CRAF Newsletter 1996/3
November 1996
The European Science Foundation is an association of its 62
member research councils and academies in 21 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.
Contents
The final meeting of Task Group 1-3 concluded at the end of October.
The results will be of concern to all who are worried by the problems
of electromagnetic pollution. The task group has produced a revised
recommendation on spurious emission limits which is to be presented
to WRC-97 in a year's time. The recommendation contains
three tables of emission limits, only the first of which (Category A limits)
are destined to appear in the Radio Regulations. These Category A limits
represent a lowest common denominator which all countries are prepared to
live with. They are generally far more relaxed that the limits which are
being set within Europe by ETSI (Category B). In some cases they even
represent a step backwards from the current limits. A more detailed
discussion will have to wait until the next Newsletter.
What will be of particular concern to radio astronomers is that the
space services were not willing to accept even Category A limits except
as "design goals" until they have made further studies. Not only this,
but they wish the spurious emission limits to be specified in a narrow
band (4 kHz). This leads to a further 24 dB relaxation of the limits on
broad band spurious emissions from systems like Iridium and ASTRA. The
space services did not give technical justification for the
privileges that they are claiming over other services. The 4 kHz
band is to be used even for earth stations (such as mobile handsets)
where it is not clear why space services need special consideration.
The ITU works by consensus so if some administrations are prepared to
support their space services in this way there is not a lot to be done.
What troubles me most is that the ITU has allocated a large number of
satellite bands (space-to-Earth) immediately adjacent to radio
astronomy bands. In fact if you count up the MHz then 80% of the bandwidth
allocated to radio astronomy on a world-wide primary basis lies adjacent to
a space-to-Earth band. Looking through the bands we see many which are
familiar problem areas, such as 1610.6-1613.8 MHz. But there are many more
problems waiting to come once the satellites are launched and start
transmitting. We are right to be worried.
To make matters worse, the satellite community is not satisfied with its
current allocations. It is actively seeking much wider bands to support
the ambitious systems of the future like Teledesic and M-Star
(son of Iridium). Is it right for these allocations to go ahead wholesale
while the issues of satellite pollution are under study and while the space
services cannot accept limits like other services, but only design goals?
Radio astronomers in all countries of the world need to be aware of these
issues and publicize them. We need tighter regulations on satellite
pollution so that progress can benefit everyone.
The main subjects discussed at the meeting concerned the threat of the Iridium
downlink to the Radio Astronomy Service in the 1.6 GHz band, CRAF's
participation in the CEPT SE28 project team ITU TG1/3 progress and
policy issues for mm-astronomy.
CRAF participated in 3 meetings of the CEPT SE28 project team since the 20th
CRAF meeting. CRAF input documents were extensively discussed in SE28. However,
the pressure by the MSS operators is steadily increasing. Radio
astronomers have to fight against the proposition of some operators that for
radio astronomers it does not matter when they wait "a little longer" for their
data and that when radio astronomers lose data because of interference,
observations can be repeated. It appears from the many discussions held so
far that Motorola does not want a discussion about technical details, i.e.
about what has been or can be implemented in its system to protect the Radio
Astronomy Service, but it only wants that radio astronomers accept its dictate.
The protection of mm-wave astronomy is becoming an issue of concern. Vehicular
radar may cause significant interference from harmonics of the transmitted
signals at frequencies above 200 GHz. Satellite-borne cloud radar systems
which are proposed for 95 GHz will have the potential to destroy sensitive SIS
receivers. The updated mm-astronomy position document addressed various aspects
of frequency protection, re-allocation and needs for different or more
frequency bands. CRAF asked IUCAF to review the mm-frequency use, needs and
priorities. It became clear that mm-astronomers have to worry already about the
frequeny use and protection for frequencies between 275 and 400 GHz.
The progress of TG1/3, which was setup for looking to harmful spurious EMI of
active users into passive services, is a matter of great concern. Its
initiative, to report at the WRC97 on spurious emission questions raised at
the WARC92, has in fact been taken over by the operators and the agenda had
been adjusted to their will. Much effort is needed to bring the task group
back on its initial track.
