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At the start of 1996 radio astronomers may be tempted to look back comfortably on the gains which they made at WRC-95. The 6.668 GHz line of methanol which had the misfortune to be discovered only in 1991 has now received recognition in the Radio Regulations though a new footnote. Many of our established frequency bands also received footnotes to provide protection against out-of-band emissions from satellites. These footnotes contain direct reference to ITU-R RA Recommendation 769, which specifies the interference thresholds. Gone are the days when these protection criteria for radio astronomy were hidden in Report 224 of the CCIR. They are now referred to directly in the Radio Regulations.
But it would be wrong to think that "the system" will now take care of us. The threat from satellites and their unwanted emissions is actually stronger than before, in some quarters. Iridium is due for launch at the end of this year. Motorola propose to operate in the secondary downlink band 1610-1626.5 MHz allocated at WARC-92. Footnote 733E is there to protect us against the downlink, including its unwanted emissions. But what does the protection mean in practice? Motorola admit that the unwanted emissions are likely to exceed the interference thresholds for radio astronomy most of the time. But this is not harmful interference they say, this is friendly interference. Motorola have a Memorandum of Understanding with radio astronomers of the US National Radio Astronomy Observatories. Motorola will let them observe by kindly providing them with an interface so they can blank their detectors or receivers during the pulsed downlink transmissions. The lucky radio astronomers will then be able to "see through" the picket fence of friendly interference. Is this what we would like in Europe? Would we be ungrateful enough to refuse? After all, ours is only a primary allocation, whereas the friendly interference would be from the unwanted emissions of a downlink with a secondary allocation. How simple it might be if we could just listen to the dollars talking, and forget the stars.
Radio astronomers have a fight on their hands. If this goes through, other satellite operators will see the benefits of being friendly too. That alone would take out many of our frequency bands. And you can bet that satellite operators wouldn't be our only friends on such terms. We must fight for the rights of our science to coexist alongside the multinational corporations.
R.J.Cohen - Jodrell Bank
The agenda of the WRC-95 contains as major items the consideration of the report of the ITU Volutary Group of Experts, VGE, implying a complete revision of the Radio Regulations proposed by the VGE in its efforts to simplify these Regulations. Also high on the agenda are important matters related to the Mobile Satellite Service, MSS, and associated feeder links including the review of those provisions in the Radio Regulations which might hinder the timely introduction of new mobile satellite services. Attention is to be paid, however, to the existing services to which the frequency bands to be considered by WRC-95 are also allocated.
The VGE proposal does not seem to have much impact on radio astronomy. In the proposed harmonization and simplification of the Radio Regulations the Radio Astronomy Service is retained but the large amount of footnotes is simplified and combined to only a few footnotes.
The MSS allocations concern the frequency bands 149-150.05 MHz and 401-404 MHz. Satellite transmissions in these bands may cause significant interference by out-of-band emission in the RAS bands 150.05-153 MHz, 322.0-328.6 MHz and 406.1-410.0 MHz (see also section WRC 1995).
The protection of mm-astronomy is becoming an issue of concern. New cloud radar systems and vehicular radar may cause significant interference from harmonics of the transmitted signals at frequencies above 200 GHz. CRAF is preparing a document on this topic (which will be included in the 2nd edition of the see CRAF Handbook for Radio Astronomy as well). On the other hand, CRAF is in good contact with the manufacturer and operator of a new to develop cloud radar system section on (see vehicular radar).
The presure by satellite operators on the Radio Astronomy Service in the 1.6 GHz frequency area is extremely high. Motorola is approaching radio observatories to obtain a memorandum of understanding concering the coexistence of radio astronomical observations and MSS operations. INMARSAT attempts to reach its goal by "plain infiltration" in the radio astronomical community (see also sections on IRIDIUM MoU and CEPT SE28).
Pricing the spectrum has become an important issue and the developments have to be monitored carefully. In Europe there is yet no harmonization on this subject. The threat for French radio astronomy that also radio observatories would have to pay for spectrum use, has gone (hopefully for ever): a study to spectral pricing came to a result strongly opposed to this pricing.
The ESF press release on the CRAF Handbook for Radio Astronomy had a tremendous effect. In many publications (journals, newspapers) and radio programs (interviews with Kahlmann and Cohen) attention was paid to this publication. There is obviously a lot of public interest and sympathy.
Until now, more than 300 copies of the book have been distributed by the clearing house, while the ESF distributed it among its relations as well. The reactions on the book have been very positive so far: from the ITU several suggestions for the 2nd edition have already been made.
In fact, the frequency of the 3rd harmonic (228-231 GHz) is one of the most frequently observed both at Pico Veleta and Plateau de Bure. The CO transition (2-1) being at 230 GHz. In addition this is almost exactly the band allocated to radio astronomy for continuum observations. There are many continuum observations made in this band with bolometer arrays for which the interference levels are about 20 dB more severe. So there is certainly the possibility of interference.
