Committee on Radio Astronomy Frequencies
The Committee on Radio Astronomy Frequencies (CRAF) is an Expert Committee of
the European Science Foundation (ESF).
The WARC 1992 allocated the band 1610.6-1613.8 MHz to the Radio Astronomy Service on a primary basis. It also allocated the band 1610-1626.5 MHz to the Mobile-Satellite Service, MSS (Earth-to-space) on a primary basis. MSS (space-to-Earth) got a secondary status in the band 1613.8-1626.5 MHz. Furthermore, footnote 5.372 was added to the band 1610-1626.5 MHz.
The band 1610-1626.5 MHz is used by several MSS operators for the Earth-to-space
transmission path. One operator, Iridium LLC with its Iridium Satellite
System, wanted to use the band 1621.35-1626.5 MHz for space-to-Earth transmissions
(conform the USA license and allocation - Iridium LLC does not state that it will
not extend its operations to lower frequencies).
The Iridium Satellite System, which consisted of 66 satellites in Low Earth Orbits
flying at an altitude of 780 km, was designed and built by Motorola INC.
Motorola Inc. showed, before the launch of the IRIDIUM system, that the space-to-Earth
transmissions in the band 1621.35-1626.5 MHz will cause harmful interference at peak time,
in the middle of the radio astronomy band, with levels of 26 dB (21 dB) at 1613.8 MHz
(respectively 1610.6 MHz) above the interference threshold (-238 dBW/(m².Hz) as defined
by Rec. ITU-R RA.769 for spectrum line measurement) for about 20 hours per day.
Furthermore, Motorola Inc. states that, during periods of low traffic, the Earth-to-space
transmissions could meet the Recommendation ITU-R RA.769 levels for harmful interference.
In practice, this interference level is for about 4 hours per day (night hours), which is
catastrophic for many radio astronomical projects, especially for monitoring celestial
objects and for projects for which daytime observations are mandatory on physical grounds.
Already soon after the first specifications of the Iridium system were known, the radio astronomy community (represented by the Inter-Union Commission on the Allocation of Frequencies, IUCAF) discussed with Motorola Inc/Iridium LLC the protection of radio astronomy observations for possible harmful interference from MSS stations. By a letter of October 9th, 1991, to the chairman of IUCAF, Motorola Inc. stated "Motorola's goal is to share frequencies in a manner that will not interfere with radio astronomy or other MSS/RDSS services."
In Europe, CRAF tried to have technical discussions with Iridium LLC on the protection of radio astronomy for unwanted emissions from the space-to-Earth transmissions of the Iridium Satellite System. However, before 1998, Iridium LLC was not inclined to have such discussions.
In the course of time it became clear that Motorola did not implement any measures in the Iridium system to protect the radio astronomy [the system design shows that no measures have been taken to protect any other radiocommunication service, as well]. Motorola Inc. did propose some possible operational solutions. However, these would affect radio astronomy observations only and have no impact on Iridium system's operations. One of the proposed options is - for example - the installation of a beacon transmitter at a radio observatory, to enable the operator to inhibit transmissions from a mobile earth station when it "sees" the beacon. Tests and calculations have shown that if the beacon is stronger than what a radio telescope sees of 1 personal computer at about 5 km distance, harm is done. CRAF, therefore, considered this option as not realistic.
In USA, the radio astronomers could not resist the pressure and the US National Radio Astronomy Observatory, NRAO, operating the VLBA network and observatories in Green Bank and Socorro, agreed to restrict radio astronomy operations to low traffic hours: observations in the band 1610.6-1613.8 MHz are thus restricted to roughly 4 hours per day during low traffic of the Iridium system. Possibly, the interest of NRAO in this frequency band is rather limited.
Other observatories do much more work in this frequency band, in particular, Arecibo radio observatory from Puerto Rico. At Arecibo, the National Astronomy and Ionospheric Centre, NAIC, accepted in 1998 a (similar) agreement stating that about 8 hours per day (but on notification, only!) the Iridium interference will be below the level for detrimental interference as given in ITU-R Recommendation RA.769-1.
The situation in the USA is that, by not having implemented anything to protect radio astronomy against harmful interference, Motorola INC. and Iridium LLC force the radio astronomers to significantly reduce their activity in this frequency range. It seems that Motorola Inc. and Iridium LLC do not consider the ITU-RR footnote 5.372 in the way it was intended. This implies that, in USA, the Iridium system causes very significant degradation of radio astronomical research at 1.6 GHz.