Among the other topics discussed were the severe interference problems in Italy
at 1.6 GHz (which prevent Italy to be a reliable and full participant in the
VLBI operations at this frequency); interference from an unregistered satellite
system at 328.25 MHz, continuing trouble at 10.6-10.7 GHz because of the
ASTRA transmissions and a new channel 38 TV transmitter in Denmark. CRAF is
working hard on the solution of these problems.
The ITU showed interest in the CRAF EMI-database which resides at Nançay. Up to
date interference reports from European radio observatories are urgently needed.
CRAF is also investigating the possibilities to become an ITU Sector member.
This is important since various interest groups of active frequeny users have
this status already. Radio astronomy should participate as soon as possible at
the Sector member level in ITU activities.
From 28 August - 5 September, 1996, the 25th General Assembly of the URSI was
held in Lille, France. Most of the CRAF members partcipated in this conference.
Ample attention was paid to the problems of ether-pollution, harmful interference,
problems of interference in radio astronomy. Also various aspects of spectrum
management and utilization (in general) received attention.
CRAF's contributions were:
R. J. Cohen described the beauty and vulnerability of research of the OH radical
in the 1.6 GHz domain. He referred to the improvements for radio astronomy in the
frequency bands in this domain for which the it has a primary
allocation, due to the cooperation with the GLONASS administration: GLONASS
satellites moved out of these frequency bands. He also explained why the
Iridium system which will operate both up- and downlink in the 1.6 GHz domain,
will have a severe impact on radio astronomical research at 1.6 GHz.
H. C. Kahlmann (former CRAF chairman) described the work of CRAF and its current
position in Europe. CRAF being an associated committee of the European Science
Foundation has good cooperations with various administrations, the CEPT, the
European Commission (although both organizations have deviating interests).
CRAF serves IUCAF and CORF with information from the regional European scene
which may look different from that in other parts of the world, e.g. due to its
political fragmentation.
T.A.Th.Spoelstra addressed the question 'Will "Passive Services" and
Scientific Applications of Radio
Survive?' - the answer is "yes", when [1] there are
a series of "passive-exclusive" frequency
allocations to passive spectrum use; [2] when active users recognized that when
they are not able to solve their problems in 98% of the spectrum, they cannot
do this in 100% of the spectrum; and [3] regulatory issues should not be
articulated and put in rules and agreements by special interest groups of active
spectrum users, but by the "neutral effort" of the administrations of sovereign
countries working for the long term best interests of all spectrum users.
A.Winnberg described the specific situation of mm-astronomy. Untill now mm-astronomy
hardly suffered from interference, but at present various system are entering
within the horizon which carry a potential disaster to radio astronomy:
vehicular radar, satellite-borne cloud radar, etc. His plea was: let us not
make the same mistakes at mm-wave atmospheric windows as we did for lower
frequency radio astronomy: mm-astronomy is best served when the frequencies
within atmospheric windows are exclusively allocated to passive spectrum use.
Remote sensing people and radio astronomers should cooperate more closely to
achieve adequate allocations at frequencies above 275 GHz.
In addition, IUCAF had 2 meetings during this conference (for a report see
IUCAF). Items discussed during these
meetings included developments of applications operating at mm-wave
frequencies, the threat from Iridium (Motorola canceled a meeting between
IUCAF and Motorola which had been scheduled to precede the URSI General
Assembly).
This URSI General Assembly turned out to be a good opportunity for radio
astronomers working on the protection of radio astronomy frequencies to have
thorough discussions.
CRAF participated in the 7th meeting of this CEPT SE28 project team which was
held on 11-12 September 1996 in Paris at CNET [E.Gerard and T.A.Th.Spoelstra].
Around 20 participants from administrations, MSS organizations and CRAF
attended the meeting.
The major part of the discussions were focussed on the protection of radio
astronomy. SE28 agreed on the methodology to be adopted for the protection
of radio astronomy against unwanted emissions from mobile earth stations, MES.
