocommunication Sector should consider expanding the scope of this preliminary agenda item to study impacts on radio astronomy observations in the radio astronomy service primary allocation from 10.6–10.7 GHz.

PRELIMINARY AGENDA ITEM 2.4: SPACE-TO-SPACE LINKS AT 4 AND 6 GHz

WRC-31 Preliminary Agenda Item 2.4 considers “based on the results of ITU Radiocommunication Sector studies, support for inter-satellite service allocations in the frequency bands 3 700-4 200 MHz and 5 925-6 425 MHz, and associated regulatory provisions, to enable links between non-geostationary orbit satellites and geostationary orbit satellites, in accordance with Resolution 683 (WRC-23).”

Figure 3-4 shows the bands under consideration in this preliminary agenda item, along with relevant nearby allocations to science services.

Resolution 683 (WRC-23) begins by observing that many satellites in non-geostationary orbits (non-GSO satellites) operate with limited and non-real-time connectivity to Earth stations, and that inter-satellite communications would enhance the efficiency of non-GSO operations, hence significant interest in new space-to-space links in the above listed bands. The resolution goes on to note other usage of these and adjacent bands and limitations of the extent to which space-to-space links can claim protection from incumbent terrestrial services. The resolution then invites studies of potential new space-to-space usage of these bands, with

• 5925–6425 GHz being considered for communications from non-GSO to GSO satellites (thus in the same “direction” as Earth-to-space transmissions) and
• 3700–4200 GHz being considered for GSO to non-GSO transmission (thus in the same “direction” as space-to-Earth).

Pending the outcome of these studies, WRC-31 is invited to consider new allocations along these lines.

Earth Exploration-Satellite Service (Passive)

As noted in RR 5.458, the frequency band 6425–7250 MHz has been used by EESS (passive) to measure sea-surface temperatures (SSTs) on a global scale. The measurement of SST is important for detecting and forecasting meteorological events that drastically impact the safety and security of administrations and the populations of their countries. These measurements also provide an essential resource for monitoring and understanding climate variability and climate change. Passive microwave observations, with certain frequency bands having unique physical characteristics, are the only means of obtaining all-weather daily and global measurement of SST (see Table 3-2). Current measurements of SST within the 6425–7250 MHz bands are hindered by radio frequency interference (RFI), particularly near coasts where provision of accurate SSTs has the greatest impact upon the local population: RFI mitigation is vital and therefore careful studies of complementary frequency bands are needed.

At present, passive microwave sensor measurements of SST are carried out in the frequency band 6425–7075 MHz. While complementary bands needed to ensure continuity of SST measurement by EESS (passive) are being investigated, it is crucial that the current bands (6425–7250 MHz) are protected to ensure continuity of SST measurements.

The potential new allocation for non-GSO to GSO transmissions in the band from 5925–6425 MHz, is directly adjacent to the band identified for this EESS (passive) use. This adjacency raises the prospect of significant RFI into the EESS (passive) observations unless appropriate steps, such as guard bands and/or appropriate out-of-band emission (OOBE) masks are employed.

As noted in the discussion of WRC-27 Agenda Items 1.12 and 1.18, there is potential for spaceborne transmissions to contribute RFI not only in the beams used by EESS (passive) sensors for Earth observation but also in the calibration beams, directed well away

TABLE 3-2 Previous, Current, and Planned EESS (Passive) Sensors Observing in the 6425–7075 MHz Band

* The Aqua AMSR-E instrument failed in 2016; the remainder of the Aqua platform continues to operate at the time of writing.

NOTE: Acronyms are defined in Appendix C.

SOURCE: Courtesy of World Meteorological Organization OSCAR (Observing System Capability Analysis and Review Tool), n.d., “Satellite Frequencies for Earth Observation, Data Transfer and Platform Communications and Control,” https://space.oscar.wmo.int/satellitefrequencies, accessed on December 12, 2024.

from Earth. Accordingly, even when a non-GSO transmitter orbits at a higher altitude than an EESS (passive) sensor, the potential for RFI needs to be evaluated. In cases where the non-GSO transmitter orbits beneath an EESS (passive) sensor, there is strong potential for direct interference into the main observing beam. In either geometry, particular attention needs to be paid to the, admittedly unlikely, but mission-ending, situation whereby a spaceborne transmitter is sufficiently close to an EESS (passive) sensor that permanent damage is caused to the sensitive microwave receivers through beam-to-beam coupling in either the Earth-viewing or cold sky calibration views.

As noted in RR 5.437, the 4200–4400 MHz band may be authorized for EESS (passive) use on a secondary basis. Furthermore, WRC-27 Agenda Item 1.19 specifically considers a new allocation to EESS (passive) in this band, which is expected to lead to increased

use of this band for Earth observation. It will be important to ensure protection of EESS (passive) observations in this band from interference from space-to-space transmissions under consideration in the adjacent 3700–4200 GHz band.

Recommendation

Recommendation: Groups undertaking studies and considering new regulatory provisions under Preliminary Agenda Item 2.4 of the 2031 World Radiocommunication Conference should

• Ensure, through implementation of guard bands or suitable out-of-band emission limits, that interference into Earth exploration-satellite service (EESS, passive) observation in the 6425–7250 MHz band, and those planned for 4200–4400 GHz, remain below the thresholds given in Recommendation ITU-R RS.2017-0.
• Consider realistic levels of aggregate emissions in devising the protection criteria detailed in the above bullet.
• Examine the potential for permanent damage to EESS (passive) sensors from nearby active transmitters and require suitable mitigation if warranted.

PRELIMINARY AGENDA ITEM 2.9: RADIONAVIGATION AT 5 GHz

WRC-31 Preliminary Agenda Item 2.9 considers “possible new allocations to the radionavigation-satellite service (space-to-Earth) in the frequency bands [5 030-5 150 MHz and 5 150-5 250 MHz] or parts thereof, in accordance with Resolution 684 (WRC-23).”

