CRAF Newsletter 2000/3

CRAF Newsletter 2000/3

December 2000

The European Science Foundation is an association of its 67 member research councils and academies in 23 countries. The ESF brings European scientists together to work on topics of common concern, to co-ordinate the use of expensive facilities, and to discover and define new endeavors that will benefit from a co-operative approach
The scientific work sponsored by ESF includes basic research in the natural sciences, the medical and biosciences, the humanities and the social sciences.
The ESF links scholarship and research supported by its members and adds value by cooperation across national frontiers. Through its function as coordinator, and also by holding workshops and conferences and by enabling researchers to visit and study in laboratories throughout Europe, the ESF works for the advancement of European science.

On behalf of European radio astronomers, the ESF Committee on Radio Astronomy Frequencies, CRAF, coordinates activities to keep the frequency bands used by radio astronomers free from interference.


1. Chairman's Corner

When the radio astronomy service entered the Radio Regulations in 1959, the new service was allocated the frequency band 1400-1427 MHz containing the famous `hydrogen line'. This was the first time that the ITU had wanted to set aside a frequency band for purely passive use. To make the intention clear, a remarkable footnote was added, stating

"All emissions are prohibited in this band."

In subsequent years the radio astronomy service gained many frequency allocations, but even today the hydrogen line band is arguably our most important single band, protecting the spectral line emissions of the most abundant element, in its most primitive form.

Given this history, it is not surprising that some radio astronomers are gravely concerned about moves originating in the USA, which threaten the quiet in this band. This Newsletter highlights two such developments: a proposal for a satellite downlink in the band 1429-1432 MHz, and the emergence of Ultra-Wide Bandwidth devices. Both these developments have the potential to interfere with passive use of the hydrogen line band.

Elsewhere in the Newsletter, Klaus Ruf, Chairman of IUCAF, reflects on the passing of ITU-R Task Group 1/5. Despite brave individual efforts and several notable achievements, TG1/5 was unable to place suitably stringent limits on the unwanted emissions into passive frequency bands, limits that would be acceptable to the passive services using them. Let us hope that the new Task Group 1/7 will be more successful in this aspect of its work.

Finally, this Newsletter has been my last chance to speak to you as CRAF Chairman. From 1st January 2001 that privilege will belong to Wim van Driel, a man who has spent more hours observing the hydrogen 21-cm line than most. I wish him luck in his efforts to keep our frequency bands quiet.

R. J. Cohen
Jodrell Bank Observatory

2. New CRAF Chairman

Dr.W.van Driel will succeed Dr. R. J. Cohen as Chairman of CRAF from 1st January 2001.

Dr.W.van Driel was born in 1956 and studied astronomy in Leiden and Groningen (the Netherlands), obtaining his Ph.D. in astronomy at Groningen University in 1987. After his Ph.D. he was Assistant Astronomer Paris Observatory, France (1987); Assistant Professor University of Paris VII, France (1987-1988); Assistant Professor University of Amsterdam, Netherlands (1988-1992); Associated Professor University of Tokyo, Japan (1992-1994); Professor University of Paris VII, France (1994-1995); Astronomer Paris Observatory, France (from 1995) and director of the Nançay Radio Observatory, France (from 1994-2000).

His main field of interest is extragalactic astronomy, based on multi-wavelength (radio, optical and infrared) observations, in particular in the 21-cm HI line and the CO lines, as well as in continuum, with interferometers and single-dish telescopes. He has a wide range of experience in observational radio astronomy with various instruments, including those at the following observatories: Arecibo (USA), ATCA (Australia), Effelsberg (Germany), IRAM (France), Nançay (France), Nobeyama (Japan), SEST (Chile), VLA (USA) and Westerbork (the Netherlands) which led to many publications in refereed journals and conference proceedings.

Since becoming Director of the Nançay Radio Observatory in 1994, he became increasingly involved in the protection of frequency bands used for radio astronomy. He succeeded B.Darchy in CRAF after the latter retired in 1998. In 1999 he was elected a member of IUCAF representing URSI. Dr.Van Driel has participated in various meetings of IUCAF, WRC-2000, ITU-R WP7D, CEPT project teams SE21 and SE28 and conferences related with frequency management issues.