In India, radio astronomers are protected for up to 6 hours of Iridium 'quiet time'.
It should also be noted that in Australia, there exists no agreement between radio astronomers and Iridium LLC.
In Canada, there is agreement on the condition that from 1 January 2006 the levels for detrimental interference, as given in ITU-R RA769, must be respected by the Iridium system.
The European situation
In Europe, at least a dozen of radio astronomy observatories which perform scientific reasearch
in 1.6 GHz band are affected.
Beginning with 1995, CEPT SE28 project team worked on the problem of sharing of MSS
Earth-to-space and space-to-Earth transmissions with other services.
CRAF participated actively in the SE28 work. Much time was needed to address the sharing
problem between MSS and the Radio Astronomy Service. SE28 developed a coordination methodology
for the Earth-to-space case. The space-to-Earth case (secondary allocation in the band
1613.8-1626.5 MHz) developed into firm positions of Motorola Inc and CRAF (representing European
radio astronomers). In no way did Motorola indicate or explain which measures it had taken to
protect the radio astronomy against harmful interference from downlink transmissions (ITU-R
RR-footnote 5.372).
The final report produced by SE28 on this matter reflects that no technical solution to
this issue is possible.
Simultaneously with the work of CRAF in SE28 framework, IUCAF had discussions with Motorola on this matter. A meeting between IUCAF and Motorola scheduled for mid-August 1996 at Motorola HQ in Chandler-Arizona was cancelled (by Motorola), since a technical discussion on measures to protect radio astronomy was not accepted. Motorola's aim was to agree on operational aspects only, implying that radio astronomy observatories have to adapt to Motorola schedules.
The concern of CRAF is that situation when the licensing in Europe does not sufficiently consider the vulnerability of the Radio Astronomy Service in relation with satellites communication, then the radio astronomy observations in the 1.6 GHz domain are unrepairably affected. This would be a great loss for further development of the science of radio astronomy - in particular and astronomy - in general.
CRAF brought this concern to the attention of the CEPT.
Under the auspices and guidance of the
ERC Milestone Review Committee, MRC, a Framework Agreement was
reached between the European Science Foundation (on behalf of CRAF) and Iridium LLC. This
agreement being a formalization of MRC Recommendation
No. 4 in support of the licensing process for the Iridium system in Europe.
The agreement includes also that CRAF and Iridium LLC will work towards a second agreement
on regulatory and technical issues concerning the period 1 March 1999 - 1 January 2006.
It is to be noted that the European agreements between ESF/CRAF and Iridium LLC is, in many respects, better than similar agreements reached elsewhere in the world.
The Iridium frequencies are:
| uplink (system frequencies) | 1616.0 - 1626.5 MHz |
| uplink (licensed frequencies) | 1621.35-1626.5 MHz |
| downlink (system frequencies) | 1616.0 - 1626.5 MHz |
| downlink (licensed frequencies) | 1621.35-1626.5 MHz |
| gateway links | 19.4 - 19.6 GHz (uplink) |
| 29.1 - 29.3 GHz (downlink) | |
| Inter-Satellite links | 23.18 - 23.38 GHz |
Iridium LLC started operations in November 1998 but filed for bankruptcy under Chapter 11
in August 1999. On March 17, 2000, on 11.59 pm EST the system operations and maintenance had to
stop and its satellites had to be deorbited. CRAF modestly celebrated this historic event that
an active source of spectrum pollution was removed.
However, as observations at the radio observatories at Nançay and Westerbork showed,
the operations continued after March 17th, 2000. From a publication in the Dutch magazine
Intermediair (15 June 2000) one can deduce that Iridium may be given a restart at the
time a buyer for the system has been found.
On November 16, 2000, the newly organized company Iridium Satellite LLC announced to purchase all of the existing assets of Iridium LLC, including the satellite constellation and the terrestrial network. Iridium Satellite LLC will continue to provide commercial satellite communications to the US Government and plans to re-launch affordable satellite communications services to those industry segments that have a particular need for satellite communications.
By the end of March 2001, the new operator Iridium Satellite LLC relaunched the system for commercial service. The constellation is now controlled by Boeing, replacing operator Motorola under the previous regime.