Resolution 684 (WRC-23) underscores the value of radio navigation satellite services (RNSS) while observing that current allocations to RNSS may be insufficient for providing the higher positioning accuracy, availability, and robustness that may be required in the future. It goes on to note the various allocations and/or protection needs for services in the frequency range from 4990–5250 MHz, specifically noting the primary allocation to the radio astronomy service (RAS) at 4990–5000 MHz. The resolution ends by inviting studies of the technical and operational characteristics for RNSS in the band from 5030–5250 MHz, with a view to a potential future allocation to RNSS in all or part of this region to be considered at WRC-31.

Figure 3-5 shows the band under consideration in this preliminary agenda item along with nearby bands allocated (or afforded footnote protections) for scientific use. The band from 4990–5000 MHz is allocated to RAS on a primary basis with a secondary allocation to RAS extending down to 4800 MHz (and additional primary allocations in 4825–4835 MHz and 4950–4990 MHz in Argen-

tina, Australia, and Canada, per RR 5.443). The 4950–4990 MHz band also has a secondary allocation to the Earth exploration-satellite service (EESS, passive) per RR 5.339. Immediately adjacent to the band under consideration in Preliminary Agenda Item 2.9, EESS (active) has a primary allocation in 5250–5570 MHz.

Radio Astronomy Service

The 4990–5000 MHz RAS allocation is a workhorse of the world’s premier radio telescopes: it optimizes sensitivity while providing versatile access to a diversity of astrophysical emission mechanisms. Take, for example, its use in studies of the variable and transient sky, where astronomers are not only discovering “classic” explosions such as supernovae, but are also uncovering a staggering diversity of other time-variable phenomena, including the mergers of stars and the tearing-apart of stars by massive black holes. At 5 GHz, studies of these transient phenomena uniquely trace the fastest moving material—jets and blast waves that are launched at speeds approaching that of light and which interact with surrounding material, revealing the explosion’s immediate environment. In one specific example, the object GW 170817 was the first merger of two neutron stars ever detected in gravitational waves, and also the first “multi-messenger” gravitational wave event. Over days, weeks, and now years of following the outburst, astronomers have watched a radio source be born, brighten, and then fade. The 5 GHz signature heralded the birth and expansion of a highly collimated, relativistic jet of material launched when the two stars merged. Such jets are one of the most common and dramatic manifestations of accretion onto black holes, dumping vast amounts of energy into the environment, and the launch of a jet in GW 170817 is a strong piece of evidence that a black hole could have been promptly formed in the merger. Future 5 GHz observations of neutron star mergers will shed light on the nature of merger remnants and the diversity of outcomes following stellar mergers.

The sensitivity and resolution of the 5 GHz band also make it ideal for tracing star formation and accreting black holes at high redshift. Unlike observations at ultraviolet, optical, and infrared wavelengths, radio observations penetrate the enshrouding dust, enabling a complete census of star formation and accretion activity across cosmic time. Observations at 5 GHz have been used to show that, even in the early universe, star formation was distributed throughout galaxy disks (similar to the Milky Way today), and that this star formation drove outflows that polluted intergalactic space

with heavy elements. In the future, by combining 5 GHz observations with higher-frequency data, the contributions from star formation and black holes will be separated, enabling an understanding of how the supermassive black holes that lurk at the centers of most galaxies have grown since 13 billion years ago, or when the universe was less than 10 percent of its current age.

The cosmic emissions that radio astronomers receive are extremely weak, and radio astronomy observations cannot be successfully performed without access to interference-free frequency bands and protected, remote geographic locations. Because of their remarkable sensitivity, radio observatories are particularly vulnerable to interference from in-band emissions, spurious and out-of-band emissions (OOBE) from licensed and unlicensed users of neighboring bands, and emissions that produce harmonic signals in the RAS bands, even if those human-made emissions are weak and distant. The detrimental interference level for continuum RAS observations is −241 dB(W/(m² Hz)) at 4995 MHz, based on Table 1 of Recommendation ITU-R RA.769-2.

The 4990–5000 MHz band needs to remain interference-free down to these thresholds, taking into account the effects of OOBE and spurious, and aggregate emissions, including harmonics. The committee also notes the need to protect continuum observations outside of allocated bands in nationally or internationally recognized radio quiet zones (RQZs).

Earth Exploration-Satellite Service

The 5250–5570 MHz band (“C band”), directly above the band considered in Preliminary Agenda Item 2.9, is allocated on a primary basis to EESS (active). This band has been used quite heavily for both airborne and spaceborne synthetic aperture radar (SAR) systems, which have provided countless critical observations of Earth system variables. In the polarimetric SAR (PolSAR) mode, these sensors provide measurements of surface soil moisture, vegetation properties, land ice, sea ice, and snow cover. In the interferometric SAR (InSAR) mode, the observations quantify glacier movement and surface deformation and are used in support of earthquake and volcanic activity studies. The European Space Agency (ESA) C-band satellite InSAR observations made in the 1990s showed glaciers melting and breaking up. Numerous pre- and post-earthquake surface deformation maps have also been produced through InSAR observations from C-band SAR satellites. These help with damage assessment and response, as well as investigations that someday may lead to predictions of earthquake activity.

