The European Science Foundation is an association of its 62
member research councils and acedemies 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.
Unwanted emissions (rayonnements non désirés) are an increasing worry for many users of the radio spectrum, and particularly for radio astronomers. Our experiences with satellite systems such as GLONASS, GPS and now ASTRA (section 8 of this Newsletter), have made us aware of the problem. As more and more satellites come into operation there is a very real threat that no direction in the sky will be clear for us. Instead there will always be electromagnetic pollution from one satellite or another. Can we prevent this happening?
The problem of unwanted emissions was placed firmly on the agenda for WRC-97, and ITU-R Task Group 1/3 was set up to deal with it. The progress so far has not been encouraging (section 5 of this Newsletter). The Task Group faces an uphill battle, because these unwanted emissions are actually wanted by many of the people who turned up at the most recent meeting. Representatives of the space services argued that they should be exempt from the general limits on spurious emissions. What they want are much less stringent limits, the same in fact as those for radio amateurs. This will allow them to use exciting new active antennas to reconfigure the beam coverage of their satellites at will. Unfortunately for the rest of us there will be additional pollution, because nobody told the designers to worry about pollution, it seems. For radio astronomy this could be the beginning of a dark age.
One of the few hopeful signs is that other people are starting to have problems with unwanted emissions too, including people from the satellite community. Recent discussions within the European CEPT project team SE28 have uncovered a new difficulty for mobile satellite services. Unwanted emissions from the terminals of rival MSS systems, all of which are in full view of the satellite receiver, may interfere with the reception of the wanted signals from the operator's own terminals. Unwanted emissions must be kept low in this case simply to allow rival systems to operate in adjacent frequency bands.
It all comes down to the principle of being good neighbours, a principle which is at the heart of the Radio Regulations. Don't throw your rubbish over the fence into the neighbours' garden. It is better for everyone that way. Today it is radio astronomers who suffer, but in future it could become everyone's problem.
R.J.Cohen - Jodrell Bank
In WP7D much discussion was devoted to the out-of-band emissions from mobile earth stations, and methodology for calculating interference levels at radio astronomy observatories, but no conclusion could be reached yet. In view of the ongoing discussions within CEPT SE28 PT and negotiations with Motorola/IRIDIUM it was unfortunate that the issue whether it is acceptable to radio astronomy to loose 10% of observing time due to interference (due to harmful transmissions reaching the telescopes due to propagation conditions) could not be resolved.
TG1/3, which was setup for looking to harmful spurious EMI of active users into passive services, has been infiltrated by MSS operators, e.a. The result is that it is now only dealing with spurious emissions outside 250% of the necessary bandwidth. Out-of-band emission is no longer dealt with. But the constraints should be more tight for this. The operators are telling loudly that they will not meet the ITU-R levels of harmful interference.
The protection of mm-astronomy is becoming an issue of concern. Cloud radar systems and vehicular radar may cause significant interference from harmonics of the transmitted signals at frequencies above 200 GHz. CRAF is involved in this development as external consultant to an ESA/Oerlikon-Contraves project to develop a cloud radar system including protection of radio astronomy at mm-wavelengths. The status of this cloud radar project is that using filter and antenna technology developed by ESA and Oerlikon-Contraves it seems possible to control the transmissions so that these do not cause harmful interference with the RAS band around 79/95 GHz. However, this does not solve the problem of damage at the radio telescope since the receivers used have bandwidths much wider than the bandwidth allocated to RAS. CRAF will prepare a document as input to WP7D discussions on the protection of mm-astronomy.
The pressure by satellite operators on the Radio Astronomy Service in the 1.6 GHz frequency area is extremely high. This became clear during the work in the CEPT SE28 PT. Before October 1996 the project team has to come with a methodology for sharing MSS mobile earth station activity with e.g. radio astronomy.
