CRAF Newsletter 2001/2
December 2001
The European Science Foundation (ESF) acts as a catalyst for the development of science
by bringing together leading scientists and funding agencies to debate, plan and implement
pan-European scientific and science policy initiatives.
The ESF is the association of its 67 major national funding agencies
devoted to scientific research in 24 countries. The ESF brings European scientists together
to work on topics of common concern, to coordinate the use of expensive facilities, and to
discover and define new endeavors that will benefit from a co-operative approach.
The ESF represents all scientific disciplines: 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
and coordination 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.
Contents
In recent, often quite animated, discussions on various fronts between astronomers and
other users of the spectrum, the key word has again been "Rec 769" - this being short
for Recommendation ITU-R RA.769, which gives levels for interference detrimental to the
Radio Astronomy Service as a function of frequency (see section 4 of this Newsletter). Other
spectrum users sometimes appear to be unpleasantly surprised by the high sensitivity of our
radio telescopes and would like to see more "reasonable" levels applied to their emissions,
thinking we can well live with quite a few dB more of unwanted emission from their systems
into our bands. Nothing is less true, however, and the radio astronomy community has
elaborated on the significance of the Rec. 769 levels wherever necessary. In order to
be able to continue routine scientific operation of radio astronomy sites our rallying
cry is "Rec. 769 is The Limit". CRAF is pleased to note the support radio astronomy has
received on a European level, from the CEPT and various national Administrations, in our
efforts to see these levels adopted in regulatory texts, to make them mandatory, instead
of merely recommended, for the protection of radio astronomy.
Wim van Driel - Paris Observatory
The 33rd CRAF meeting was held on 22-23 October 2001 at the Observatoire de Bordeaux,
France.
Key items discussed were:
- the Iridium satellite system has become operational again (March 2001). CRAF's concern is
that the new operator, Iridium Satellite LLC, does not appear to be inclined to comply with
the ESF-Iridium framework agreement and CEPT MRC Recommendation 4 that states that from
January 1st, 2006, the emissions from this system shall be below the protection criterion
for radio astronomy in the band 1610.6-1613.8 MHz. CRAF is in contact with the CEPT General
Milestone Review Committee, GMR, on this issue.
- the development of an "ECC Report on potential for interference
of Ultra-Wideband Transmission devices into existing and planned radio Services". This report
is in preparation in CEPT project team SE24. The protection criteria for radiocommunication
services (potentially) suffering from Ultra Wide Band (UWB) transmissions are developed and taken
as constraints for the (possible) implementation of UWB technology. A regulatory constraint is
footnote S5.340 of the ITU Radio Regulations, which states for specific frequency bands
that all emissions are prohibited. The ECC report will be limited to frequencies between 1
and 5 GHz: a study for other frequency bands will be done at a later stage.
- GONETS: Russia is planning to launch a MEO satellite system which will operate in the band
387-390 MHz (space-to-Earth). Coordination discussions have already started in the United Kingdom.
CRAF is concerned about the impact this system may have on European radio astronomy.
- CEPT working group SE28 noted that, judging by the limited number of officially reported
interference events from MSS systems to the Radio Astronomy Service in Europe, these
systems are apparently able to operate with unwanted emission levels in the radio astronomy
frequency bands more stringent than those brought forward by satellite operators as their
practical operational limitations in various discussions. "Several examples have also shown
that, when necessary, an appropriate design of the satellite is enabling to reduce the unwanted
emissions down to the Rec. ITU-R RA.769 limits" (from document 8D/191).
- CRAF took notice of the draft CEPT Briefs on all WRC-03 agenda items adopted by the CEPT
Conference Preparatory Group (CPG) and appreciates the importance shown by European
countries to the protection of radio astronomy. The current draft Briefs are well in
line with the radio astronomy views.
Footnote S5.340 of the Radio Regulations is very simple to read and to understand:
"All emissions are prohibited in the bands ...", and there follows a list of frequency bands,
which are of such importance to radio astronomy and passive remote sensing, that no other
(transmitting) service is allowed to use them. Strangely enough, this footnote recently led to
many discussions in a number of fora, including Study Group 1 of ITU-R. This discussion was
triggered by the envisaged introduction of Ultra-Wideband (UWB) technology. And in fact, the
language of the footnote is very straight forward, but the problems lie in the details.