The Canadian Space Agency (CSA) Radarsat program and the ESA ERS-1, ERS-2, EnviSAT, and Sentinel-1 programs are among the prominent providers of C-band SAR data. The CSA programs started with Radarsat-1 (1995–2013, 5.3 GHz center frequency and bandwidth of 30 MHz) and continued to Radarsat-2 (2007–present, 5.405 GHz center frequency and bandwidth of 100 MHz) and the Radarsat constellation mission (2019–present, 5.405 GHz center frequency and bandwidth of 100 MHz). The ESA missions started with ERS-1 in 1991–2000 (5.3 GHz center frequency and bandwidth of 15.55 MHz) and continued with ERS-2 (1995–2011, same frequency band as ERS-1), then Envisat, and most recently, the Sentinel-1 A/B constellation (with satellites launched in 2014 and 2016, both of which are still active at the time of writing, 5.405 GHz center frequency and 100 MHz bandwidth). The continuous coverage of the globe with these satellite SAR missions, starting from the early 1990s and continuing through the present time, has uniquely enabled enormous scientific and operational progress. Measurements from these sensors have been used in thousands of peer-reviewed scientific publications and have generated operational products in support of decision-making in agriculture, natural hazard prediction, disaster response, surveillance and security, wildfire monitoring, and burned-area mapping, among others. These application areas rely on the continued availability of these data sets.

Although SAR systems have their own source of microwave energy transmission onboard, they are still quite sensitive to radio frequency interference (RFI). SAR processing algorithms rely on precise knowledge of received signals and on coherence between transmitted and received signals across the time length of each synthetic aperture. This reliance originates from the need to compute the autocorrelation (also known as matched filtering or pulse compression) between the received signal and a replica of the transmitted signal as part of the SAR processing algorithms. RFI can cause distortion in the received signal spectra in such a way that the amplitude and/or phase of the various spectral components of the received signal (usually a chirp) are substantially altered from those of the nominally expected spectrum, causing detrimental defocusing of the SAR image. If the interfering source is strong enough, it can also cause saturation in the receiver, entirely burying the original radar signal. The effects of RFI typically show up in SAR imagery as either blurred or bright bands along the affected range lines, making them unusable. If filtering schemes are applied to exclude the affected edges of the involved frequency bands (therefore reducing the total bandwidth), the net result is a reduction of image resolution

because the range resolution of a SAR system is determined by the effective processing bandwidth. In the extreme case where the RFI signal is strong enough to saturate the receiver, no amount of filtering can salvage the signal even at the expense of reduced resolution.

Although, as noted above, EESS (passive) has a secondary allocation in 4950–4990 MHz band per RR 5.339, the committee is unaware of any current or planned use of this band.

Finding and Recommendations

Recommendation: Groups studying potential future use of the 5030–5250 MHz band by RNSS under Preliminary Agenda Item 2.9 of the 2031 World Radiocommunication Conference should include consideration of what out-of-band emission (OOBE) limits will be needed to ensure that

• Interference into the radio astronomy service (RAS) observations at 5000 MHz and lower frequencies falls below the thresholds given in Recommendation ITU-R RA.769-2.
• Interference into the Earth exploration-satellite service (EESS, active) observations in the immediately adjacent 5250–5570 MHz band falls below the thresholds given in Recommendation ITU-R RS.1166-5.
• The above-noted thresholds are not exceeded when aggregate transmissions from realistic numbers of transmitters impact an RAS or EESS (active) sensor.

Finding: Wider bands than are primarily allocated to RAS are required to fully realize the scientific potential of radio astronomy.

Recommendation: Administrations should use local coordination—for example, through radio quiet zones—to protect radio astronomy observatories, taking into account not only terrestrial emitters but also spaceborne emitters.

PRELIMINARY AGENDA ITEMS 2.10 AND 2.11: NEW COMMUNICATIONS LINKS FOR THE EARTH EXPLORATION-SATELLITE SERVICE

WRC-31 Preliminary Agenda Item 2.10 considers “a possible new primary allocation to the Earth exploration-satellite service (Earth-to-space) in the frequency band 22.55-23.15 GHz, in accordance with Resolution 664 (Rev.WRC-23).”

WRC-31 Preliminary Agenda Item 2.11 considers either “an upgrade of the secondary allocation to the Earth exploration-satellite service (space-to-Earth) in the frequency band [37.5-40.5 GHz] or possible new worldwide frequency allocations on a primary basis to the Earth exploration-satellite service (space-to-Earth) in certain frequency bands within the frequency range [40.5-52.4 GHz], in accordance with Resolution 685 (WRC-23).”

Note that protection of or advocacy for bands used for space-to-Earth and Earth-to-space communications in support of the Earth exploration-satellite service (EESS) do not fall in the scope of the statement of task for this report, which focuses on the bands used for the scientific observations themselves—that is, those allocated for EESS (passive) or EESS (active), as well as for the radio astronomy service (RAS). Accordingly, the committee’s views on these preliminary agenda items are focused on their implications for those “scientific” bands, specifically, for example, the protection of RAS observations from new space-to-Earth downlinks.

Figure 3-6 shows the bands under consideration in these preliminary agenda items, along with relevant nearby bands allocated for scientific use and/or afforded “all emissions prohibited” protection under RR 5.340.

For Preliminary Agenda Item 2.10, Resolution 664 (Rev. WRC-23) invites studies on spectrum requirements for and sharing and compatibility between nearby EESS (Earth-to-space) and the existing services. These studies are expected to take into account various existing allocations (including several primary allocations to RAS and EESS (passive), protected by RR 5.149 and RR 5.340, respectively). The Resolution states that studies should ensure the protection of these services and use relevant technical and operational parameters of their current and planned use. It ends by inviting WRC-31 to consider, based on the results of the studies, a new worldwide primary allocation to EESS (Earth-to-space) in the frequency band 22.55–23.15 GHz.