3. Report of 31st CRAF meeting (9-10 November 2000)

The 31st CRAF meeting was held on 9-10 November 2000 at the Satellite Geodetic Observatory in Penc, Hungary. The main items discussed concerned an evaluation of the results of WRC-2000, the planning of work for WRC-03, the update of the CRAF terms-of-reference and the coordination of aeronautical earth stations (AESs) at 1.6 GHz.

  • Chairman: the meeting concluded after proper search that Dr.W.van Driel (Nançay) become the next Chairman of CRAF. The European Science Foundation has subsequently appointed Dr. van Driel to the position from 1st January 2001.
  • CRAF Terms-of-Reference: an update of the CRAF Terms-of-Reference was discussed and adopted by the meeting and sent to the European Science Foundation for consideration by the ESF Executive Board.
  • CRAF status in CEPT: The CEPT and the European Science Foundation agreed on a letter of understanding implying that CRAF will have a formal observer status within the CEPT ERC family.
  • Preparation of WRC-03: CRAF identified the agenda items of the WRC-2003 which are relevant for radio astronomy. These agenda items can be found in CRAF Newsletter 2000/2. For each relevant agenda item, a coordinator was identified. Within the CEPT, it is not yet certain which working group or project team will address which agenda item.
  • Aeronautical Earth Station coordination: In reaction to the ERC CEPT WG SE liaison statement to ETSI TC-SES, CRAF wrote a letter to WG SE (with copy to ETSI TC SES) expressing its concerns on the proposed values for the ETSI standard for the aeronautical earth stations at 1.6 GHz, i.e. in the band 1610.6-1613.8 MHz. WG SE and CRAF agreed that SE28 will develop an improvement of these values for the maintenance of the standard from the results of a study between CRAF and GLOBALSTAR (similar to the work done by CRAF and INMARSAT in 1999).

    4. ITU-R Task Group 1/5

    When satellite communications began in the second half of the last century, radio astronomers got very concerned. Rightfully: observational astronomy developed, perhaps 3000 year ago, because it is dark at night. Even today, an artificial sun illuminating us at nighttime would kill optical astronomy. Humans have eyes to see the light, so we intuitively understand the problems that optical astronomy has with excessive lighting. But the effect on radio astronomy of an excess of artificial radio emission from the sky is exactly the same.

    As a result of the actions of concerned radio astronomers, the World Administrative Radio Conference 1979, WARC-79, added a note to the "Table of Maximum Permitted Spurious Emission Levels", saying: "Since these levels may not provide adequate protection for receiving stations in the radio astronomy ..... service, more stringent levels might be considered in each individual case.....". At the same time, our colleagues from the satellite communications community exempted their service, in another note, which was added to the same table and which said: "These levels are not applicable to stations in the space services, but the levels of their spurious emissions should be reduced to the lowest possible values compatible with the technical and economic constraints...."

    Basically, this is where we still stand today.

    The reason to refer back to WARC-79 is Recommendation 66, which was initially adopted by that conference, which was recently revised for the third time by WRC-2000, and which provided the mandate of two Task Groups, TG1-3 and TG1-5. TG1-5 had its seventh and final meeting at the end of October in Geneva. It has been a very successful Task Group, which reached conclusions on a number of important issues related to "Unwanted Emissions": Satellite services are no longer exempted from spurious emission limits, the table of maximum levels of spurious emissions was extended to the highest frequencies and important recommendations dealing with out-of-band emissions, spurious emissions, spectra and bandwidth of emissions and measurement methods were adopted. Only a few items were not covered but unfortunately real protection of radio astronomy was one of them. TG1-5, at the final meeting (Geneva, 23-31 October 2000), also adopted recommendations on the protection of safety services and passive services and this again is a great step forward. The unwanted emission levels, however, which active services are allowed to transmit, may result in interference to radio astronomy within its allocated bands. Most dangerously, some unwanted satellite emissions may result in pfd levels at our radio telescopes many tens of dB in excess of the interference threshold levels necessary to protect radio astronomy. And this problem was to be looked at in a so-called band-by-band study, because general limits may not be stringent enough to protect us. (WARC-79: "... since these levels may not provide adequate protection ... more stringent levels might be considered in each individual case..."). The band-by-band study has been suffering from a nutritional imbalance, because only the party interested in results provided input: the other party, which could very well live without such results, was reluctant to feed the study with numbers. Instead the weight and cost of additional filtering was stressed. (WARC-79: "...spurious emissions should be reduced to the lowest possible values compatible with the technical and economic constraints...").

