The Committee on Radio Astronomy Frequencies (CRAF) is a committee of
the European Science Foundation (ESF).
A cloud-radar is one of the instruments investigated within the ESA Earth Observation Prepatory Program (EOPP). It is a potential candidate for a future Earth Observation mission. Such an active instrument is of high interest as it can provide data about the vertical profile within the cloud structure. It will operate in the band 94 - 94.1 GHz, with a transmit power (pulsed) near to 1 kW, using a highly directive sensor (an antenna with about 2.5 m diameter reflector).
The frequency band near 95 GHz is of interest to other (passive) users: e.g. the radio astronomy community has strong interest in observations in the mm-wave domain. The spectral line observations are done to study the physical conditions within the source of the related line emission. This work is done at a number of mm-wave observatories spread all over the world. mm-Wave radio telescopes make use of a ultra-sensitive wide band receivers. At present, mm-wave astronomical observations are done at frequencies up to about 1 THz. However, the unique information each spectral line provides about the physics and also the chemistry of the Universe implies that observations in one frequency domain cannot be a substitute for those in a different frequency domain. At present more than 100 different molecules have been detected and are subject to daily investigation. The spectral line observations reveal not only the physical conditions within the radio source but also about the kinematics in the celestial object within which the source resides and about even the distance scale of the Universe as a whole (as derived from maser emission).
The spectral power flux density, spfd, generated on the Earth surface by an orbiting cloud-radar can reach such a level, which would lead to interference with significant consequences for the radio astronomical observations: If the radio telescope pointing direction and the direction of pointing of the radar would be co-aligned (which could occur with a certain low probability) the interference could even be destructive for the ultra sensitive receiver front-end, if no precautions are taken, although frequencies of operations are slightly different from the radar frequency. An interference situation could occur also when a sidelobe-sidelobe coupling scenario would be present, in certain situations it could lead to interferences, causing a loss of observations and degradation of data quality.
This situation was well recognized in ESA and in order to study such interference scenario and to investigate possible means to improve upon such situation, a study was initiated by EOPP, for which Oerlicon Contraves SpA is responsible. The activity is being carried out in close cooperation with CRAF.
Within the scope of this activity, it has been possible to explore the potential and limitations of certain band stop filter configurations, which could provide reduction of the signal level into the ultra sensitive radio telescope receiver. This approach is one of the possible approaches to prevent interference and more work is needed to provide operational scenarios for users of the electromagnetic spectrum without interferences.
Initial results for such filter configuration are promising and an initial proof of capabilities demonstrated by a realised bread-board component, is available. In fact, this activity has contributed to significant progress in the accurate design and realisation of specialised millimetre wave components, not only for this application: For the first time it was possible to design, accurately manufacture and test a millimetre wave (near 95 GHz) stop band filter with indicatively promising results. Design tools were derived from antenna design tools already developed for highly accurate corrugated horn antennas, with a capability to analyse a number of different configurations for the separate corrugations and with a capability to analyse a high number of such cascaded corrugations (as in a corrugated horn). Based on mode matching approaches, these tools are well advanced, making use of a variety of modesets and in fact numerical approaches for situations as encountered in non-rotational structures (like available in polarisers) were already handled as well. This approach has been necessary, as the usual approaches in filter design do not normally foresee to handle configurations as employed here. In fact, the use of coaxial mode sets as needed in ring-loaded corrugations, still has to appear in several filter design tools. Band stop filters are also not frequently encountered and their application at these frequencies makes the situation not more easy in realisation and so as a result the design process deserves to some extent some respect. A short trade-off study led to a proposed design which was investigated in more detail and which has been realised.
A report of the final meeting of Phase-I of ESA/Oerlikon-Contraves/CRAF project can be found in Newsletter 1997-2.
At WRC97, after much discussion, a primary allocation was made to the Earth exploration-satellite (active) service for spaceborne cloud radars in the band 94-94.1 GHz, where the radio astronomy service has a secondary allocation. Radio astronomy also has primary allocations in the adjacent bands 92-94 GHz and 94.1-95 GHz.
Since cloud radar transmissions in the band 94-94.1 GHz that are directed into the main beam of a radio telescope could damage its receivers, space agencies operating these radars and the radio observatories concerned should mutually plan their operations so as to avoid such occurrences to the maximum extent possible, according to Footnote 5.562A of the ITU Radio Regulations.
In July 2004, radio astronomers were informed by NASA of the planned launch in April 2005 of the 22 months’ duration CloudSat cloud-mapping experiment, which will carry a pulsed, 1.8 kW narrow-band, 94.05 GHz nadir-pointing (downward-looking) radar. Power levels of the CloudSat radar are such that they could burn out an SIS-type junction deployed on a typical radio telescope if it is observing at zenith in the 94-94.1 GHz band during an overflight. Moreover, an SIS receiver operating in this band will probably be saturated during a (near) overflight, wherever the telescope is pointed.
Acting on behalf of the worldwide radio astronomy community, IUCAF, in consultation with CRAF, brought its concerns to the attention of the Space Frequency Coordination Group (SFCG), which subsequently adopted SFCG Resolution 24-3 on this issue at its meeting in September 2004.
Since it is not possible to turn off the CloudSat radar transmissions as the satellite passes over a radio astronomy site, observatories will need to know when exactly to avoid observing. Further practical information can be found on the IUCAF website on Cloudsat.
The European sites concerned are Bordeaux (France), Effelsburg (Germany), Metsähovi (Finland), Onsala (Sweden), Pico Valeta (Spain), Plateau de Bure (France), Sardinia (Italy) and Yebes (Spain.)