The frequency band 25.5–27 GHz is allocated worldwide to EESS (space-to-Earth) on a primary basis, but it currently does not

have a paired band for potential associated Earth-to-space links. Resolution 664 (Rev. WRC-23) notes that an EESS (Earth-to-space) allocation in the frequency band 22.55–23.15 GHz would allow for uplinks and downlinks on the same transceiver, increasing efficiency and reducing satellite complexity, and further, such an allocation would allow for its use for satellite tracking, telemetry and command in combination with the existing EESS (space-to-Earth) allocation at 25.5–27 GHz.

The frequency band 22.5–23.15 GHz is currently allocated to the fixed, inter-satellite, and mobile services on a primary basis and is also allocated to the Space Research Service (SRS) (Earth-to-space) on a primary basis, paired with the SRS (space-to-Earth) allocation in the frequency band 25.5–27 GHz.

Considering passive scientific use, 22.21–22.5 GHz is allocated to RAS and EESS (passive) on a primary basis, and the frequency bands 22.81–22.86 GHz and 23.07–23.12 GHz are noted for RAS use under RR 5.149 (whereby administrations are urged to take “all

practicable steps” to protect RAS observations); while 23.6–24 GHz is allocated to EESS (passive) and RAS on a primary basis and protected under RR 5.340 (“all emissions prohibited”).

Resolution 664 (Rev. WRC-23) further recognizes that the possible development of EESS (Earth-to-space) in the frequency band 22.55–23.15 GHz should not constrain the use and development of EESS (passive) operating in the frequency band 23.6–24 GHz, and that protection of the RAS sites operating in the relevant frequency bands may be achieved through sufficient geographic separation from EESS Earth stations.

For Agenda Item 2.11, Resolution 685 (WRC-23) discusses the need for either

• A review of the existing allocation to EESS (space-to-Earth) in the frequency band 37.5–40.5 GHz, and sharing and compatibility studies as necessary, in order to determine the feasibility of upgrading the existing secondary allocations to primary status, or
• The identification of frequency bands within the frequency range 40.5–52.4 GHz for sharing and compatibility studies as necessary, to determine the feasibility of creating new primary allocations to EESS (space-to-Earth) within 40.5–52.4 GHz. These studies are to be done while ensuring the protection of the primary services.

The committee notes here that the use of square brackets placed around the frequency bands listed in the preliminary agenda item and in Resolution 685 (WRC-23)—“[37.5–40.5 GHz]” and “[40.5–52.4 GHz]”—is understood to mean that WRC-27 will consider and review the inclusion of these frequency bands, which may therefore change before a final WRC-31 agenda item is agreed upon.

As noted in the discussion of Preliminary Agenda Item 2.11, above, the frequency band 37.5–40.5 GHz is allocated worldwide to EESS (space-to-Earth) on a secondary basis and to several other services on a primary basis. The frequency band 40–40.5 GHz is allocated worldwide to EESS (Earth-to-space) on a primary basis. An additional frequency allocation to EESS (space-to-Earth) above 37.5 GHz would allow its use for payload data transmissions in combination with the 40–40.5 GHz band, as well as enabling uplinks and downlinks on the same transceiver, increasing efficiency and reducing satellite complexity.

Resolution 685 (WRC-23) further notes the “importance of appropriate regulatory status and certainty to accommodate the

requirements of future Earth observation missions”; and further notes that this might require primary allocation to EESS (space-to-Earth) in certain frequency bands above 37.5 GHz.

Earth Exploration-Satellite Service (Passive)

Preliminary Agenda Item 2.10 invites WRC-31 to consider, based on the results of studies, a new worldwide primary allocation to EESS (Earth-to-space) in the frequency band 22.55–23.15 GHz. The adjacent frequency band 22.21–22.5 GHz is allocated to EESS (passive) on a primary basis, while the 23.6–24 GHz is allocated to EESS (passive) on a primary basis and afforded RR 5.340 “all emissions prohibited” protection.

WMO OSCAR Space¹ lists more than 50 current and planned Earth observing missions, including from the United States (Department of Defense (DoD), National Oceanic and Atmospheric Administration (NOAA), National Aeronautics and Space Administration (NASA)), Europe (European Space Agency (ESA) and European Meteorological Satellite agency (EUMETSAT)), China (China Meteorological Administration (CMA)), and Russia (Russian Federal Service for Hydrometorology and Environmental Monitoring (RosHydroMet)) utilizing the 22.21–27.0 GHz passive microwave frequency band. These measurements are central for operational weather forecasting and nowcasting, climate science, and scientific research, and essential for numerous other meteorological applications.

The microwave radiometer measurements at or near 23.8 GHz constitute one of the longest continuous records of atmospheric-integrated water vapor content measured from space, starting with the ESA ERS-1 mission in 1991. The development, deployment, and operation of these satellite missions, together with their long-term calibrated and validated climate data records, represent a significant investment and commitment by numerous national governments and international agencies. Furthermore, many private commercial companies use these vital measurements for weather forecasting, and companies are also expanding into the commercial EESS sector.

These microwave sensors are in a low Earth orbit (primarily polar), and any given satellite/sensor will, at best, provide global coverage twice per day. The goal is to have at most a 4- to 6-hour revisit time to observe the same point on Earth by a satellite observ-

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¹ See World Meteorological Organization, “OSCAR: Observing Systems Capability Analysis and Review Tool,” https://space.oscar.wmo.int/observingmissions/view/13, accessed October 14, 2024, for more details.

ing the same or similar weather information. The current 4 to 6-hour revisit rate is achieved through the shared use of satellite observations provided by multiple space agencies. The observations are distributed as part of the World Meteorological Organization (WMO) Integrated Global Observing System, which provides a framework for the integration and sharing of observational data from national meteorological and hydrological services and other sources.

It should be noted that weather is not stationary, and accurate weather forecasts depend on having global observations for accurate initialization of numerical weather forecast models. Often the most valuable (impactful) observations are those that observe weather features upstream of the forecast area.

