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CHAPTER 2

Radio Astronomy
and Spectrum Management

Introduction

Spectrum management is a combination of administrative and technical procedures which are necessary to ensure the efficient operation of radio communications, taking into account legal, economic, engineering, and scientific aspects for the use of the radio frequency spectrum. Current spectrum management policies are under increasing strain as the public’s demand for existing wireless services grows, and new spectrum-related technologies and applications emerge.

The recognition of radio astronomy as a radio service with equal status to broadcasting or mobile radio communications was an historic step since it provided a legal basis under which radio astronomy could seek protection against any “harmful” interference. Radio regulations provide two levels of protection: primary and secondary. Radio services allocated on a primary basis have equal rights within the same band. Secondary services are on a non-interference basis to the primary services. Footnotes to the allocation table draw attention to the use of a specific band by providing for an additional allocation, alternative allocation, or a different category of service.

The radio astronomy community has been actively participating in almost all national and international spectrum management forums to protect its spectrum allocations. This is quite evident at national and international meetings where their efforts are quite visible to retain and protect their interference-free bands in the face of expanding telecommunications systems and services.

Protection in Frequency Bands Shared with Other Radio Services

The radio astronomy service, being a passive radio service searching for and receiving extremely low power levels from cosmic radio sources, is very concerned about transmissions from the active radio services. Many of the frequency bands allocated to the radio astronomy service are also allocated to other radio services which transmit in those bands. Thus, the radio astronomy service is particularly vulnerable to harmful interference from transmitters in shared frequency bands. In particular, radio astronomy observatories find frequency sharing difficult, if not impossible, when operations on shared frequencies are within direct line-of-sight.(1)

The National Science Foundation (NSF), in their comments to an NTIA Notice of Inquiry regarding spectrum requirements,(2) responded to questions about sharing with other radio services. Obviously, the radio astronomy service can easily share with other passive radio services such as the Earth exploration-satellite service (passive) and the space research service (passive). Any line-of-sight sharing between the radio astronomy service and stations in the active services is not conducive to radio astronomy observations because of the extremely sensitive receivers employed. Geographical sharing between the radio astronomy service and some active services is possible, provided separation distances are great enough. Because airborne or satellite-borne transmitters can cause harmful interference to radio astronomy observatories (even with hundreds to thousands of kilometers separation), geographical sharing is easier with terrestrial stations. The fixed service is a good candidate for sharing with radio astronomy service, while geographical sharing is also feasible with uplinks in the fixed satellite service. Most radio astronomy observatories are located inland, and, as a general rule, radio astronomy can share with Earth stations in the maritime mobile service. Finally, geographical sharing with the mobile services is feasible only under special circumstances.(3)

The protection of radio astronomy observations from harmful interference in shared bands is accomplished by the establishment of coordination zones for terrestrial services. In these coordination zones, terrestrial transmissions are allowed only after technical means can be found to avoid harmful interference to the radio astronomy service.

Protection from Transmitters in Adjacent Bands

As a result of the sensitivity of its receivers, the radio astronomy service also experiences harmful interference from transmitters not in the same band.(4) This type of interference is called adjacent-band interference. Interference can also result from harmonic and intermodulation signals. Usually, low-level signals from a transmitter (modulation sidebands, etc.) that fall within the radio astronomy band can cause this adjacent-band interference. Protection measures exist that address radio astronomy requirements for protection from transmitters in adjacent bands.(5) Further, radio astronomy receivers may contribute to this type of interference if its response to signals outside of radio astronomy bands is not sufficiently low.

Ultra high frequency (UHF) television, airborne, and spaceborne systems have been found to be troublesome to the radio astronomy service. In particular, transmitters on satellites or aircraft present a problem when they are transmitting in radio line-of-sight. Satellite transmissions, especially those associated with television and sound broadcasting, may cause severe interference to radio astronomy. By the very nature of satellite broadcasting systems, large areas of the Earth are illuminated and line-of-sight conditions exist. Terrestrial interfering sources are normally in the far-side lobe region of a radio telescope while satellite transmissions are likely to be received in the main beam and near-side lobes that have higher gains.

Sharing in Radio Astronomy Bands Below 40 GHz

In the most used portion of the radio frequency spectrum, radio astronomy allocations (primary or secondary) are relatively small. For instance, below 2 GHz, allocations total approximately 2.7 percent and between 0–5 GHz, approximately 2.2 percent. The following paragraphs briefly describe the frequency bands where the radio astronomy scientists conduct observations and research.