    Nevertheless, we have achieved something over the past twenty years, and we have the chance to further achieve solutions tailored to the need of individual frequency bands, in cooperation with the active services. One of the last actions taken by TG1-5 was the organization of the remaining work. The band-by-band study will continue in another Task Group, TG1-7. The tradition of excellent chairmen will be continued, from TG1-3 to TG1-5 and now to TG1-7. And since TG1-5 has cleared our way, we may now concentrate on the protection of passive services from unwanted emissions, in TG1-7. The work is still complicated, but it is do-able.

    K. Ruf
    Max Planck Institut fur Radioastronomie

    5. 1.4 GHz and WRC-03 agenda item 1.16

    The frequency band 1400-1427 MHz is exclusively allocated to the passive services and it is of fundamental importance for radio astronomy. This band contains "the 21-cm" line, as it is called for short, of neutral hydrogen (HI) at 1420 MHz rest frequency. Undoubtedly, this spectral line is the one most often used for radio astronomical observations world-wide. It was theoretically predicted in 1944 by the Dutch astronomer Henk van de Hulst and its experimental discovery in 1951 opened the way to the study of cold gas between the stars.

    Now, after 50 years of intensive observations, the study of this line has taught us many very fundamental things about the Universe, with more very likely still to come. The 21-cm line traces cold, neutral atomic interstellar gas in galaxies and is found throughout their disks, usually till far outside their optical outlines. HI line observations have provided us with a view of galaxies complementary to that obtained in visible light. Among the fundamental studies made using the line, we can cite:
    - the mapping of the structure and rotation of the entire Milky Way galaxy, unimpeded by the dust clouds that obscure optical observations,
    - the determination of the rotation speeds of spiral galaxies far beyond their stellar disks, which clearly showed the presence of a dominant component of "dark matter" surrounding the visible objects and of still unknown composition,
    - determination of accurate redshifts of large galaxy samples, enabling us to study the global expansion of the Universe, and also local deviations of this expansion near local mass concentrations, like clusters of galaxies, allowing the measurement of their total masses.

    The protection of this important band for the Radio Astronomy Service has been under discussion for some time in WP7D and 8D. Other services also have interest in the 1.4 GHz frequency domain: WRC-2003 agenda item 1.16 (Resolution 800) requests all parties concerned "to consider allocations on a worldwide basis for feeder links in bands around 1.4 GHz to the non-GSO MSS with service links operating below 1 GHz, taking into account the results of ITU-R studies conducted in response to Resolution 127 (Rev.WRC-2000), provided that due recognition is given to passive services, taking into account No. S5.340". Resolution 127 focuses on the band 1390-1393 MHz for the uplink and on 1429-1432 MHz for the downlink, while the reference to footnote S5.340 asks WRC-03 to take into account that "All emissions are prohibited in the band 1400 - 1427 MHz".

    The straightforward phrasing of footnote S5.340 gives the impression that, in principle, no interference is to be expected, or accepted, in this passive band. Using the ITU Radio Regulations, Rules of Procedure and the view of the ITU-R Radio Regulation Board, the footnote can be interpreted as follows: S5.340 is meant to guarantee that the production of radiation by radio transmitters in the 1400-1427 MHz band is prohibited by all means. To resolve cases of harmful interference between services in adjacent bands and allocated services in the 1400-1427 MHz band, emission overlapping the latter band shall be attenuated sufficiently to eliminate interference which is detrimental for the service(s) protected by this footnote.