In February 2003 the FCC agreed to an expansion of the frequencies used by the Iridium satellite system from 1621.35-1626.5 MHz downward to 1618.25 MHz. In June 2004, the FCC issued a Further Notice of Proposed Rulemaking "to explore whether CDMA and TDMA MSS operators could share an additional 2.25 MHz of spectrum at 1616.0-1618.25 MHz". This significant change constitutes a change in the conditions for the Agreements completed between Iridium and ESF/CRAF and the related CEPT MRC Recommendations regulating the protection of radio astronomy in Europe from interference from the Iridium system. This frequency extension was motivated by the frequency need of the Iridium system for military operations during the war in Iraq.
Within Europe, further discussions with Iridium Satellite LLC took place towards the guarantee that from January 1st 2006 the interference from the Iridium satellite system into the band 1610.6-1613.8 MHz will be below the protection criteria for radio astronomy as given in Recommendation ITU-R RA.769 . These discussions had not any practical results.
In the CEPT countries, the operations of the Iridium satellite system are subject to the obligations in the above mentioned Agreements and relevant CEPT MRC Recommendations which are still in force. These documents include the need for the Iridium satellite system to comply with the spfd -238 dB(Wm-2Hz-1) limit on unwanted emissions into the 1610.6-1613.8 MHz radio astronomy band from January 1st, 2006, in the absence of an alternative solution.
A report from January 2004 on monitoring measurements of Iridium satellites made at CEPT Leeheim Satellite Monitoring Station convincingly shows the occurrence of unwanted emissions in the 1610.6-1613.8 MHz RAS band. The strongest unwanted emission seen corresponds to a level 25 dB above the threshold level for detrimental interference in astronomical spectral line observations, as given in Recommendation ITU-R RA.769.
At the 12th ECC meeting held in Portugal in October 2005, it was noted that Iridium Satellite LLC
has an Agreement with Iridium, which has not been executed as one might expect, whilst the justification
behind the recently traced Iridium transmissions has also not been delivered, including no specific
values for daytime operations. Consequential to this, no alternative solutions or improvement of
the Iridium satellites as must be done (compliant with the Interim Agreement before 1 January 2006)
have been found. This situation implies that according to the Framework Agreement
the RA.769-1 levels will apply from 1 January 2006.
At the same ECC meeting, it became apparent that some CEPT Administrations
already granted, based on a request from Iridium, authorisation for the use of
an extension in 1.6 GHz band on a temporary basis
for the relief operations, recovery work and humanitarian reasons in the Gulf States, following the hurricane
disasters in 2005 in US.
The ECC Chairman noted the status on this issue and concluded that an ECC body has to take over the
responsibility for further work on the issue. WG SE PT40 was requested to identify a method of
measurement as assumed in the Recommendation ITU-R RA.769, which is specifying limits, and to
investigate how this can be applied by Leeheim Monitoring Station. The SE Chairman agreed to take
the issue on board.
Monitoring observations at the Nançay Observatory in France confirmed that Iridium is using frequencies
below 1621.35 MHz, as is seen in the following figure (courtesy Nançay Observatory).
These observations showed also that, during the summer of 2005, the Iridium Space-to-Earth transmission
were observed at a frequency as low as 1616.0 MHz implying that apparently Iridium uses for its operations
the range 1616.0 to 1626.5 MHz.
In the previous Nançay measurements, the lower recorded edge was 1620.3 MHz, as in the Leeheim measurements (see above).
This observation raised much concern within CRAF.
In the beginning of 2006 none visibile improvement of the quality of the 1610.6-1613.8 MHz RAS band has been noted (as expected, according to Agreement).
In November 2006, Leeheim performed measurements of 5 contiguous 20 kHz channels,
around 2 frequencies (1612.6 MHz and 1613.19 MHz) within RAS band (160 channels), during the whole (visible)
passage of 4 Iridium satellites (Iridium 30, Iridium 33, Iridium 77, Iridium 86) emitting in the band
1618.25 – 1626.5 MHz (core + extension bands), each satellite at one different
time of the day. By simultaneous use of another antenna and measurement device, in order to provide an indication
of the in-band Iridium satellite emissions, each out-of band measurement was accompanied by a
measurement taken at the same time, of the Iridium satellite emissions in its band 1618.25-1626.5 MHz.
The the description of the measurements, as well as the graphical results of the measurements, were included
in a report which is available in the ERO website, in the Meeting Documents downloadable area, SE Working Group,
CEPT satellite measurement reports, as “Iridium measurements (November 2006)”.
Last modified: March 16, 2007