According to the ITU Radio Regulations (article S1.138), emission is defined as
"radiation produced, or the production of radiation, by a radio transmitting station. For
example, the energy radiated by the local oscillator would not be an emission
but a radiation." Is radiation allowed in
passive exclusive frequency bands? The Radio Regulations give no answer to this question.
If it comes to interpreting the ITU Radio Regulations, the Radio Regulations Board, RRB, is
the ultimate authority. Its interpretation of FN S5.340 can be summarized as follows
(cf.: the Rules of Procedure of the ITU on this issue):
"2. Emissions in bands where uses other than those authorized are prohibited
2.1. The provisions listed below relating either to frequencies or bands to be used for safety
and distress communications or allocated for passive usage prohibit any other use:
a) Provisions relating to safety and distress communications:
aa) Appendix S13 (Part A2) (Non-GMDSS): § 13, 15(1), 16(1), 17A, 18(1);
ab) Appendix S15 (GMDSS), Tables S15-1 and S15-2 (frequencies marked with an asterisk (*)
to indicate that any emission causing harmful interference to distress and safety
communications is prohibited).
b) Provisions relating to passive usage: Nos. S5.267 and S5.340.
2.2. The Board considers that, in view of this prohibition, a notification concerning any
other use than those authorized in the band or on the frequencies concerned cannot be accepted
even with a reference to No. S4.4; furthermore the administration submitting such a
notice is urged to abstain from such usage.
In the case of assigning frequencies in a band to which S5.340 applies to radio
stations the notes Nos. 2.1b and 2.2 shall apply."
Alright, this is what we already understood from reading FN S5.340. But if we consider
frequency use adjacent to the bands of concern, which may spill over into the passive band?
The following decision by the RRB shall apply:
"To resolve cases of harmful interference between services in adjacent bands it was decided
that, irrespective of the phenomena at the origin of the interference (out-of-band emission,
intermodulation products, etc.), the administration responsible for the emission overlapping
a non-allocated band shall use appropriate means to eliminate the interference."
And here we find a clue to the answer to our question. Irrespective of the phenomena at the
origin, overlapping emission must be suppressed if it leads to interference. So, clearly,
transmissions are also not allowed, if they cause interference. CRAF recognises that
it is very hard to find a radio frequency band that is 100% free of any (man-made) emission
to any arbitrary level. The dilemma is that these emissions are not allowed in purely passive
bands, at least as soon as they cause interference.
CRAF recognises that the Radio Regulations do not preclude sovereign countries from developing
coordination scenarios for different services suitable to the specific interests of that
country, as long as radio services in other countries, operating in accordance with the
Radio Regulations, are not affected. This may even mean, for sufficiently large countries
that they may determine and implement separation distances required for the adequate
protection of a passive radio station, while enabling active applications in the same
band, even if footnote S5.340 applies. The interpretation would then be: "All emissions
crossing national borders are prohibited in the bands....". The Radio Regulations don't
give guidance, and also the RRB does not comment on this problem, simply because it is not
within its scope.
We note that unresolved questions like the ones above are apparenlt brought up only
to create confusion. CRAF favours the development and implementation of new technology,
especially if this technology helps to use the radio spectrum in a more efficient manner.
However, CRAF wishes to point out that UWB transmissions are incompatible with footnote
S5.340 if the bandwidth used for the transmissions covers any of the frequency bands
to which this footnote applies. CRAF cannot support any weakening of this footnote to enable
the introduction new technology.
The methodology to determine the levels of interference detrimental to radio astronomy is
given in Recommendation ITU-R RA.769. This Recommendation also contains a table of
power flux-density, pfd, and spectral power flux-density, spfd, values for the protection of
radio astronomy which must not be exceeded.
The pfd and spfd levels presented in Recommendation
ITU-R RA.769 have traditionally served to indicate the levels of detrimental
interference for radio astronomy stations. Although these numbers may appear extremely
stringent for certain active operators in the
spectrum, they are essential for the protection of radio astronomy operations. While the
technical characteristics of the radio astronomy stations have changed dramatically in recent
years, the numbers of Recommendation ITU-R RA.769 have not changed since 1979.