Additionally, EESS (active) instruments that use radar (in other frequency bands) to measure the solid earth (geoid), ocean topography (sea-surface height and significant wave height), sea-ice elevation, and precipitation typically include an EESS (passive) sensor in the 23.6–24.0 GHz band. Those companion passives sensors measure the total atmospheric water vapor amount, used to correct the refraction-induced path delay in the radar signal.

Preliminary Agenda Item 2.11 considers two options for upgrading the secondary allocation to primary EESS (space-to-Earth) in the frequency ranges below, while ensuring protection of the primary services.

Option #1 – 37.5–40.5 GHz

The adjacent 36–37 GHz EESS (passive) band is allocated on a co-primary basis with fixed, mobile, and SRS (passive). There are more than 25 current or planned satellite missions utilizing the adjacent 36–37 GHz band, including sensors from (but not only) the United States, Europe, China, Japan, and Russia. In addition, some of these sensors incorporate channels that extend above 37.5 GHz.

The 36–37 GHz band is a cornerstone for EESS (passive) remote sensing of atmospheric water vapor, snow water equivalent (SWE), precipitation, cloud properties, sea-ice concentration and type, snow cover, sea-surface temperature, ocean surface winds, and ocean topography. The 36–37 GHz window channel on microwave radiometers also supports radar altimeters; one such example is the Micro-Wave Radiometer (MWR) on the Sentinel-3(A-D) ocean and land imaging satellite series. These altimeters also provide a substantial contribution to solid earth (geoid) science.

Option #2 – 40.5–52.4 GHz

The 50.2–50.4 GHz frequency band has a primary allocation to EESS (passive) and is protected under RR 5.340 (“all emissions prohibited”). This band is also associated with mandatory limits, stated in ITU-R Resolution 750 (Rev. WRC-19), which governs the level of unwanted emissions from fixed-satellite service (FSS) Earth-to-space links (note this preliminary agenda item concerns space-to-Earth links). The adjacent 52.6–54.25 GHz band has a primary allocation to EESS (passive) and is protected under RR 5.340 (“all emissions prohibited”).

As discussed above for WRC-27 Agenda Items 1.1 and 1.3, the 50.2–50.4 GHz oxygen absorption band is critical for lower atmospheric temperature profiling and surface emission characterization, and it also provides crucial surface characterization for precipitation intensity at the surface (liquid or solid). The 52.6–54.25 GHz oxygen absorption band is critical for lower atmospheric temperature profiling in nearly all-weather conditions. Both frequency bands are essential components of the 50–60 GHz temperature sounding spectral region and are crucially important for numerical weather prediction (NWP) forecast accuracy and climate assessment.

Numerous current and planned EESS (passive) microwave sensors use these frequency bands (see Table 2-1 (50.2–50.4 GHz) and Table 2-2 (52.6–54.25 GHz)).

Both the 50.2–50.4 GHz and the 52.6–54.25 GHz passive microwave frequency bands have a very long history for operational observations, beginning in 1978 with the Microwave Sounding Unit (MSU) launched on the NOAA TIROS-N (Television and Infra-Red Observation Satellite) and extending to the present. The development, deployment, and operation of these satellite missions, together with their long-term calibrated and validated climate data records, represent a significant investment and commitment by numerous national governments and international agencies. Multiple international space agencies have planned continuity missions that include these frequency bands, extending out through at least 2042.

The committee notes that the RosHydroMet/Roscosmos² Meteor-M N2 satellite series (with six planned missions extending beyond 2031) with a passive microwave imaging/sounding radiometer has two window channels, at 42.0 GHz and 48.0 GHz,

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² See World Meteorological Organization, “Space Agency: Roscosmos,” OSCAR: Observing Systems Capability Analysis and Review Tool, https://space.oscar.wmo.int/spaceagencies/view/roscosmos, accessed November 5, 2024, for more details.

each with 400 MHz bandwidth. These frequency bands are located directly within the bands under consideration in WRC-31 Preliminary Agenda Item 2.11.

As noted in the committee’s views on WRC-27 Agenda Item 1.1, weather and climate observations have an effective usable “lifespan” far beyond their initial use in numerical weather analysis and forecasting applications. They are used in Earth System Reanalyses (e.g., NASA Modern Era Retrospective analysis for Research and Applications, version 2 (MERRA-2) or European Centre for Medium-Range Weather Forecasting (ECMWF) Reanalysis, version 5 (ERA-5)) which enhance understanding of Earth system processes, and for long-term climate studies. As such, the “value” of a particular frequency band or set of observations is not ephemeral, rather, it endures forever forward in time.

Radio Astronomy Service

The frequency range 22.21–22.5 GHz is allocated to RAS on a primary basis, and the frequency bands 22.81–22.86 GHz and 23.07–23.12 GHz are noted for RAS use under RR 5.149 (where “administrations are urged to take all practicable steps to protect the radio astronomy service from harmful interference”); while 23.6–24.0 GHz is allocated to RAS on a primary basis and protected under RR 5.340 (“all emissions prohibited”). These frequencies include the scientifically important water vapor line at 22.235 GHz and a series of ammonia lines across 23.69–23.87 GHz. These ammonia lines are helpful for constraining the temperature of interstellar gas, and for understanding the interstellar medium from which stars form.

Within the frequencies above 37.5 GHz considered in Preliminary Agenda Item 2.11 there is significant RAS use. Allocations to RAS in this range are 42.5 GHz–43.5 GHz (RR 5.149), 48.94–49.04 GHz (RR 5.149), 49.04–49.05 GHz (RR 5.555), and 51.4–52.6 GHz (RR 5.556, under national arrangements). RAS facilities within the United States or with U.S. involvement that use these frequencies include the Atacama Large Millimeter Array (ALMA) in Chile, which has a 35–50 GHz receiver band, Q-band (39.2–49.8 GHz) at the Green Bank Radio Telescope in the National Radio Quiet Zone (NRQZ) in West Virginia, and at the Very Large Array (VLA) in New Mexico (40–50 GHz). The Simons Observatory in Northern Chile also has a band centered at 39 GHz.