    Discussions at a more pragmatic level in WP7D have led, on the other hand, to the acceptance by the radio astronomy community that strict, 100% protection of RAS bands is not feasible in practice, and of related "percentages of acceptable data loss" due to emissions into astronomy bands, which amount to 2% maximum per satellite system, with a total aggregate maximum loss of 5% in a given band. The exact interpretation of these percentages on a practical basis is still not fixed, however, and discussions are continuing on, e.g., the question whether the 2% maximum data loss applies to any given square degree area on the sky, or to an average over a long string of observations made all over the sky. The latter interpretation could well imply considerably higher percentages of data loss near the horizon, where satellites tend to cluster by projection effect, compensated by better protected areas elsewhere in the sky.

    The adopted maximum acceptable loss percentages should be seen as a delicate compromise, accepted by the radio astronomy community worldwide, as represented by IUCAF (which at present has 3 members from CRAF), and applicable to all RAS bands, irrespective of the intensity of their use by astronomers, and irrespective of their inclusion in footnote S5.340. The latter apparent discrepancy between the seemingly firm language of footnote S5.340 and the data loss percentages accepted by the Radio Astronomy Service needs further clarification, in principle without renegotiation of the percentages.

    6. High Altitude Platform Stations

    The agenda of the World Radiocommunication Conference 2003, WRC-03, has as item 1.13 "to consider regulatory provisions and possible identification of existing frequency allocations for services which may be used by high altitude platform stations, taking into account No. S5.543A and the results of the ITU-R studies conducted in accordance with Resolutions 122 (Rev.WRC-2000) and 734 (WRC-2000)". Agenda item 1.33 asks "to review and revise technical, operational and regulatory provisions, including provisional limits in relation to the operation of high altitude platform stations within IMT-2000 in the bands referred to in No. S5.388A, in response to Resolution 221 (WRC-2000)".

    The fact that two agenda items of this WRC address the issue of High Altitude Platforms, HAPs, shows the expectations of this technique. According to article S1.66A of the ITU Radio Regulations, a high altitude platform station is "a station located on an object at an altitude of 20-50 km and at a specified, nominal, fixed point relative to the Earth". Because of this definition, a HAPs operates in the Fixed Service but its transmissions can have similar impact on radio astronomy to airborne or space-borne transmitters.

    The WRC-97 identified the frequency bands 47.2-47.5 GHz and 47.9-48.2 GHz for HAP applications subject to the provisions of Resolutions 122 (footnote S5.552A). These frequency bands are close to the radio astronomy band 48.94-49.04 GHz to which footnote S5.340 applies. The first system which will use these frequencies is Sky Station. The allocation scenario implies an immediate compatibility issue between radio astronomy and Sky Station. Since WRC-97, CRAF had approached Sky Station several times with the request for cooperation on the issue of protection of radio astronomy. Until now, without any positive action from the operator. Nevertheless, CRAF remains alert and ready to participate actively in discussions and possible studies.

    A discussion during the WRC-2000 initiated by the proposal of Japan to use HAP stations also at lower frequencies, such as ~30 GHz, led to WRC-03 agenda item 1.13. Radio astronomy also has major interest in this frequency area, namely the band 31.2 - 31.8 GHz. HAP stations are also considered as a possible platform for IMT-2000 (UMTS) in the frequency bands 1885 - 1980 MHz, 2010 - 2025 MHz and 2110 - 2170 MHz. Although these frequencies are not adjacent to radio astronomy frequencies, CRAF will monitor the developments very carefully and contribute to the various discussions when necessary.

    7. Ultra-Wide Band transmissions (UWB)

    Ultra-Wide Band, UWB, transmission technology is a relatively new development which may have the capability to provide significant benefits for public safety, businesses and consumers. The available UWB proposals and ideas ensure that existing and planned radio services, particularly safety services, are adequately protected. While comprehensive tests have not been completed, UWB devices appear to be able to operate in spectrum already occupied by existing radio services without causing interference, which would permit scarce spectrum resources to be used more efficiently.

    UWB transmission is a signal whose fractional bandwidth, dbw, is larger than 0.25, where the fractional bandwidth is defined as

    dbw = 2(fH - fL)/(fH + fL)

    where fH is the upper frequency of the -20 dB emission point and fL is the lower frequency of the -20 dB emission point. Specifically, it is proposed to define an UWB device as any device where the fractional bandwidth dbw is greater than 0.25 or occupies 1.5 GHz or more of spectrum. It is also proposed to base the definition of an UWB device on the -10 dB bandwidth rather than the -20 dB bandwidth, because UWB devices will operate so close to the noise floor that in many cases it will not be possible to measure the -20 dB bandwidth.