Routine operations of radio astronomy stations support scientific research on signals that
are much weaker than the spfd levels presented in Recommendation ITU-R RA.769 -
for example, in astronomical terms, the level for emissions detrimental to radio astronomical
HI-line observations in the 21-cm band (i.e. 1400-1427 MHz) is 125 Janskys
(1 Jansky = 10-29 Wm-2Hz-1), while the sensitivity of routine
observations of this line are expressed in milliJansky.
In practice, man-made in-band signals that exceed the levels of detrimental interference given in
this Recommendation cannot be easily removed from the data by mitigation techniques, if at all.
These thresholds of interference to the Radio Astronomy Service as calculated from the
methodology given in this Recommendation are a realistic assessment of the values of unwanted
emissions above which radio astronomical data are degraded or completely obliterated.
Therefore the levels presented in this Recommendation should not be considered as a
"starting position" for negotiations and studies between passive service operators and
active service operators.
Although these levels are inadequate to protect all radio astronomy experiments, they do serve
as adequate "upper boundaries" for protecting general operations of the current radio astronomy
stations and of the next generation telescopes. The figure below explains the degradation in
sensitivity of radio astronomy observations due to interference exceeding the level of
detrimental interference of Recommendation ITU-R RA.769. When discrepancies exist between
"what is desired" by passive services and "what can be achieved" by active services, an
increase of the levels in Recommendation ITU-R RA.769 cannot be a viable mitigation
option if radio astronomy is to remain a viable branch of science.
Loss of radio astronomy channel capacity due to interference
Further sensitivity improvements on current telescopes and the introduction of the next
generation telescopes will only further increase the difference between the operational
sensitivity levels and the detrimental levels given in Recommendation ITU-R RA.769.
It is crucial for preserving the future of radio astronomy as a science not to increase
the levels given in this Recommendation.
Within the scope of the Ultra-Wideband technology, short range radar (SRR) applications are
developed for implementation in automobiles. Recently, such a system was proposed for the
frequency of 24.15 GHz with a bandwidth of about 5 GHz for each radar system (e.g a spread
spectrum concept), but at very low emission levels to avoid the need for licenses. These
emissions out of the existing Short Range Device band are at or below the spurious levels
according the defined emission mask. The emissions outside the proposed emission mask are
reduced below -50 dBm. It should be noted that the compatibility of SRR with other
radiocommunication services has not yet been studied.
In the USA, it is duscussed whether the condition of footnote S5.340 should be relaxed
for the band 23.6-24.0 GHz
to enable the 24 GHz Short Range Radar to develop further the market, implying a
significant reduction of the protection of the passive services in that band. Such a scenario
applies, of course, to the US territory only and has no status in Europe.
SRR industry had performed tests at Haystack radio observatory to quantify the impact of this
application to radio astronomy observations. The tests were done at ~24 GHz. After scrutinizing
the test setup and the test results, CRAF could only conclude that such tests have no value for
coordination purposes or standard development. CEPT WG SE project team SE24 endorsed the CRAF
position that tests using radio telescopes shall not be used for coordination purposes or
standard development, since such tests are necessarily done in the near field of a radio
telescope and cannot be interpreted for the purpose the test was set up.
ETSI had recently developed a standard for SRR at 24 GHz and is working on a system
reference document for SRR operating at 24 GHz. The standard has already been sent out for public
consultation. CRAF identified that this system reference document does not comply with
the ITU Radio Regulations in a number of aspects.
During discussions within CEPT great concern was expressed that the standard is already
in public consultation before the system reference document was adopted. It was also noted
that on issues like this, CEPT WG FM must first give clearance to proceed with work in the
selected frequency band(s). The European Common Allocation table identifies the band 76-77 GHz
for vehicular radar applications and no frequency around 24 GHz. So far, only the commercial argument of cheap components
supports the choice for 24 GHz.