Scientific applications in these frequency bands include observations of cold molecular gas in the interstellar medium and are particularly important for understanding star formation (and through

that planet formation). Some of the isotopes of the CS molecule create spectral lines found in the 48.94–49.04 GHz band. These measurements can be used to constrain the isotope abundance of carbon, a useful check for theories of nucleosynthesis in stars and their links to star-formation rates. The spectral region from 30–50 GHz contains the strongest lines of the cyanoacetylene molecule (HC₃N), which is useful for understanding the collapse of molecular clouds to protostars and is an important constraint for the temperature of extremely cold interstellar gas. There are also a set of silicon monoxide (SiO) masers (microwave amplification by stimulated emission of radiation) in the 42.5–43.5 GHz band, observations of which have been useful for understanding the gases in both the envelopes of evolved stars and in regions of star formation. This frequency range is also important for our understanding of the first light in the universe, coming from the cosmic microwave background (CMB). The most serious hindrance to the full exploitation of existing and future CMB data is the understanding of the galactic foreground. To create images that isolate emission from the CMB alone, measurements at multiple frequencies, including those at 40–50 GHz, are required. These lower frequencies (for the CMB) in the frequency range of Preliminary Agenda Item 2.11 are particularly important to identify and remove foreground synchrotron emission.

Recommendations

Recommendation: Groups undertaking studies of new Earth-to-space transmissions in the 22.55–23.15 GHz band under Preliminary Agenda Item 2.10 of the 2031 World Radiocommunication Conference should

• Ensure that out-of-band emission (OOBE) limits for new Earth-to-space transmissions are sufficient to keep interference into the 23.6–24.0 GHz band, allocated to the Earth exploration-satellite service (EESS, passive) and the radio astronomy service (RAS), below the thresholds given in Recommendations ITU-R RS.2017-0 and ITU-R RA.769-2, respectively.
• Consider the impact of aggregate emissions in ascertaining the OOBE limits described above.
• Identify the separation distances between EESS (Earth-to-space) ground stations and RAS facilities that would be sufficient to ensure protection of observations in the overlapping 22.81–22.86 GHz and

• 23.07–23.12 GHz bands within which, under RR 5.149, administrations are urged to take “all practicable steps” to protect RAS observations.

Recommendation: Groups undertaking studies of new space-to-Earth usage of selected bands within 37.5–54.2 GHz and considering new/revised allocations under Preliminary Agenda Item 2.11 of the 2031 World Radiocommunication Conference should

• Refrain from recommending new space-to-Earth allocations in or directly adjacent to bands in this range having a primary allocation to the Earth exploration-satellite service (EESS, passive) or the radio astronomy service (RAS), or afforded “all practicable steps” protection under RR 5.149. The two bands listed under RR 5.340 (“all emissions prohibited”) in this range should be given particular consideration in this regard.
• Ensure that guard bands, out-of-band emission (OOBE) masks or other suitable measures are required to keep interference from new space-to-Earth transmissions into any bands allocated to RAS below the thresholds given in Recommendation ITU-R RA.769-2.
• Ensure that guard bands, OOBE masks or other suitable measures are required to keep interference from new space-to-Earth transmissions into any bands allocated to EESS (passive), through reflection of Earth’s surface, below the thresholds given in Recommendation ITU-R RS.2017-0.

PRELIMINARY AGENDA ITEMS 2.12 AND 2.13: EARTH EXPLORATION-SATELLITE SERVICE (ACTIVE) AT 3 AND 10 GHz

WRC-31 Preliminary Agenda Item 2.12 considers “possible new allocations to the Earth exploration-satellite service (active) in the frequency bands [3 000-3 100 MHz] and [3 300-3 400 MHz] on a secondary basis, in accordance with Resolution 686 (WRC-23).”

WRC-31 Preliminary Agenda Item 2.13 considers “studies on coexistence between spaceborne synthetic aperture radars operating in the Earth exploration-satellite service (active) and the radiodetermination service in the frequency band 9 200-10 400 MHz, with possible actions as appropriate, in accordance with Resolution 722 (WRC-23).”

Figure 3-7 shows the bands under consideration in these WRC-31 preliminary agenda items.

For Preliminary Agenda Item 2.12, Resolution 686 (WRC-23) discusses the use of Earth exploration-satellite service (EESS, active) sensors for measuring ice boundaries, ice type and age, ocean wave

structure and associated near-ocean-surface wind speed and direction, and for mapping of ocean circulation. The resolution observes that the 3100–3300 MHz band is already allocated to EESS (active) on a secondary basis, and that both altimetry and synthetic aperture radar (SAR) measurements are currently made in this band. The resolution indicates that bandwidths of at least 400 MHz are preferable for providing high-resolution SAR observations, and that such observations are intended to be made away from populated regions of the globe, being primarily made over oceans and seas. The resolution goes on to detail other services allocated to or afforded protections in the adjacent 3000–3100 MHz and 3300–3400 MHz bands, including radio astronomy observations in 3332–3329 MHz and 3345.8–3352.5 MHz, which administrations are urged to take all practicable steps to protect, per RR 5.149. The resolution then invites study of the potential to provide new secondary allocations to EESS (active) in the 3000–3100 MHz and 3300–3400 MHz bands.