The 13.36–13.41 MHz and 25.55–25.67 MHz Bands. These two bands are in the HF portion of the radio frequency spectrum. In the United States, the band 13.36–13.41 MHz is allocated for radio astronomy. Under Footnote G115, however, assignments in the fixed service are permitted in the continental United States, and will be protected for national defense purposes or if they are used only in an emergency where no other means of communications exist. Worldwide, this band is shared with the fixed service. The 25.55–25.67 MHz band is allocated for the radio astronomy service and has fixed and mobile allocated bands just below it, with the broadcasting service allocation just above. Radio communications in these bands for long distances rely primarily on propagation via the ionosphere. Radio astronomical observations from the Earth are possible only when the electron density is sufficiently low and relatively free of irregularities, so that radio signals are not significantly refracted. These two HF bands are some of the preferred frequency bands for continuum observations.(6) Also, these bands are very important for research of radiation from the planet Jupiter and from the Sun. Jupiter is the only radio planet observable from the ground. Its study is a unique means of developing theoretical models for the radio emissions of other planets. The Sun is the nearest star, and its study enables a closer understanding of the radio emission mechanisms of all other stars.(7)

The 37.5–38 MHz and 38–38.25 MHz Bands. The 37.5–38 MHz band is allocated to the radio astronomy service on a secondary basis in the United States and worldwide. The non-government land mobile service has primary allocation in the United States with fixed and mobile services having primary status in the band worldwide. Radio astronomy has primary allocation in the 38–38.25 MHz band and shares this allocation equally with the fixed and mobile services. International Footnote 547 applies to both these bands and urges administrations to take all practical steps to protect radio astronomy when making frequency assignments to other radio services. These two bands are in the table of radio astronomy preferred bands for continuum observations.(8) Further, like the two radio astronomy bands above, these two bands are very important for observations of radiation from the planet Jupiter and from the Sun.(9)

The 73–74.6 MHz Band. This band is allocated to the radio astronomy service on a primary basis in the United States and throughout Region 2. Regions 1 and 3 have these bands allocated to the fixed and mobile services as the primary services. International Footnote 568, however, urges administrations to take all practical steps to protect radio astronomy from harmful interference, though the radio astronomy service has no protection. As with the bands above, continuum observations are performed in this preferred band.(10) This band is also used for research and monitoring of the interplanetary “weather” structure in the solar wind by an international network of radio astronomy instruments.(11)

The 406.1–410 MHz Band. Radio astronomy shares primary allocation status in the band 406.1–410 MHz with the fixed and mobile (except aeronautical mobile) services in the United States and worldwide. In the United States, this band is one of the principal bands supporting Federal land-mobile communications. This band is also one of the radio astronomy service's preferred frequency bands for continuum observations.(12)

The 608–614 MHz Band. There are ITU regional allocation differences for this band. Radio astronomy is the primary allocation in Region 2. In Region 1, the broadcasting service is the primary service; while in Region 3, the fixed, mobile, broadcasting and radionavigation services share primary allocations. Because of international footnotes, different levels of protection are provided radio astronomy in this band. This band is also one of the radio astronomy service's preferred frequency bands for continuum observations.(13)

The 1400–1427 MHz Band. Radio astronomy shares primary allocation status in this band worldwide with the Earth exploration-satellite (passive) service and the space research (passive) service. Both line and continuum observations are performed in this band. Though no allocation status is provided, recognition of observations of very great importance to radio astronomy is provided by footnotes at lower frequency bands (1350–1400 MHz) due to Doppler shifts to lower frequencies. The band is a preferred frequency band for both spectral line and continuum observations.(14) This is also the most important band for studies of the hydrogen line and is used to study how our Galaxy and other galaxies are made, structured, rotate, etc.(15)

The 1610.6–1613.8, 1660–1670, and 1718.8–1722.2 MHz Bands. These three bands allow radio astronomy observations to be made of the hydrogen and hydroxyl radical spectral lines.(16) These lines are important because of their abundance in the universe, allowing study of the spatial structure (stellar and expansion velocities) of the galaxies as well as development and validation of theories on the origins and evolution of the universe.(17) Although the observing requirements are similar in these three bands, the sharing problems are quite dissimilar. For instance, radio astronomy in the 1610.6–1613.8 MHz band has a primary allocation and shares primary status with the mobile-satellite, aeronautical radionavigation, and radiodetermination-satellite services. The nature of the other radio services in this band make observations extremely difficult due to the line-of-sight transmissions. The frequency bands 1610.6–1613.8 MHz and 1718.8–1722.2 MHz, discussed below, are identified as two of the 47 frequency bands (below 275 GHz) which contain rest frequencies and Doppler-shifted frequencies of the astrophysically most important spectral lines identified by the International Astronomical Union (IAU).(18)

