For a long time, mm-radio astronomy has been relatively safe from man-made interference which has been continuously worsening for cm-radio astronomy in the last decades. Due to the high demand for spectral bands for new activties, astronomers are more and more obliged to share spectrum with other users. Now, a major perturbing - and potentially destroying - emitter is about to appear on the mm-radio sky.
The Cloudsat satellite is planned to be part of a
five-satellite-constellation (Aqua, Cloudsat, Calipso, Parasol, Aura)
which will monitor the Earth's atmosphere to study the environment,
meteorology and climate. From this constellation, only Cloudsat has a
significant impact on Radio Astronomy. Its cloud profiling radar
operates at 94.05 GHz and will point directly downwards to generate a
central footprint of approx. 3 km diameter. The instrument will orbit
at an altitude of 705 km, and the pulsed signal will be about 1.8
kilowatts at maximum (3.3 
s pulses, max. 4240 pulses per second).
This power is by a large factor sufficient to burn out the SIS junction of a radio telescope pointed at Zenith during satellite transit (no matter to which frequency the receiver is tuned), and to detect the satellite through the telescope sidelobes as soon as it clears the horizon, no matter where the telescope will be pointed. Continuum observations with large spectral bands which contain this frequency will have to be checked for parasites and may suffer from non-linear saturation effects.
Details on the efforts which were made by the radio astronomical community to prevent this threat can be found on the NRAO web pages http://www.iucaf.org/CloudSat/. Details on the technical impact, and how it threatens large instruments which are in development and what countermeasures can be envisaged, can be found in the ALMA Memo 504 (http://www.alma.nrao.edu/memos/html-memos/alma504/memo504.pdf).
As of the edition of this Newsletter, the final launch date was not yet fixed; the Cloudsat Homepage speaks of late summer or autumn 2005.
An immediate reaction to the presence of Cloudsat will be of course to avoid parking antennas in Zenith position, or when that cannot be avoided (e.g. during PdBI antenna configuration changes) to close the central hub. The nominal lifetime for the satellite is 2-3 years, and its orbit will lead to six or seven passes per day over most observatories. Typically the satellite will be above the horizon for 10-15 minutes, so that a maximum of time lost will be about 2 percent. This pattern of transits repeats every 16 days. It is difficult to predict how serious the effects will be as they depend on the telescope sidelobes far from the telescope beam, and the rejection of the filters of the receivers and their saturation properties. Observers are encouraged to report interference. With experience it may be possible to draw up observing guidelines. Considering that more satellites operating at mm wavelengths (and more powerful ones) are on the drawing boards, however, it is a precursor of a future where mm-wavelengths will be reached by a rising tide of interference.
Other than for optical astronomy, public awareness of this kind of ``invisible'' light pollution is low. This means that there is little public pressure to limit radio interference, and sometimes a lot of economic interest to create services which cause interference as a byproduct.
The Cloudsat project shows that even within the scientific community, it is possible to create negative - and potentially destructive - side effects for other branches of science. It also shows that sometimes assurances that satellites could be turned off during transit over known telescope sites do not make it into the final design phase. For future instruments, it would be preferable to limit the mutual negative impact of scientific projects. To this end NASA will provide prediction tools.