For Preliminary Agenda Item 2.13, Resolution 722 (WRC-23) observes that SAR is amongst the most widely used type of EESS (active) sensor, and that the 9.2–10.4 GHz band is allocated to EESS (active) for such use, having been broadened from an initially narrower 9500–9800 MHz following WRC-07 and WRC-15. This band is shared with the radiodetermination service, including radiolocation and radionavigation-satellite services (RNSS). As noted in the resolution, previous studies had indicated that interference between EESS (active) transmissions and operations in those other services should be minimal. However, given the growing usage of spaceborne SAR in this band, the resolution invites studies of the technical and operational capabilities of SAR in the 9.2–10.4 GHz band, and of the coexistence between SARs and radiodetermination services operating in the same band.

Radio Astronomy Service

As noted above, in the higher of the 3000–3100 MHz and 3300–3400 MHz bands covered by Preliminary Agenda Item 2.12, “all practicable steps” RR 5.149 protection applies in two narrower bands from 3332–3339 MHz and 3345.8–3352.5 MHz. A third RR 5.149 band, from 3260–3267 MHz, lies in the existing secondary allocation for EESS (active) that sits between the two bands under consideration in Preliminary Agenda Item 2.12. Together, these three RR 5.149 bands protect radio astronomy service (RAS) observations of methylidyne (CH). In molecular clouds, CH density typically tracks that of molecular hydrogen (H₂), which lacks a dipole

moment and is hence invisible to radio telescopes. For this reason, CH serves as an important tracer of H₂ abundance.

In the 9.2–10.4 GHz band covered by Preliminary Agenda Item 2.13, there are no allocations or footnote protections for radio astronomy. Nevertheless, observing is carried out at these frequencies on an opportunistic basis. Examples of molecular species that are observed at these frequencies are formamide (NH₂CHO); cyanotetraacetylene (HC₉N); 1,3 butadiynyl (C₄H); 1,3,5 hexatriynyl (C₆H); cyanoethylnyl (CCCN); ethyne isocyanide (HCCNC); methanol (CH₃OH); 2,4,6-heptatriynenitrile (HC₇N); and deuterated water vapor (HDO).

An important consideration for protecting radio observatories from EESS (active) sensors is the possibility of receiver saturation or even damage caused by reception of intense radar pulses by a radio telescope. Power flux-density (pfd) limits for emissions into certain bands from a space station are given in RR 21.16. Resolution 722 (WRC-23) states (in noting b) that “No. 21.16 provides the power flux-density limit at Earth’s surface produced by emissions from EESS (active) in the frequency band 9900–10400 MHz with respect to the protection of the fixed service.” At elevations more than 53 degrees above the horizontal, the referenced threshold is −66.6 dB(W/m²) average power in a reference bandwidth of 1 MHz. This level, intended to protect horizontally directed stations of the fixed service, could easily be damaging to a sensitive radio astronomical receiver on a large telescope in a main-beam-to-main-beam coupling scenario. For example, assuming a telescope aperture area of 8000 m² (comparable to the Green Bank Telescope), and a duty cycle of 0.01 for the radar transmitter, the peak power spectral density delivered to the receiver would be 175 mW/MHz, a level which could be expected to be damaging even for a RAS receiver principally intended to observe in the near-adjacent RAS allocation from 10.6–10.7 GHz. Consequently, although protection of RAS observatories is not discussed in Resolution 722 (WRC-23), coordination with RAS observatories needs to be considered to assess the hazard level and, as appropriate, adopt measures to either to avoid the telescope pointing at the SAR transmitter or to avoid the SAR transmitter transmitting toward the telescope.

Earth Exploration-Satellite Service

Currently, within the “S-band” (2–4 GHz), the 3100–3300 MHz band is allocated on a secondary basis to EESS (active). Multiple scientific and operational applications utilize this band. Over land,

these include observations of crops, soil moisture, coastal zones, ice edge detection, and snow accumulation. Over the oceans, this band is used for observations related to ocean waves and ocean surface winds.

Notably, the upcoming National Aeronautics and Space Administration (NASA)-Indian Space Research Organization (ISRO) Synthetic Aperture Radar (NISAR) mission, a joint venture between NASA and ISRO, will include an S-band SAR. The NISAR S-band operates in the 3162–3237 MHz band. S-band falls in between two other popular SAR frequency bands for terrestrial applications, which are L-band (~1.2 GHz) and C-bands (~5.3 GHz). The common rationale for using S-band is that it reaches a balance between the vegetation and surface penetration properties of L-band and the ionospheric insensitivity of C-band.

The current allocated bandwidth of 200 MHz affords a maximum possible range resolution of r = c/2B = 75 cm, where c is the speed of light in vacuum and B is the total bandwidth. A higher total bandwidth at S-band, such as that proposed by Preliminary Agenda Item 2.12 (400 MHz), would enhance this resolution by a factor of two, which would be 37.5 cm for a single-look image product. SAR products are almost always “multi-looked” to reduce speckle noise, which degrades the image resolution. The variance of the speckle noise distribution decreases linearly with the number of multi-look samples. It is therefore desirable to take many multi-look samples to beat down speckle noise. For example, the prospective NISAR system is designed to produce S-band images with 3–24 m resolution (depending on operating modes), meaning that it could have very few look averages. Increasing the bandwidth (and therefore single-look resolution) by a factor of two will result in higher-quality image products, especially for applications such as ice-edge detection and coastal zone monitoring, where increasingly higher resolutions are desirable.