The meeting discussed the document on the methodology of calculation to
provide separation distances by default between a radio astronomy site and
MES (which implies that outside that area around a radio astronomy observatory
no restriction applies to the operation of mobile earth stations, MES). The
document has been prepared by CNET. The methodology is based a Monte Carlo
simulation. This is based on the principle of sampling random variables from
the defined probability distribution of traffic, frequency channel occupancy
and number of calls. An executable version of the software is expected to be
available soon at France Telecom.
Two examples were calculated: for TDMA and CDMA. However,
due to lacking quantitative information from one MSS operator (Motorola)
not more than rough examples could be presented.
In addition the meeting concluded that the protection of radio
astronomy should be provided by setting site by site separation areas around
each radio astronomy observatory depending on the channels used by the MES.
Such calculations should be undertaken in collaboration by MSS operators and
radio astronomers.
SE28 also concluded that around each radio astronomy observatory
measurements should be undertaken to determine the MES effects on radio
astronomy observations. This should be done in collaboration between MSS
operators and radio astronomers.
The meeting prepared a draft ERC report on the sharing between MES/MSS and
radio astronomy. Also draft recommendations to ITU WP7D and ITU WP8D were
prepared. These documents explain the method of calculation as mentioned above.
The chairman agreed to circulate the draft as soon as approved at the SE WG
meeting in early October so that radio astronomers can see it before the
WG7 meeting in Geneva.
No progress has been made on the item of unwanted emissions from satellites
into radio astronomy observations. This is due to the fact that again
Motorola did not provide the promised input information for this study.
Motorola said that a new and more realistic solution may be proposed at the
next SE28 meeting.
SE28 also discussed the compatibility between TDMA and CDMA systems in adjacent
bands and the sharing between several TDMA and/or CDMA systems. For the
former, it appears that the spurious emissions of the TDMA satellite downlink
will cause interference to the CDMA ground receivers and that further studies
are necessary. For the latter, sharing between CDMA systems should be possible
while sharing between TDMA and CDMA is not feasible. But sharing between 2
CDMA systems should be possible.
Also unwanted emissions from MES/MSS satellites systems into Fixed
Service, GSO MSS and GLONASS were discussed to some extent. Some refinements
of the calculations are still needed. This is partly due to lacking information
about the different systems involved.
SE28 will meet again on 5-6 December 1996 in Paris and 20-21 January
1997 in Dwingeloo.
Until now mm-wave radio astronomy did not suffer much from harmful interference.
But, because of various technological developments one cannot expect that this
situation will last much longer: There
are currently several proposals for "unlicenced" equipment to be permitted in
the USA, such as collision avoidance vehicular radar. There are plans for a
satellite-borne cloud radar operating at 94 GHz and several proposed satellite
projects will use frequencies around 60 GHz for intersatellite communications.
The spectrum occupancy at frequencies above ~ 50GHz is such that at present
there are also several allocations in the mm bands
for space-to-Earth satellite transmissions which are not used yet
(Broadcasting, Amateur, and Radio
Navigation services). But nobody should expect that this will remain the case.
All the mentioned future applications have the potential to severely limit
the access of radio astronomy to the millimetre spectrum. This has significant
impact on the progress of astronomical research, since many spectral lines
from many molecules are detected at mm-wavelengths. Observations of these
spectral lines are of fundamental importance to our knowledge of physical
and chemical
processes in the Universe. Understanding this, it is recognized that the
increasing potential of harmful interference comes at a time when the
Radio Astronomy Service actually is in need of increased bandwidth for high
sensitivity continuum observations and for studies of line emission from objects
at high redshift. Clearly, increased protection for the Radio Astronomy
Service is necessary if these astronomical investigations are to go ahead.
CRAF is working on criteria and methods to establish this protection. It is recognized
that perhaps the methodology to protect mm-wave astronomy might differ from
"common" radio astronomy at lower frequencies, but much has to be studied
in more detail. CRAF is eager to receive any comments and recommendations
helpful for this study.