In the 1660–1670 MHz band, radio astronomy has primary allocations status but the band is segmented into three sub-bands. Sharing in this band is with three different radio services. Radio astronomy shares the first segment (1660–1660.5 MHz) with the mobile-satellite (Earth-to-space) service.(19) In the second segment, 1660.5–1668.4 MHz, the radio astronomy service shares primary allocation status with the space research (passive) service. In the last segment, 1688.4–1670 MHz, primary allocation status is shared with the meteorological aids (radiosonde) service. While the 1660–1670 MHz band is identified by the IAU as a preferred frequency band for continuum observations, it is also useful for spectral-line observations.(20)

In the United States, radio astronomy observations are made in the 1718.8–1722.2 MHz band on an unprotected basis. By Footnote US256, agencies providing other services in this band in geographical areas near designated radio astronomy sites are asked to “bear in mind” that their operations may affect those observations, and are encouraged to minimize potential interference to those observations insofar as it is practicable.(21)

The 2655–2700 MHz Band. Radio astronomy has a primary allocation status in the upper 10 megahertz of this band and shares this band with the Earth exploration-satellite (passive) service and space research (passive) service. In the remainder of the band (2655–2690 MHz), radio astronomy has secondary allocations, with primary allocation status to the fixed and broadcasting-satellite services within the United States and fixed, mobile, fixed-satellite, and broadcasting-satellite services for other regions. This band is also one of the radio astronomy service's preferred frequency bands for continuum observations(22) partly because of its low galactic background radiation and usefulness in studying both ionized hydrogen clouds and the general diffuse radiation of the Galaxy.(23)

The 4825–4835, 4950–4990, and 4990–5000 MHz Bands. These three bands are extremely useful in studying the brightness distributions of both galactic and extragalactic objects such as ionized hydrogen clouds and supernova remnants.(24) The 4800–4990 MHz band is allocated worldwide to radio astronomy on a secondary basis; however, in the United States, portions of it are also recognized for radio astronomy observations. For radio astronomy service, the band 4825–4835 MHz has no stated allocation. Footnote US203 indicates that radio astronomy observations are made at frequencies 4825–4835 MHz and users are urged to make every practicable effort to avoid the assignment of frequencies of the fixed or mobile services in this band. No allocation is stated for the radio astronomy service in the 4950–4990 MHz band. Footnote US257, however, recognizes that radio astronomy observations may be made in the band at certain radio astronomy observatories and encourages the avoidance of frequencies assignments in designated areas by the fixed and mobile services, as well as the aeronautical mobile service. Any harmful interference is to be remedied to the extent practicable.(25) In the United States, 4990–5000 MHz is allocated to the radio astronomy service on a primary basis. In other parts of the world, radio astronomy is on an equal primary basis with the fixed and mobile services. This band, as well as the entire 4800–4990 MHz band, is also one of the radio astronomy service's preferred frequency bands for continuum observations.(26)

The 10.68–10.7 GHz Band. This band is allocated on an equal primary basis to three passive radio services: radio astronomy, Earth exploration-satellite (passive), and space research (passive) services. However, the fixed and mobile (except aeronautical mobile) services, by International Footnote 834, shares primary status with radio astronomy in approximately 30 countries throughout the world. This band is also one of the radio astronomy service's preferred frequency bands for continuum observations.(27)

The 14.47–14.5 GHz Band. The allocation status for radio astronomy is not stated in the national allocation table. Non-government allocation to the fixed satellite service is primary. Nevertheless, the secondary government users in the fixed and mobile services are urged to make every practicable effort to avoid the assignment of frequencies in this band.(28) Radio astronomers identified the spectral line in this band as one of the radio frequency lines of greatest importance to radio astronomy at frequencies below 275 GHz.(29)

The 15.35–15.4 GHz Band. Worldwide, radio astronomy shares this band on a co-primary basis with the Earth exploration-satellite (passive), and space research (passive) services. In the U.S. National Table of Frequency Allocations, the radio astronomy service is afforded protection from extraband radiation.(30) Further protection is granted to the radio astronomy service by Footnote US246 whereby no station will be authorized to transmit in this band.(31) This band is also one of the radio astronomy service's preferred frequency bands for continuum observations.(32)

The 22.21–22.5 GHz Band. In this band, radio astronomy has a primary allocation status and shares with the fixed, mobile (except aeronautical mobile), Earth exploration-satellite (passive), and space research (passive) services. The 22.21–22.5 GHz band is identified by radio astronomers as a preferred band for continuum observations.(33)

The 23.6–24 GHz Band. Worldwide, this band is allocated primary status to the passive radio services: radio astronomy, Earth exploration-satellite (passive), space research (passive) services. This band is also on the list of preferred bands for continuum observations.(34)

The 31.3–31.8 GHz Band. The 31.3–31.8 GHz band is a preferred band for continuum observations.(35) In the United States, radio astronomy has primary allocation status and shares it with the Earth exploration-satellite (passive) and space research (passive) services.