The 9200 MHz–10.4 GHz band (part of the “X-band,” 8–12 GHz) is already allocated on a primary basis to EESS (active). The majority of X-band EESS (active) systems are SARs, including the constellation of commercial satellites operated by Capella Space in the United States, the German Aerospace Agency (DLR) TerraSAR-X, the PAZ system from the Spanish Space Agency, and the Italian Space Agency COSMO-SkyMed constellation of four satellites. With a long heritage of EESS (active) space and airborne operations, X-band SAR systems are primarily used for high-resolution surveillance, tracking, surface and mobility monitoring, defense, and terrain mapping applications. They are used in stripmap, spot-

light, and interferometric SAR modes. Such applications require a large bandwidth to achieve their sub-meter resolution requirements. Their large bandwidths and the nature of products needed to meet their application requirements in some cases also dictate the need for high transmit powers. The transmit power of these SAR systems ranges from 800 W (TerraSAR-X) to 4 kW (COSMO-SkyMed). Similar systems are also operated for/by the defense agencies in the United States. The prevalence of X-band systems, and the reliance of security and surveillance application on their data, may pose a significant challenge in reducing their transmit power requirements or in the reduction of their allocated bandwidth.

Finding and Recommendations

The committee supports the new allocations under consideration in Preliminary Agenda Item 2.12.

Finding: The planned limitation of the EESS (active) observations in the 3000–3400 MHz bands to oceanic regions minimizes the scope for interference into RAS observations under Preliminary Agenda Item 2.12, provided the EESS (active) transmissions are disabled over land.

Recommendation: Administrations authorizing any over-land or near-land Earth exploration-satellite service (active) observations resulting from Preliminary Agenda Item 2.12 of the 2031 World Radiocommunication Conference should require operators to coordinate observations with radio astronomy service facility operators on a global basis.

Recommendation: Groups undertaking studies under Preliminary Agenda Item 2.13 of the 2031 World Radiocommunication Conference, in addition to ascertaining the current extent of interference from the Earth exploration-satellite service (active) into operations under the radiodetermination services, should determine the minimum acceptable limits for transmit power levels and duty cycles of X-band synthetic aperture radar systems that avoid compromising the quality of products required by their applications.

Recommendation: Groups undertaking studies under Preliminary Agenda Item 2.13 of the 2031 World Radiocommunication Conference should consider the potential for damage to

radio astronomy service facilities from spaceborne synthetic aperture radar sensors, and urge administrations to require coordination accordingly.

Recommendation: Groups responsible for developing final agenda items and associated resolutions for the 2031 World Radiocommunication Conference (WRC-31) should amend Resolution 722, and/or any successor resolution that defines a related finalized WRC-31 agenda item, to invite studies of the damage hazard and of suitable coordination mechanisms between the Earth exploration-satellite service (active) and the radio astronomy service.

PRELIMINARY AGENDA ITEM 2.14: BROADCAST AND MOBILE SERVICES IN 470–694 MHz

WRC-31 Preliminary Agenda Item 2.14 is “to review spectrum use and needs of applications of broadcasting and mobile services and consider possible regulatory actions in the frequency band 470-694 MHz or parts thereof, in accordance with Resolution 235 (Rev. WRC-23).”

Figure 3-8 shows the band under consideration in this WRC-31 preliminary agenda item along with an overlapping band allocated to radio astronomy.

Resolution 235 (Rev.WRC-23) invites the ITU Radiocommunication Sector (ITU-R) to reconsider use of the frequency band 470–694 MHz (or parts thereof), which is currently allocated to terrestrial television broadcasting, and in some ITU regions, telecommunications and/or radionavigation. This preliminary agenda item also explicitly acknowledges the allocation to the radio astronomy service (RAS) in the band 608–614 MHz, which varies by country/region (also known as “Channel 37” in the United States), but is a primary allocation in the African broadcasting area, China (within which there is also a primary RAS allocation to 606–608 MHz), India, and Region 2 and secondary in most other regions. The detrimental interference level for continuum RAS observations is −253 dB(W/(m² Hz)) at 611 MHz, based on Table 1 of Recommendation ITU-R RA.769-2.

Radio Astronomy Service

Radio emission from neutron stars—extreme remnants of dead stars that represent the densest matter in the universe—is most effectively observed around frequencies of several hundred megahertz, where the emission is strongest. Take for example, the Canadian Hydrogen Intensity Mapping Experiment (CHIME), a radio telescope in British Columbia, Canada, that observes continuously between 400–800 MHz. Since 2018, CHIME has been monitoring pulsars (rapidly rotating neutron stars that are also extremely accurate “clocks”) to measure the gravitational wave background of the universe and probe the population of merging supermassive black holes. CHIME has also been a game changer in the understanding of fast radio bursts (luminous, short-lived bursts of radio continuum emission), demonstrating that they originate on neutron stars and can be observed in distant galaxies and can therefore act as a cosmological probe. In the future, the “holy grail” for tests of Einstein’s theory of general relativity will be the detection of a pulsar in orbit around a black hole, and searches for such systems often make key use of the band in question in this preliminary agenda item, 470–694 MHz.

While the most salient radio astronomy concern is protection of the bands allocated to RAS, the committee also notes the need to protect continuum observations outside of allocated bands in nationally or internationally recognized radio quiet zones. It is also important to protect against out-of-band emissions and spurious emissions, including harmonics, in the 608–614 MHz band.

Finding and Recommendations

Recommendation: Those undertaking studies under Preliminary Agenda Item 2.14 of the 2031 World Radiocommunication Conference should

• Ensure the continued protection of the radio astronomy service observations when considering allocations in and adjacent to the 608–614 MHz band.
• Require out-of-band emission masks and/or guard bands to ensure that the Recommendation ITU-R RA.769-2 interference thresholds are not exceeded in the 608–614 MHz band, including consideration of aggregate interference from multiple transmitters.

Finding: Wider bands than are primarily allocated to RAS are required to fully realize the scientific potential of radio astronomy.

Recommendation: Administrations should use local coordination—for example, through radio quiet zones—to protect radio astronomy observatories, taking account of not only terrestrial emitters but also spaceborne emitters.