The next meetings of CRAF are:
There were 20 participants, including the Secretary A.Nalbandian, the Chairman ITU-R Study Group 7 H.G.Kimball, 12 radio astronomers, 4 people from the space services, and 2 participants from national administrations. It was reported that the ITU-R Handbook on Radio Astronomy which was published last year has already been adopted as a course text by D.Backer (University of California, Berkeley, USA).
The Working Party considered 12 input documents, including material carried over in the Chairman's report from the previous meeting.
Four drafting groups were established. One drafting group worked on revising the preliminary draft new recommendation on protection of the radio astronomy service from unwanted emissions. This had been sent back by ITU-R SG7 for WP7D to reconsider, and a new version was produced. A second drafting group produced a draft new recommendation on protection of mm-wave observatories, including the possibility of establishing protection zones around individual observatories within which all mm-wave bands are protected.
Two of the input documents were going forward to the meeting of ITU-R Task Group 1/3 in Paris on 1-4th April, and these were considered by another drafting group and forwarded as liason statements from WP7D to TG1/3. Ten of the participants went on to Paris for the meeting of TG1/3, including 7 radio astronomers.
The possible use by cloud radar systems of a frequency band at 95 GHz was considered. Because of the difficulty of filtering at such a high frequency the proposed system will have the ability to destroy all mm-wave SIS receivers if beam-to-beam coupling occurs. A liaison statement was drafted to WP7C. W.A.Baan, IUCAF chairman, was nominated as Rapporteur to WP7C on this issue.
The issue of sharing of the Radio Astronomy Service with the Mobile Satellite Service (MSS), including unwanted emissions, was considered by a drafting group, but unfortunately time did not permit some of the issues to be fully agreed within the Working Party. A draft liaison statement to WP8D and a document on the out-of-band emissions from mobile earth stations, and methodolgy for calculating interference levels at radio astronomy observatories were both carried over to the October meeting of WP7D. In view of the ongoing discussions within CEPT PT SE28, and negotiations with Motorola/Iridium it was unfortunate that the issue of whether radio astronomers can accept 10% loss of time through interference could not be resolved. R.S.Roger was nominated as Rapporteur to WP8D (mobile satellite services) on the issue of sharing with MSS below 1 GHz.
Another document carried over to the October WP7D meeting was a preliminary draft contribution to the World Radio Conference 1997 - Conference Preparatory Meeting on the issue of potential adjacent band interference from satellites into the passive band 15.35-15.40 GHz. The report on the GLONASS-Radio Astronomy Joint Experiment (done in November 1992) was carried across (for the second time), and the input document on solar power satellites was also carried across.
As usual the Working Party reviewed its questions. Under the new ITU rules any question which does not lead to further output within 6 years is earmarked for deletion. Three radio astronomy questions fall into this category:
The next meeting of WP7D will be in Geneva, 8-16th October 1996.
Outside the WP7D meeting there were three evening meetings of IUCAF, which considered problems of protecting mm-wave astronomy, which spectral lines to seek new allocations for, and the issues arising from MSS sharing and the Motorola MoU with the NRAO. It was decided to send an IUCAF contact group to discuss the latter issues directly with Motorola.
R.J.Cohen, Jodrell Bank
The purpose of TG 1/3 is to revise recommendation ITU-R SM.329-6 on spurious emissions. This was the fourth meeting of TG 1/3 and there will be one more meeting in Santa Rosa, CA (Oct.24-30, 1996) before the WRC-97 at which the results of TG 1/3 are tentatively expected to be considered. There were 50 attendees at the Paris meeting, the largest delegations being from Japan and the U.S. with 10 members each. The following radio astronomers, members of WP7D, were present: W. A. Baan (U.S.), R. J.Cohen (U.K.), K. Kawaguchi (Japan), R. S. Roger (Canada), K. Ruf (Germany), A. R. Thompson (U.S.), and J. B. Whiteoak (Australia).
Four ad hoc groups have been formed to deal with input to the task group. These are: (A) preparation of revision of ITU-R SM.329-6, (B) assessment of limits, (C) measurement of spurious emission, (D) protection of passive services. Most of the activity occurs in Ad Hoc B.