    UWB applications can be generally grouped into the following categories: Medical applications, Consumer communications applications, Automotive applications, Consumer and industrial construction applications, Ground penetrating radar (GPR) systems (which are used to trace land-mines, for example), Industrial liquid level gauges, High performance data communications systems.

    Currently, studies are going on to determine the preferred frequencies for UWB applications. For some applications these may be well below 2 GHz, while other are less frequency specific. Power levels ranging from 10 W peak power to about 1 kW or even higher average power are considered, but until now there is no clear picture on this issue.

    Although it is obvious that this frequency selection must take into account existing radiocommunication services in the considered frequency ranges, the available documentation suggests that the attention to passive services is quite minimal.

    Comments by CRAF:

    On the presently available UWB information and proposals, CRAF has the following concerns:

    [1] To base the definition of an UWB device on the -10 dB bandwidth rather than the -20 dB bandwidth will probably give a dbw value which is not suited in coordination with passive services, because of their different protection requirements. The sensitivity requirements of a passive service might even require specification of the UWB spectrum mask down to a value lower than -20 dB.
    [2] Some passive applications such as pulsar research are particularly susceptible to pulsed interference.
    [3] An Ultra-Wide Bandwidth device generates wanted emission over a range of frequencies used by radiocommunication services which each have different requirements and protection criteria and related regulations. Because such wide bandwidths are used, it will be hard to avoid generating intentional emissions in passive frequency bands to which footnote S5.340 applies, where it states that in the frequency band "all emissions are prohibited". This intentional emission does not fall within the category `unwanted', `spurious', or `out-of-band'. Therefore, the UWB device must reduce its emission in a band covered by footnote S5.340 to below the pdf level acceptable for the victim service for all the time, i.e. effectively being switched off in that band.
    [4] Considering the wide bandwidths involved, it is extremely difficult to find a frequency interval suitable for UWB applications without encountering a band to which footnote S5.340 applies. It is not sure at this moment whether adequate UWB-filter technology exists to reduce the interfering signal falling into a passive exclusive band to below the level of harmful interference applicable to that band.
    [5] The protection requirements are obviously different for different radiocommunication services. The considerations and proposal presently available do not consider the specific protection requirements for passive services. Also it must be taken into account that the aggregate pfd level of the interfering signal must be below the level of harmful interference for that service. It is far from certain that this can adequately be accomplished by UWB systems.

    7. Abbreviations used in this Newsletter

    AES = Aeronautical Earth Station
    ATCA = Australia Telescope Compact Array
    CEPT = Conference of European Post and Telecommunication administrations
    CRAF = Committee on Radio Astronomy Frequencies (ESF)
    ERC = European Radiocommunications Committee (CEPT)
    ESF = European Science Foundation
    ESO = European Southern Observatory
    ETSI = European Telecommunication Standardization Institute
    GPR = Ground Penetrating Radar
    GSO = Geostationary Satellite Orbit
    HAP = High Altitude Platform
    IMT-2000 = International Mobile Telecommunication System
    IRAM = Institut de Radioastronomie Millimétrique
    ITU = International Telecommunication Union
    ITU-R = International Telecommunication Union - Radiocommunication Sector
    IUCAF = Scientific Committee on the Allocation of Frequencies for Radio Astronomy and Space Science
    MSS = Mobile-Satellite Service
    NRAO = National Radio Astronomy Observatory (USA)
    SE = Spectrum Engineering (CEPT)
    SEST = Sweden ESO Submillimetre Telescope
    TG = Task Group (ITU-R)
    UMTS = Universal Mobile Telecommunication System
    UWB = Ultra-Wide Band
    VLA = Very Large Array (NRAO)
    WG = Working Group
    WARC = World Administrative Radio Conference (ITU-R)
    WRC = World Radiocommunication Conference (ITU-R)


    Last modified: December 21, 2000