After some further discussion it was concluded that CEPT will study the issue further. As long
as this study is not completed and a decision on the frequency assignment has been made, ETSI
will put the standard 'on hold'. SE24 will work on the technical aspects of the issue and WG FM
will address the frequency assignment issue.
Note: in an earlier version of this Newsletter it was said that the FCC had relaxed
the condition of footnote S5.340 for the band 23.6-24.0 GHz. This remark appears to be
based on incorrect information given to CRAF.
As noted above, the protection criteria used for radio astronomical measurements and the
methodology to determine the power flux density levels of emission detrimental to radio
astronomy are given in Recommendation ITU-R RA.769. These criteria are generally used
in Administrative compatibility studies involving radio astronomy stations, since the generic
telescope characteristics assumed in this Recommendation, such as integration time
and antenna pattern are considered as "adequately representative" of what is actually used in
radio astronomy.
It should be pointed out, however, that the parameter values used in practice cover a wide
range of values.
Each radio telescope has its own technical and operational characteristics, none of
them identical: "each radio telescope is its own prototype" sums this up. Furthermore, each
radio telescope was built for a specific kind of research, setting its initial technical
characteristics, which continue to be modified through the
implementation of the latest technology with adequate flexibility to serve a specified range
of research projects.
The flexibility required for most radio astronomy telescopes implies that it is virtually
impossible to define precisely their characteristics for compatibility studies as a
contribution to the Administrative coordination process. This applies also to the
development of technical standards for radio emitting devices.
Recommendation ITU-R RA.769 was developed to avoid the burden on Administrations of having
to determine the detailed telescope parameters for individual telescopes in order to implement
the protection of the Radio Astronomy Service, and to develop
instead a generic scenario from very different and usually incomparable individual
options.
In cases where it is deemed necessary that tests involving specific radio telescopes
should be made (such as for very specific questions by a national regulatory authority), the
following range of questions needs to be posed
What question needs to be, and could be, answered by the anticipated tests?
What technical and operational conditions are required to lead to interpretable results
from the anticipated tests?
Can these conditions be realised at the radio astronomy station under practicable
circumstances?
What kind of information will tests add to an Administrative coordination process or
compatibility studies using ITU-R Recommendations?
Can the interpretation of the measurement data be accurate and representative for the issue
at hand and reliable vis-à-vis the model assumptions in Recommendation
ITU-R RA.769?
Very likely, such tests will show that the sensitivity of a specific radio telescope differs
from the levels given in Recommendation ITU-R RA.769, as its characteristics differ from
the "model assumptions" used for this Recommendation. The sensitivity of some radio telescopes
may be such, that for them the "769-levels" could be relaxed somewhat, while for others these
levels should be tightened significantly.
Any test will provide information only on the impact of a specific transmitter used on
a particular radio telescope under certain conditions, and cannot be expected to yield
a generically applicable result for use in compatibility studies as part of a coordination
process, or in standard development.
CEPT = Conference of European Post and Telecommunication administrations
CPG = Conference Preparatory Group (CEPT)
CRAF = Committee on Radio Astronomy Frequencies (ESF)
ECC = Electronic Communications Committee (CEPT)
ESF = European Science Foundation
ETSI = European Telecommunication Standardization Institute
FCC = Federal Communications Commission (USA)
FM = Frequency Management (WG FM of CEPT)
GMDSS = Global Maritime Distress and Safety System
GMR = General Milestone Review Committee (CEPT)
ITU = International Telecommunication Union
ITU-R = International Telecommunication Union - Radiocommunication Sector
MEO = Medium Earth Orbiting Satellite
MRC = Milestone Review Committee (CEPT)
MSS = Mobile-Satellite Service
pfd = power flux density
RTTT = Road Transport & Traffic Telematics
SE = Spectrum Engineering (CEPT)
spfd = spectral power flux density
SRD/MG = Short Range Device Maintenance Group (CEPT)
SRR = Short Range Radar
UWB = Ultra-Wide Band
WG = Working Group
WRC = World Radiocommunication Conference (ITU-R)
WRC-03 = WRC 2003
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Editorial Group: R.J.Cohen, P. Scott, W.van Driel
Last modified: February 7, 2002