The Bands above 40 GHz. For the radio astronomy service, there are allocations above 40 GHz for both continuum and spectral-line observations. Some of these allocations are shared with passive radio services and others are shared with active radio services. Until recently, there has been relatively few active systems operating above 40 GHz and interference to radio astronomy observations has been minimal.

Increased Sharing

It has been acknowledged that there are significant public benefits from the use of the radio spectrum for scientific research, such as radio astronomy. Radio astronomy is a worldwide effort. The United States and other administrations that participate in radio observations share their findings for the good of humankind. Fortunately, radio astronomy shares well with many other services, the exceptions being air- and space-borne emitters. Further, the relatively small number of radio astronomy observatories, and their remote locations, limit the impact to active radio services when spectrum managers protect the radio astronomy sites from interference.

Both NTIA and the FCC have encouraged more flexibility in the use of spectrum allocations. In keeping with this policy, it seems appropriate that a passive service be allowed to share allocations in those cases where impact to other services is minimal. Therefore, the spectrum requirements of the radio astronomy service should be considered whenever spectrum is reallocated in the vicinity of the preferred radio astronomy bands.


Endnotes

(1)NSF Comments at 6–7, Sep 30, 1992.

(2)Current and Future Requirements for the Use of Radio Frequencies in the United States, Notice of Inquiry and Request for Comments, Docket No. 920532-2132, 57 Fed. Reg. 25010 (1992).

(3)See NSF Comments, supra note 26, at 6–7, Sep 30, 1992.

(4)Id., at 9.

(5)See International Telecommunication Union, ITU Radiocommunication Sector 1994 RA Series Recommendations, Recommendation 517–2, Protection of the Radioastronomy Service from Transmitters in Adjacent Bands, at 8 (1994).

(6)See International Telecommunication Union, ITU Radiocommunication Sector 1994 RA Series Recommendations, Recommendation 314–8 , Table 3, Frequency Bands Allocated to the Radioastronomy Service That Are Preferred for Continuum Observations, at 3 (1994).

(7)See European Science Foundation, Committee on Radio Astronomy Frequencies (CRAF), CRAF Handbook for Radio Astronomy, at 40 (2d ed. 1997).

(8)See ITU Recommendation 314–8 , supra note 31, Table 3, at 3.

(9)See CRAF, supra note 32, at 40-41.

(10)See ITU Recommendation 314–8 , supra note 31, Table 3, at 3.

(11)See CRAF, supra note 32, at 41.

(12)See ITU Recommendation 314–8 , supra note 31, Table 3, at 3.

(13)Id.

(14)Id., Table 1, at 2 and Table 3, at 3.

(15)See CRAF, supra note 32, at 43-44.

(16)See ITU Recommendation 314–8 , supra note 31, Table 1, at 2.

(17)Critical Doppler redshift measurements are made in these bands. See CRAF, supra note 32, at 44-45 and 61-65.

(18)See ITU Recommendation 314–8 , supra note 31, Table 1, at 2.

(19)This band was reallocated by the WRC–97 from the aeronautical mobile–satellite (Earth-to-space) service to the mobile–satellite (Earth-to-space) service. See the Final Acts of the World Radiocommunication Conference (WRC-97) Geneva, 1997, at 30.

(20)Electronic mail transmission from Dr Martha Haynes (Professor of Astronomy, Cornell University), Spectrum Planning and Policy Advisory Committee (SPAC) member, to Joseph Camacho, National Telecommunications and Information Administration (NTIA), Strategic Spectrum Planning Program (Sep 9, 1997), (on file with NTIA).

(21)See NTIA Manual, supra note 10, § 6.1.1, at 6–11.

(22)See ITU Recommendation 314–8 , supra note 31, Table 3, at 3.

(23)See CRAF, supra note 32, at 46-47.

(24)See CRAF, supra note 32, at 48-49.

(25)See NTIA Manual, supra note 10, § 4.1.3, at 4–108.

(26)See ITU Recommendation 314–8 , supra note 31, Table 3, at 3.

(27)Id.

(28)See Footnote US203, NTIA Manual, supra note 10, § 4.1.3, at 4–104.

(29)See ITU Recommendation 314–8 , supra note 31, Table 1, at 2.

(30)See Footnote US74, NTIA Manual, supra note 10, § 4.1.3, at 4–101.

(31)See Footnote US246, NTIA Manual, supra note 10, § 4.1.3, at 4–107.

(32)See ITU Recommendation 314–8 , supra note 31, Table 3, at 3.

(33)Id.

(34)Id.

(35)Id.

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