After several unsuccessful attempts the Earth exploration (active) satellite CloudSat is expected to be launched on 28 April 2006. The CloudSat operates in the frequency band 94 - 94.1 GHz that has been primary allocated for the Earth exploration (active) satellites according to the ITU footnote 5.562, sharing a secondary allocation for the radio astronomy service. The contiguous bands 92 - 94 GHz and 94.1 - 95 GHz continue primary allocated for radio astronomy service, although there is a potential risk for harmful interferences.
The CloudSat has a nearly circular orbit at 705 km altitude with a
99 min. period that repeats every 233 orbital revolutions. The orbital
trajectories describe a reticular grid around the Earth that repeats
every 16 days. The transmitted signal has the form of short pulses of
1.8 kW peak, with approx. 3.3 s duration and 250
s repetition rate.
The antenna gain is 64 dBi giving a peak surface power flux density
over all emitted frequencies of -31 dB(Wm
). The emission is to the
nadir with a maximum deviation of
.
The first effect to consider with respect to the CloudSat emission is
the possible damage of the receivers due to the direct impact when the
antenna points to the zenith and the satellite crosses just
above. Considering the effective collecting area of the 30m telescope
of 700 m and the flux density of -31 dB(Wm
) the power concentrated
into the mixer reaches 556 mW. This power is in the order of millions
times more than the standard required LO power for normal operation
and enough to permanently damage the junction.
We have protected the receivers against this situation by installing
an automatic closing mechanism on the vertex window for antenna
elevations higher than . In order to minimize a possible
failure on the system, two redundant processes are in charge. The
first process is implemented at the level of hardware by means of the
PLC (programmable logic controller) of the servo system. When the
antenna prelimit-up in elevation is reached the electrical power that
keeps the vertex open is switched off, and the vertex is then
closed. The second process, implemented by software, is permanently
running in a VME machine, monitoring the antenna parameters: antenna
elevation bigger than
, elevation prelimit-up and working
limit-up, and if any of these three conditions is reached the vertex
is commanded to close. Any failure of the previous operation is
notified to the operator by the acoustic alarm in the control desk
alarm system.
Making pointing scans on Venus at 94.05 GHz we have measured the total power collected by the receiver with the vertex open and closed. We conclude that the attenuation introduced by the vertex closed is, at least, of 30 dB, enough to protect the receivers.
According to the transmit signal characteristics of the CloudSat
(http://www.iucaf.org/CloudSat/CloudSat-TechDetailsV3.pdf) the
satellite antenna gain at 1from the peak emission drops by more than
40 dB and the expected power reaching the receiver seems not
dangerous. With the protection implemented, the vertex is closed within
from the zenith, outside that cone the vertex can remain open since
the level of power flux density from the CloudSat, even if the 30m is
pointing to it, is not harmful for the receivers.
In the foreseen CloudSat ephemeris data
(http://www.iucaf.org/CloudSat/CloudSat-Approaches-WebPage.htm) the
closest approach estimate to the zenith of the 30m every 16 days cycle
is very favourable, with no orbits or tracks in a 30 km radius from
the zenith (). The situation is less favourable at Plateau de Bure
site with two orbits at less than 15 km in a similar time period. The
final CloudSat tracks will be similar, but not identical.
The second effect to consider with the CloudSat is the harmful
interference in the adjacent bands 92 - 94 GHz and 94.1 -
95 GHz. According to the ITU Recommendation 769 and CCIR 224-7 the
harmful interference level for Radioastronomy Continuum Observations
at 89 GHz with 2000 seconds of integration time is a power flux
density of -125 dB(Wm).
The signal transmitted by the CloudSat has a minimum attenuation
outside the window 94 - 94.1 GHz of 50 dB. On the other side, far
side lobes more than 10away from the peak have a minimum
attenuation respect to the peak of 75 dB. Then, the maximum power flux
density reaching the observatory when observing outside the band 94
- 94.1 GHz and farther than 10from the satellite will be
dB(Wm
), well below the threshold recommended
by the ITU. In addition, if this maximum value is averaged over 2000
second (or 250
s) the averaged power flux density is even 18.4 dB
smaller or
dB(Wm
).
Nevertheless, if the observation is done in the window GHz or
with the antenna pointing closer than 10from the satellite, the
harmful threshold of -125 dB(Wm
) at the observatory could be
exceeded. In the first case with
dB(Wm
) and in
the second case with
dB(Wm
).
A good criteria to guarantee that the running observation has not harmful interference due to the CloudSat would be monitoring (and notifying in the control room) the short periods when the CloudSat is above the horizon, informing of the instantaneous azimuth and elevation of the satellite. If the observation is outside the band 94 - 94.1 GHz and the antenna is pointing farther than 10away from the satellite no interference must occur. On the other hand if this is not the case the situation must be analysed carefully or the observation be even rejected.