The point of major importance to WP7D is the specification of the general limit on spurious emissions, and in particular on emissions from satellites. These specifications fall within what is referred to as the Category A limits in the draft recommendation, and are given in terms of power into the antenna of the interfering service. The general limit is specified as a power level lower than the transmitted power (P Watts) by a minimum of (43 + 10 log P) dB, or 70 dB, whichever is less stringent. In the draft version of the recommendation from the previous TG 1/3 meeting space services came under the general limit. At the present meeting, an input document from Japan (1/3/23) proposed that for space services the 70 dB be replaced by 50 dB, thus allowing up to 20 dB higher spurious emission levels for space services transmitters with output powers greater than 5 W. This proposal was accepted at the meeting.
Cases of interference from satellite transmitters to radio astronomy have proven to be almost entirely due to broadband emissions resulting from digital modulation, or from intermodulation when many signal channels are put through the same power amplifier. The required limit here is in terms of power spectral density into the antenna and, although not given specifically in the Category A limits, it is obtained by dividing the power limit by the resolution bandwidth specified for the measurement of the emission. The resolution bandwidth for general limits is 100 kHz for frequencies from 30 MHz to 1 GHz and 1 MHz above 1 GHz. At the present meeting another exception was made for the space services, for which the resolution bandwidth became 4 kHz. This has the effect of increasing the allowable power spectral density by 14 dB for frequencies below 1GHz and by 24 dB for frequencies above 1 GHz. Thus the combined effect of the changes in power level and resolution bandwidth is to relax the stringency of the limit for broadband emissions by up to 44 dB for space services relative to the general standard.
With regard to interference to ground-based antennas, the limits for transmitters in space should be more stringent than those for terrestrial transmitters because for signals from space one does not have the effects of shielding by terrain features and Earth curvature. The limits presently proposed are therefore the opposite of what would be required to protect victim services. At the meeting there were two input documents from the U.S. concerned with effects on radio astronomy, one on spurious emissions from satellites and one on spurious emissions from terrestrial transmitters. Although the radio astronomy case was clearly stated and widely understood in the meeting, representatives of satellite broadcasting and the MSS refused to consider any compromise.
The current draft revision of ITU-R SM.329-6 contains an annex giving threshold values of interference for radio astronomy and passive sensing. The values for passive services are described as "permissible" interference, and this wording was questioned during discussion in the meeting. A liaison statement was sent to WP7C concerning this and other points that would be helpful in the interpretation of interference requirements for passive sensing.
The radio astronomers clearly stated their dissatisfaction with the present state of affairs in TG 1/3. A statement was included in the Chairman's report of Ad Hoc B saying that: "the RA community has strong reservations with regard to the Category A standards being considered here. We understand that Category A is the best we can expect from this meeting. However, none of these levels provide any serious protection for the radio astronomy service as mandated by Rec. 66. Furthermore, services which are of greatest concern to the radio astronomy service, like the space services are being excepted from the general rule on spurious emissions. The passive services' only objective here is to keep some parts of the spectrum clean from unwanted emissions. This will become extremely difficult in the years to come as these Category A limits become standard."
In an informal discussion on the day following the close of the meeting A. Azoulay, chairman of TG 1/3, proposed writing to the chairmen of study groups 1, 4, 7, and 8, seeking advice on how the impasse between radio astronomy and space services can be resolved. A parallel issue relates to the mandate of TG 1/3. Mr. Azoulay is considering to request from SG 1 a modification of the mandate of TG 1/3, in order to continue the study of spurious emissions but also to start work on out-of-band emission issues.
It remains to be seen whether any progress can be made towards improving the situation at the next meeting of TG 1/3, which is in October at Santa Rosa, CA.
TG 1/3 EVALUATION AND FUTURE WORK
From the onset it has not been sure how much the Radio Astronomy Service, RAS, would get out of TG 1/3, since the true harmful limits of RA.769 are too far out of reach for most spectrum users. However, since every little bit helps, the RAS would certainly gain from the TG 1/3 effort.
