1、STDmCEPT ERC REPORT 50-ENGL 3977 - 2326434 0033296 126 W ERC REPORT 50 European Radiocommunications Committee (ERC) within the European Conference of Postal and Telecommunications Administrations (CEPT) X, - ERC REPORT ON INTERFERENCE CALCULATIONS FROM MSS SATELLITES INTO RADIO ASTRONOMY OBSERVATION
2、S Moscow, September 1997 STD-CEPT ERC REPORT 50-ENGL 3997 M 232b4L4 0033297 062 Copyright 1998 the European Conference of Postal and Telecommunications Administrations (CEPT) STD-CEPT ERC REPORT 50-ENGL 1997 2326434 0013298 TT9 m ERC REPORT 50 ERC REPORT ON INTERFERENCE CALCULATIONS FROM MSS SATELLI
3、TES INTO RADIO ASTRONOMY OBSERVATIONS 1 INTRODUCTION . . 1 2 LEVELS OF INTERFERENCE PRODUCED BY MSS SATELLITES . 1 2.1 CASE OF THE 2.5 GHz BAND 1 2.2 CASE OF THE 1.6 GHz BAND 1 2.3 MITIGATING FACTORS . 2 2.3, I . 2 2.3.2 Thermal Noise 3 2.3.3 Interference spectrum . . . . . . . . . . . . . , . , . .
4、 . . . , . . . . . . . . . . . . . . . . . . . . . . . . 3 2.3.4 Polarisation . . . . . . . . . . . . . . 2.3.5 Summary of mitigation techniques and factors according to Motorola and the radio astronomy communiy.4 IMPACT OF THE INTERFERENCE . . . . . . 4 Radio Astronomy sidelobe levels . . . . . . .
5、 _ 18dB 13 dB c 3 IMPACT OF THE INTERFERENCE Radio astronomers have explained within SE28 the difficulties they have to face with satellites from the GLONASS, GPS, and ASTRA networks. For GPS, all new satellites will have sufficient filtering. Concerning ASTRA, the recent problem of interference in
6、the 1 O GHz band has not yet been resolved. For GLONASS, there is a step by step plan described in the MOU between IUCAF and the GLONASS Administration to reduce GLONASS interference and the first step (GLONASS frequency reconfiguration) in of this plan is being implemented. The GLONASS Administrati
7、on has agreed to investigate the ways of reducing out-of-band emissions in the frequency band 1610.6 - 1613.8 MHz to the levels of -238 dBW/(mHz). Radio astronomers are expecting that such improvement will be implemented on the GLONASS system, the IUCAF/GLONASS MOU stating that “the GLONASS Administ
8、ration agrees to investigate the ways of reducing out-of-band emissions in the frequency band 16 10.6- 16 13.8 MHz to the levels indicated in Rec.769 and to communicate their proposed solution of this problem at a future meeting“ (between IUCAF and GLONASS). At present, the estimated pfd level is -2
9、01 dBW/(m2.Hz) from one GLONASS satellite and some -198 dBW/(m2.Hz) from the whole constellation i.e. 40 dB above the ITU Interference threshold. In practice, the measured interference temperature is mitigated by about 7 dB due to radio astronomy far side lobe levels and up to 3 dB due to the nature
10、 of interference spectrum. These factors have not been put in the table because of the concern from the radio astronomers that these factors might not apply to the case of Iridium. Radio astronomers indicated that it is still possible to make low sensitivity observations in a narrow band but that hi
11、gh sensitivity or wide bandwidth measurements still show strong interference. STD-CEPT ERC REPORT 50-ENGL 1777 2326414 0033303 1i9b ERC REPORT 50 Page 5 The estimated levels of interference from Iridium satellites are well below the current GLONASS levels experienced by radio astronomers in the band
12、. If the existing GLONASS satellites were simply moved to centre frequencies below 1604.8 MHz as indicated for the GLONASS configuration after a date between 2005 and 2008, the Iridium levels would be a few dB below the levels of GLONASS. Radio astronomers stress that, before the fill application of
13、 the IUCAF/GLONASS agreement, interference from GLONASS and Iridium constellations will be cumulative and would lead to an increase in the number of observations lost. Motorola state that in the final frequency configuration, Iridium levels are expected to be 7 dB below those of GLONASS in the middl
14、e of the band and would only be a small fraction of the interference power. Furthermore, since the two systems operate on either side of the RAS band, the interference maximal do not coincide and therefore it is by no means clear that the effect will be cumulative in this way. Radioastronomy is prot
15、ected by ITU-R Radio Regulations Footnote S5.372 which states that “ harmful interference shall not be caused to stations of the radio astronomy service using the band 1610.6-1613.8 MHz by stations of the RDSS and MSS services“, and by the fact that interference is caused by unwanted emissions. The
16、radio astronomers further stressed the secondary status of the downlink MSS compared to their own primary status. This last argument was questioned by Iridium, which stated that primary/secondary status refers to in-band rather than adjacent band coexistence. 4 POSSIBLE WAYS OF SHARING As mentioned
17、in section 2.3, radio astronomers have to exclude data when a satellite passes within the main beam of the telescope, and Motorola proposes that, during busy hours, data could also be excluded for a few additional percent of the time when the satellites are just outside the main beam of the radio te
18、lescope. Motorola further proposes that the loss of radio astronomy observation time using this technique could be halved (to about 2.5 %) if the Radio Astronomy stations could be synchronised on the satellite transmissions in order to enable observations during roughly half of the time when a satel
19、lite is not transmitting. In addition, ITU-R Rec. RA.769 levels will be met during 4 hours per 24 hours day, when the traffic level is down. Motorola stressed the fact that some observatories are observing in the frequency band 1610.6-1613.8 MHz only during a few percent of time and that a MOU had b
20、een signed between NRAO (National Radio Astronomy Observatory in US) and Motorola Satellite Corporation Inc. where radio astronomers state that they accept to evaluate the “blanking“ solution. Motorola further states that with this scheme (blanking, main beam avoidance and all other mitigation facto
