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This web document is intended for VLA observers using the 18-21 cm
(L-band) system. For standard VLA issues please
consult the VLA Observational Status
Summary. The information provided here will likely migrate to that
document eventually. But since this is a new capability, we think that it
is important for this information to have a special place on the web
for a time.
In late 1998 the Iridium constellation of 66 communication satellites were
activated full-time. Since then, despite the fact that the company operating
operating the satellites has changed several times, the signals from Iridium
have never been turned off or been interrupted. The downlink signals from
Iridium span the range 1621.35 - 1626.5 MHz and are quite strong. Since, at
any moment, there are a minimum of six satellites above the horizon, the probablility
of having the VLA receivers inundated at any given time with these signals is quite
high. The important OH transition at 1612.231 MHz lies very close to the downlink
signal, and significant sideband transmission from Iridium can sometimes be seen as low as
1619 MHz.
The Iridium downlink causes two major problems for the VLA: 1) Observing directly
in the downlink frequency range causes the data itself to be corrupted and 2) the downlink
signal beats with the noise tube calibration cycle and causes data even outside the
downlink frequency (if it is somewhat close by) to be nearly uncalibratable (see this
brief 1998 report by
Greg Taylor on calibration in the
presence of Iridium). Using the standard front-end filters of the VLA, even the most
narrow standard filter of 12.5 MHz, because of radio-frequency tuning limitations, are
not sufficient to relieve the effects of the Iridium transmission.
Beginning in early 2001, we installed three prototype narrow-band filters on three
antennas of the VLA, in order to test the possibility of mitigating the effects of
Iridium downlink signal. After some initial tests, it was decided to install similar
filters on all the antennas. This was completed in the late fall of 2002, and tests
with the entire array have been underway since. We feel that the system is now serving
the purpose of alleviating the effects of the Iridium downlink signal as best we can
with the current system. Here are the details of the filters: bandpass filters with
the center frequency at 1611.6 MHz, and the -3 dB points (the half-power points on the
filter shape) at 1610.0 and 1613.2 MHz.
The method used to invoke the filters is a bit different than the standard 50, 25
or 12.5 MHz filters. An observer has to select a special "ROT" file which the system
knows about in order to use the narrow-band filters. Jobserve and Observe both allow
the observer to type in the name of a ROT (or IF) file. This can be found on the
source page, at the bottom right. The name of the ROT system file which invokes the
filters is "1612LROT". Here is an example of an observe
file in plain text which invokes the filters in this manner.
In the signal path, the iridium filter is located immediately
following the receiver, so all signal outside the filter band is
lost. Therefore, with the filter in place, it is not possible to
observe simultaneously at 1612 MHz in one IF pair and outside the
iridium filter band in the other IF pair.
For the latest information on the use of these narrow band
filters, please contact Gustaaf van Moorsel (gvanmoor@nrao.edu).
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