Up to this moment, the RAS has not really stated what it wants from TG 1/3, except that two recent papers argued for tighter standard than Category A (preferably along the lines of Category B), in particular for the space services.
It should also be noted that the suppression of spurious emission is "one of the ways of protecting the passive services". Other ways that would help the RAS are: - more wisdom in allocating radar and space service bands; i.e. do not allocate a space downlink adjacent to a passive service, - abandon the practise of slicing up the spectrum as has been done at lower frequencies and allocate adjacent frequencies to services that can share, and - set up Coordination/Protection/Quiet zones for radio observatories in order to facilitate local coordination.
At this point in the proceedings of TG 1/3, it is necessary to produce RAS proposals that specifically present the requirements for the RAS and the compromises it can live with. The time is right for this. For instance, if RAS accepts to use the VLBI harmful limits in more general environments (rather than RA.769 limits), the discrepancy between the wishes of the RAS and the other users may not be as extreme.
R.J.Cohen (Jodrell Bank, U.K.), A.R.Thompson (Charlotteville, U.S.A.) and W.A.Baan (Arecibo, U.S.A.)
The Italian branch of the Swiss company Oerlikon-Contraves with which ESA collaborates in this project, asked CRAF to cooperate in an investigation of possible ways to protect radio astronomy in the 3-mm window and to suppress spurious emission of the cloud radar system at higher frequencies. This request arose from the awareness that the cloud radar system might cause harmful interference to the radio astronomy service and ESA and Oerlikon-Contraves aim to protect radio astronomy.
CRAF joined this project as an external consultant and it instructed its member Anders Winnberg of Onsala Space Observatory to assist Oerlikon-Contraves with information about mm-wave astronomy techniques and methods.
So far, three meetings of representatives of ESA, Oerlikon-Contraves and CRAF have been held: one at IRAM, Grenoble, on January 22 and 23, 1996, and two at ESTEC on March 1 and on May 20, 1996.
The purpose of the meeting at IRAM was to give the Oerlikon-Contraves and ESTEC representatives an opportunity to visit a modern mm-wave observatory and to be informed about the high technical standard and competence of such an institute.
The meeting was of an informal nature and, in addition to reports about mm-wave hardware and methods, visits to the receiver laboratories and semi-conductor manufacturing rooms were arranged. The second day was devoted to a visit to Plateau de Bure, the site of the IRAM mm-wave array at an altitude of 2 500 m. The main conclusions of the meeting was that it would be technically very difficult to ensure an interference-free operation of mm-wave telescopes in the presence of cloud radar signals. There is even a real possibility of destroying an SIS element in the case of a direct (or near direct) mainlobe-to-mainlobe hit of a radar pulse into a radio telescope.
The second meeting, held at ESTEC Noordwijk, the Netherlands, was of a formal character and served the purpose of a first Progress Meeting. Since during meeting, a number of questions could not be answered, the meeting ended with a long list of action items.
CRAF experiences this attitude of industry as very positive since by experience industry does often not prefer to implement measures to protect radio astronomy by default.
The third meeting was a MidTerm Report (MTR) also held at ESTEC, Noordwijk.
Oerlikon-Contraves presented a long series of calculations of various types of filters for installation in radio astronomy receiver systems. The characteristics of some of these filters look very promising but the computer programme used did not take account of ohmic losses (this is very difficult) and one certainly expects that the performance of these filters will turn out to be considerably poorer in reality. It was decided that a breadboard version of a waveguide corrugated filter should be manufactured and tested at the Oerlikon-Contraves premises in Rome and at Arcetri Observatory later this year. It is possible that these activities imply that new components need to be made to be tested in a follow-up activity.
CRAF suggested that superconducting materials should be tested as well.
It was pointed out by CRAF during the meeting that one possibility of protecting radio astronomy receivers would be to block the receiver, probably at IF (it cannot be done at the front end), during the short duration of a radar pulse (~ 3 micro-seconds). It would then be important that the radar transmitter changes its pulse repetition frequency continuously rather than abruptly in such a way that a separate radar detection receiver at the observatory site can determine the PRF and predict the time for the next pulse accurately.