21、rs), only a few additional percent of observatibn time would be lost, which is not significant, and the remaining observations could be carried out with an interfering level down to 2.5 dB below ITU-R Rec. RA.769 level at 1610.6 MHz and up to 5 dB above the ITLJ-R Rec. RA.769 level at 1613.8 MHz. Si
22、nce radio astronomers doubt these mitigation factors, the proposal to restrict observations to a 4 hour slot per night is strongly opposed by European radio astronomers. The reasons given by the radio astronomy community is the impossibility of making periodic observations of time-varying phenomena
23、(on the scale of hours and days) and the smaller time available for other programmes. The threat is most serious for single dish instruments with limited sky coverage like Arecibo, in US (not a co-signatory of the NRAO MOU) and Nancay, in France. This is a particular concern for Nancay, where almost
24、 half of the time is dedicated to the OH bands near 16 13 MHz and near 1660 MHz. The blanking proposal is also strongly opposed by European radio astronomers because it prevents observations of short period time-varying phenomena. The radio astronomers are concerned that another operator, despite th
25、e fact that, for the time being, only Iridium has declared that they will use this allocation, might also use this downlink allocation without being synchronised with Iridium satellites, which would further reduce the available time for radio astronomy observations. Motorola recognises the validity
26、of this argument for the blanking technique but states that, if the main beam avoidance technique is applied, the lost observation time due to Iridium would remain small even without the blanker. Other technical solutions have been suggested within SE28, like the improvement of satellite linearity a
27、nd traffic limitations. But Motorola stated that considerable efforts have already been made to minimise the emissions and considered that further improvement would bring unacceptable economic and technical burdens. ERC REPORT 50 Page 6 5 CONCLUSIONS SE28 examined many technical aspects of the issue
28、 of interference from MSS satellites into radio astronomy. The findings of SE28 are that: - The MSS satellites transmitting at 2.5 GHz should be able to provide sufficient filtering so that the power of the second harmonic is below the interference threshold of the radio astronomy observatories. Glo
29、balstar states that they will meet the ITU-R Rec. RA.769 level by 20 dB. - The unwanted emissions of the Iridium MSS satellite constellation, operating at 1.6 GHz (estimated from simulation up to -215 dBW/(m2.Hz) at peak time in the middle of the radio astronomy band, source Motorola) will be 26 dB
30、(21 dB) at 1613.8 MHz (respectively 1610.6 MHz) above the interference threshold (-238 dBW/(mZ.Hz) as defined by ITU-R Rec. RA.769 for spectrum line measurement) for about 20 hours per day. - These estimated levels are well below the current GLONASS levels experienced by radio astronomers in the ban
31、d. If the existing GLONASS satellites were simply moved to centre frequencies below 1604.8 MHz as indicated for the GLONASS configuration after a date between 2005 and 2008, the Iridium levels would be a few dB below the levels of GLONASS. However, the GLONASS Administration has agreed to investigat
32、e ways of reducing out-of-band emissions in the frequency band 1610.6 - 1613.8 MHz to the levels of -238 dBW/(mZ.Hz). Radio astronomers are expecting that such an improvement will be implemented on the GLONASS system, the IUCAF/GLONASS MOU stating that “the GLONASS Administration agrees to investiga
33、te the ways of reducing out-of-band emissions in the frequency band 1610.6-16 13.8 MHz to the levels indicated in Rec.769 and to communicate their proposed solution of this problem at a future meeting“ (between IUCAF and GLONASS). Radio astronomers also stress that the interference from both GLONASS
34、 and Iridium are above the ITU-R Rec. RA.769 levels and that the time cumulative effect will reduce the amount of time available for observation. Motorola stresses that fewer satellites of Iridium will be in view than for GLONASS and therefore, together with the lower level of Iridium interference,
35、that the incremental effect of Iridium over GLONASS will be very small. - Motorola has indicated several mitigating factors and proposed one mitigation technique which they believe will allow radio astronomy observations to proceed at the sensitivity assumed by ITU-R Rec. RA.769 assuming a total mit
36、igation factor of up to 23.5 dB with only a small loss in observation time. This is disputed by radio astronomers who consider that Motorolas proposals will not apply to all radio telescopes and in all circumstances and could result in a total mitigation factor of less than 8 dl3. Real measurements
37、from orbiting Iridium satellites are needed to confm or deny the conflicting claims. Such tests should be carried out with fully loaded satellite transmitters simulating maximum totminimum traffic conditions. Therefore, SE28s opinion is that no further work is possible before real measurements are m
38、ade to properly assess the impact of unwanted emissions from Iridium downlinks on the radio astronomy service in the 1610.6 to 1613.8 MHz band. Such measurements are planned at the US Greenbank Observatory. CEPT should encourage ESFICRAF and Motorola to collaborate in investigating the feasibility o
39、f such measurements in Europe. The launch of the first Iridium satellites with full simulation of maximum and minimum traffic loads will make it possible to accurately determine the various mitigation factors discussed above. When the measured data are made available to CEPT, WG SE should review the
40、 applicability of the mitigation techniques and determine the actual interference levels into radio astronomy observations and forward its conclusion to WG FM and ERC. In the light of these conclusions, the Milestones Review Committee, which was established by Decisions of ERC and ECTRA, will then be able to take a final decision in accordance with the principies laid down in clause 3.2.6, paragraph 4 in the minutes of the 20Ih ERC meeting.