At Pico Veleta there is a road at 1-3 km distance which has a clear line of sight and is very popular with tourists (at least in the summer). This is the "highest road in Europe" and is an increasingly popular tourist attraction. Plateau de Bure is perhaps better protected, although to the south there is a line of sight to Sisteron at about 50 km - with additional attenuation of 34 dB + 50*0.05 dB = 36.5 dB to be applied to the numbers given by GM. Neglected is then any protection given by the radiation pattern of the radar in the vertical direction since the Plateau only subtends about 2.5 degrees at Sisteron. On that basis the interference from one car would still be 3.5 dB above the spectral line harmful limit of -197 dB/m2/Hz (ITU-R RA 769) for General Motors' proposed permissible spurious level at 1 km distance, assuming clear line of sight to the observatory's 0 dB sidelobe, one radar-equipped car and 0.05 dB/km absorption. Sisteron is at present the end of an autoroute, but this autoroute (A51) will probably be extended across the Alpes through Grenoble to become one of the main N-S routes in Europe. Construction work has already begun, although the ecologists have legal cases pending and are blocking some of the sites. Hence we can expect an intense traffic at Sisteron in a few years with interference levels many dB above the limit for spectroscopy.
According to CRAF the radars should be filtered by at least 40 dB more than currently envisaged. Even this may not suffice for the continuum observations which may require 20 dB more protection. So 60 dB may be a more correct estimate of the protection needed in the 1 mm band.
In return for accepting the interference for 50 percent of the time, Motorola would provide a receiver system that would be able to serve as a clocking device for stopping the correlator during the 90 ms that Iridium uses the band as a downlink. This system could be supplied to each observatory in the form of a "study grant".
Motorola has recently visited Australia, Canada and Jodrell Bank. Visits to Arecibo and to The Netherlands, Germany, and Sweden are also in planning stages. The only existing MoU is with NRAO which had been signed on June 12th, 1994. No other observatory has signed.
MSS (uplink) has a primary allocation shared with RAS in the band 1610.6 - 1613.8 MHz [MSS has a primary allocation for the uplink in the band 1610 - 1626.5 MHz], while the MSS downlink has a secondary allocation in the band 1613.8 - 1626.5 MHz. Also FN733E holds. Shortly before WARC92 Motorola approached radio astronomy observatories for a memorandum of understanding (MoU). Calculations done for Jodrell Bank show that during about 4 hours in the night the interference would be below -240 dB W m-2 Hz-1. Else it is at least about -210 dB W m-2 Hz-1, being similar to the GLONASS interference. Any spectrum at 1612 MHz would show spurious features of a few tenths of a Kelvin. The MoU proposed by Motorola to Jodrell Bank is similar to the one signed by NRAO. If accepted the best situation would be a window of about 4 hours in the night during which sensitive radio astronomical observations can be done.
CRAF members have unanimously rejected the proposal for such a MoU in Europe.
CRAF will investigate this development and aim for clarification.
Radio astronomy could achieve some improvements, even though not on the agenda, in the form of "consequential changes". It almost appeared that protection of radio astronomy was the least controversial topic of the conference. Of course, new allocations to the Radio Astronomy Service could not be made, as this would be a substantial change the the table of frequency allocations, which must be put onto the agenda years before the conference. But protection of radio astronomy reached a new quality, as in several footnotes up to 15 GHz the detrimental effect of unwanted out-of-band emissions of satellite transmitters is pointed out.
In detail the results are:
During the SE17 meetings there were usually about a dozen people including from Iridium and from INMARSAT. About 25 participants participate in SE28. They are from administrations, MSS organizations and CRAF.
The meetings dealt so far with a discussion of the assumptions for radio astronomy, like e.a. antenna sidelobe levels, antenna gains, elevation angles used, S/N degradation factors due to RFI, etc.
CRAF explained that the levels of harmful interference to the Radio Astronomy Service (ITU-R RA 769) are in fact realistic and quite friendly to active services. CRAF also presented a document on various experiences with harmful interference from satellites (e.g. GLONASS, GPS, ASTRA).
Different systems for inhibiting emissions from MSS were discussed like e.g. the solution
CRAF has unanimously rejected the MoU with Motorola. CRAF indicated further that a preliminary analysis of the beacon solution carried out at Jodrell Bank indicates two problems: one concerns the effect of the beacon on the radio astronomy receiver, and the other concerns the unwanted emissions from the beacon into the band 1610.6-1613.8 MHz. At Jodrell Bank, a 1 mW beacon would be sufficiently powerful to cause problems in the receiver and would need to be filtered out at RF implying loss of sensitivity. Not all European observatories have been consulted yet. Furthermore, more input is needed from the MSS operators and RAS observatories like specific numbers, multiple beacon installations. It was said that the implementation of the beacon solution is very site dependent. But note: the beacon solution helps only for the uplink problem: not for the downlink.
CRAF explained the difficulties of sharing between RAS at 1610.6-1613.8 MHz and MSS downlink at
1613.8-1626.5 MHz. It is noted remarkable that a
During a regulatory discussion it was made clear that the ITU-R Radio Regulations imply that a newcomer in a frequeny band has to adjust his system in order not to interfere with those applications already using this band. In an allocated band it is: first come - first serve.