However, this option may require more than only blanking the receiver during the pulse. The front-end remains completely open with this method and the system remains vulnerable to damaga even though the receiver system is blanked.
A.Winnberg and J.E.B.Ponsonby, Onsala Space Observatory
The main discussion during the meeting was devoted to further tuning of the methodology which should be used for the calculations of the protection and sharing criteria which have to be presented to the CEPT in October 1996. Considering radio astronomy the following matter were discussed:
1.6 GHz MSS: over Europe: the number of filed systems is not yet known. At present INMARSAT is representative for Europe.
It should be noted that the beacon concept is based on reciprocity of propagation conditions. The beacon will have to be at a different place (more than several km away from the radio telescope, as experience at Westerbork showed) and at a different frequency than the radio telescope. In addition there may be large differences between the coupling from a small beacon into a MES compared with the coupling into a very large radio telescope.
In addition, the meeting noted the following warnings: be aware of unwanted emissions from beacons; and: how do "solutions" account for a varying density of mobiles?
Motorola stated that the power for one beacon will be 1 mW for the IRIDIUM system. CRAF learned that a non-negligible amount of power is emitted outside the narrow beacon band.
Experimental evidence at Effelsberg and Westerbork shows that a beacon should be weaker than -100 dBm. ESA informed CRAF that it has tested, in a different context, one of its transmitters and found that the residual noise outside the carrier frequency does not become minus infinity, but remains at a level of -70 to -90 dBc, which is not enough to protect a radio astronomy observatory, if the beacon is mounted on-site. One might gain a number of dB if the beacon is mounted off-site in a place (or in various places), which are terrain-shielded from the observatory antenna. If, however, this solution is feasible to efficiently protect radio astronomy observations, it would require more studies and also 'live tests'.
CRAF doubts whether at present Motorola is able to make beacons as desired by radio astronomy and required to protect radio astronomy observations. On technical ground CRAF rejected the suggested beacon solution.
Motorola showed that the IRIDIUM system has been designed to enable (at present) protection below the levels of harmful interference for Radio Astronomy (ITU-R RA769) for low traffic hours (i.e. about 16% of the day) only. This interference is generated by out-of-band downlink transmissions in the band 1613.8-1626.5 MHz where MSS has a secondary allocation into the band 1610.6-1613.8 MHz where the Radio Astronomy Service has a primary allocation. This explanation shows that the MoU between NRAO and Motorola on the use of the 1.6 GHz radio astronomy band does in fact not imply an agreement between NRAO and Motorola but simply that NRAO accepted the dictate of Motorola: IRIDIUM has been designed that way. Accepting this by the Radio Astronomy Service would imply that in the band 1610.6-1613.8 MHz the primary service radio astronomy has to accept harmful out-of-band emission from an application having a secondary allocation in an adjacent band, i.e. operating the band 1616.5-1626.5 MHz as is the plan for IRIDIUM downlink operations in Europe (see also: J.E.B.Ponsonby, Nature, Vol. 381, p.550, 13th June 1996).
Motorola showed no inclination at all to discuss the specifications of IRIDIUM and to discuss to what extent modifications (when needed) might be added to the system in order to protect radio astronomy. What Motorola did say was that it does not know how to filter the handsets. CRAF pointed out that by written letter of October 9th, 1991, Motorola guaranteed to protect radio astronomy without any limitations. The present situation is in contradiction with this. CRAF brought this under the attention of Motorola, but the latter had no comment.
The first (three) IRIDIUM satellites will be launched in September 1996.
This meeting gave the opportunity to discuss various urgent problems in European radio astronomy with many key representatives. Some of these problems are:
|EMI Event Duration||12 minutes|
|Pulse Duration||~ 1 second|
|Pulse Interval||70.6 seconds|
|Interval after which EMI reappears||98.570+/- 0.003 minutes|
|Interpretation||satellite at 700 km altitude|