Some comments, constraints and criteria can already been given:
It has been proposed several times that some entire atmospheric mm windows
should be reserved exclusively for RA - in a manner analogous to Natural Parks
or the Antarctic Continent.
A radical step, which has also been advocated, is to allocate whole atmospheric
windows to radio astronomy.
The justification for this is based on recent discoveries which
have led to a new
awareness of the potential of the millimetre waveband for astronomy, and has
precipitated the planning of several large instruments (see below). For
example, it is now known that galaxies and quasars can be detected at high
redshifts so that their line emission from carbon monoxide (CO), the most easily
detected molecule, can appear in any part of the atmospheric windows, depending
on the exact redshift. It seems increasingly likely that studies of these
objects will be relatively easy at millimetre wavelengths.
One reason is just the higher intrinsic sensitivity attainable
at millimetre wavelengths. This is because the thermal emission from these
objects displays a spectrum which rises rapidly with increasing frequency.
Furthermore, a larger observing bandwidth can be used which for line observations
leads to a further increase in sensitivity proportional to the square root of
frequency.
In addition, the continuum emission from dust in these objects and others
in our own Galaxy can be detected by wide-band bolometer detectors which for the
highest sensitivity must utilise a large fraction of a whole atmospheric window
(typically 50 GHz).
Yet another important field of study which calls for broadband continuum
observations in the millimetre bands is that of the cosmic background radiation.
This has its (very broad) peak near 2 mm wavelength. The Sunyayev-Zeldovich
effect shows a pronounced absorption at about 2 mm.
Also the density of spectral lines in the mm bands is very high. For
example, the frequency range 218 - 263 GHz has been surveyed in the star forming
region Sgr B2 N and about 1800 spectral lines from more than 50 different
species have been detected, implying a mean density of about 40 lines per GHz.
Each such line is potentially a source of astronomical information.
Future large instruments will be designed to explore
distant galaxies in more detail with enhanced sensitivity and angular
resolution. For example, to study the epoch of galaxy formation at redshifts of
3 or greater, the CO (1-0) line has moved out of the mm windows and the higher
order transitions emitted at higher frequency must be used (at rest frequencies
of 230, 345, 460 GHz etc). In principle just to study CO in this way requires
an octave in frequency so that as one transition moves out of the band at the
lower frequency end, the next higher transition enters the top end of the band.
The present allocation in the 3mm window only covers the z range out to about
0.1 - and does not take these considerations into account.
The blanket protection of a whole window may still be relatively easy
for the 0.8 mm window and shorter, but protections for the more important 1, 2,
and 3 mm windows will probably have to be "partial" (i.e. in terms of frequency
band allocation) to respect the requirements of
other services. Even this limited goal requires large changes in the existing
allocations. However, this may still be feasible as the Radio
Astronomy Service is the only service which routinely is using these bands at
present. Facing the threats appearing within the mm-astronomy horizon, one
has to ask severel questions: Which windows are most
important? Would it be sufficient to ban satellite transmissions in selected
windows, and rely on "quiet zones" for protection from Earth-based emissions?
If so, what about aircraft emissions etc? Is it possible to use parts of
several windows instead of a whole window for high redshift observations?
At the same time we should keep in mind that far-advanced plans for
satellite observatories for the sub-mm range exist. The most imminent
satellites of this type are the American SWAS and the Swedish ODIN. They will
observe spectral lines of oxygen and water, i.e. the very same species that divide
the mm band into a number of windows seen from the ground. ODIN will be
equipped also with a receiver for the 119-GHz oxygen line. Therefore protection
is needed for at least the 119-GHz line. The atmospheric absorption around
60 GHz is also due to oxygen but it is a complicated band consisting of many
densily packed lines. However, due to this the band can be used for determining
the temperature profile in the troposphere and the meteorologists are fighting
for access to this band.
