1612 MHz OH and Continuum near 1622 MHz at the VLA

                               or

              Calibration in the Presence of Iridium
                        August 12, 1998

   The full-time activation of the Iridium constellation of
communication satellites in September, 1998 will have a significant
impact on VLA observations of the 1612 MHz transition of OH.
Observers are advised to make use of the 25 MHz front end filter, and
make sure that the Iridium downlink from 1621-1628 MHz does not pass
through the correlator.  Even with these precautions there will be
interference when a satellite flies through the near
sidelobes.  Moreover, the 90 millisec uplink/downlink cycle of the Iridium
constellation will continuously beat against the 102 millisec
calibration cycle of the VLA.  When any Iridium satellite is above the
horizon and transmitting (100% of the time after September 1998) this
beating is sufficiently strong to corrupt the on-line system
temperature corrections applied to the data and to cause all data to
be flagged due to wild system temperature variations.  For this
reason, the normal calibration cycle must be turned off during all
1612 MHz observations (or any other observation that include the
frequencies between 1620-1630 MHz in the 50 or 25 MHz front-end used
for measuring the system temperature).  This will result in some
degradation in the accuracy of the absolute flux density calibration
as shown in this plot of calibration error with elevation.



   There are a couple ways to recover accurate flux density
calibration.  One way is to schedule the primary flux calibrator
(e.g. 3C286) at an elevation comparable to that of the secondary flux
calibrator(s) used.  Another option is to spend a few minutes at an
adjacent frequency (say 1665 MHz) that is clear of RFI on both the
primary and secondary flux calibrators in order to obtain an accurate
estimate of the flux density of the secondary calibrator(s).

   To turn off the calibration cycle a request should be made to the
array operator to set columns 64 and 65 blank in a special Front End
Parameters file (better known as the SYSLROT file).  An example of
such an instruction is shown below, along with a short segment of an
OH observe file:

//* *** Special Instructions
//*
//* *** uses GT0LROT:  with iridium filter in (59-60=T) and                   
//* *** cals off (64-65 both blank).                                          
//* *** 
2148+611      22 07 35 21 48 16.0454 +61 07 05.838C    LL   C   4444   2.23   
//DS 2AD H      10   6         6         6         6                           
//LO     -3.2   -3.2      3790      3790              1111            GT0LROT  
//FISVTL        000000.000          000000.000    1612.231000    1612.231000  

This example also shows how to ask for the iridium filter which has
been installed in 3 antennas.  This is a room temperature band pass
filter cutting off all signals above 1616 MHz.  Those planning a 1612
MHz observation or continuum observation near 1620 MHz are encouraged
to contact the under-signed (gtaylor@nrao.edu).

   Even with the above precautions, large amounts of data may be
corrupted at some level.  Tests are in progress to determine the
typical amount of affected data and the optimal observing strategy
to minimise the impact of Iridium.  

                                              - Greg Taylor

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