CRAF investigates several changes with respect to the present situation. These
have regulatory and technological aspects. CRAF's aim is a minimum but necessary
change to the existing situation to
guarantee access to the nearby and distant Universe for studies of carbon
monoxide and dust emissions. On the other hand CRAF recognizes the need of
well-balanced articulation of possible changes: just as radio astronomers do
not want to be harmed by active spectrum users, CRAF does not accept unjustice
to these other spectrum users. Therefore, CRAF stimulates where possible
efforts to develop
new hardware and software tools for proper frequency protection and spectrum
engineering.
Technological development
may open different new and so far unknown possibilities to protect radio
astronomy. CRAF's efforts
are extended to frequencies up to 1 THz. It does this work in close collaboration
with IUCAF. By this it attempts to
enable new facilities to work through open windos towards the Universe. At
present,
the following major new millimetre-waveband observing facilities have been
planned:
1) The Millimeter Array of the US National Radio Astronomy Observatory (MMA).
40x8m diameter antennas for 30-366 GHz range.
2) The Large Millimetre and Submillimetre Array of the National Astronomical
Observatory of Japan (LMSA). 50x10m diameter antennas for 35-500 GHz range.
3) The Submillimeter Array of the Smithsonian Institution (SMA). 6x6m diameter
antennas for 150-890 GHz range.
4) The European Large Millimetre Array (LSA). 50x16m diameter antennas for
80-360 GHz range.
5) The US-Mexican 50m Telescope. Radome enclosed for frequencies up to 230 GHz.
6) The Spanish 40m Telescope. Free standing for frequencies up to 120 GHz.
Interference from a satellite system operating at 328.2 MHz is causing harmful
interference to observations in many countries, e.g. as observed in Westerbork (The
Netherlands) and India. A joint effort of the Dutch and German administrations
in collaboration with the satellite monitoring station in Leeheim (Germany)
led to the conclusion that at this frequency radio astronomy is suffering from
an unregistered satellite system.
Summary of the characteristics of the interferer is given in the following tables.
Characteristics of 328.2 MHz interference
| Frequency | 328.25 MHz |
| Bandwidth | ~ 38 kHz |
| Maximum pfd | -126 dBW/m*m |
| Polarization | possibly LIN |
| Modulation | Digital Modulation |
| Type of transmission | data bursts |
Orbital elements
| Perigee | 610 km |
| Apogee | 780 km |
| Inclination | 87 degrees |
| Period | 98.4 min |
Concerning the modulation, Leeheim reports in summary that two
frequencies (representing the shift between the upper and lower frequency pairs
(shift = digital information) are symmetrically modulated with respect to a
fictitiously suppressed carrier frequency. The frequency difference between the
upper and lower frequency pairs is 4750 Hz and 2375 Hz, respectively.
Data burst occur every 72 seconds with a duration of 1.806 to 1.839 seconds.
Every data burst starts with a frequency offset of 4750 Hz and a duration of
100 ms. The information is transmitted with pulse duration of 2 or 6 ms.
By means of calculations the orbital values were compared with approximately
70 satellites which have similar orbital characteristics. Since the interference
in Westerbork was noticed for the first time, Leeheim mainly focussed its work
on all satellites launched between early September 1992 and late November 1992. As
the space station currently in operation and the space station monitored back
then do not necessarily have to be identical, these comparative measurements were
continued up until 1996.
The calculations mentioned did not enable the administrations to identify the
interfering satellite. Hence, it can be concluded that the satellite under
observation is likely to be operated by the military or by an intelligence agency
(according to the interpretation of Leeheim). Orbital elements of such satellites
are unknown.
Given the increasing number of unregistered satellite systems, we may also
conclude that the coordination of this system has not been carried out properly.
CRAF informed the ITU Radiocommunication Bureau, RB, via the Dutch
administration of this problem. The CEPT
European Radiocommunication Committee, ERC, had been informed by CRAF directly.
This problem has been discussed further during a CEPT FM
meeting in Stockholm (September 23-27, 1996).
The problem is twofold: [1]
radio astronomy suffers harmful interference within a band in which the Radio
Astronomy Service has a primary allocation; [2] regulatory bodies should be
aware of the threat of not properly coordinated (= illegal) space stations.
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