NATIONAL RADIO ASTRONOMY OBSERVATORY
                          Green Bank, WV


MEMORANDUM                                           Sep. 9, 1998

TO:    R. Fisher, R. Lacasse, R. Norrod, M. McKinnon, C. Niday, T. Weadon
FROM:  R. Lacasse
SUBJ.: Minutes of Interference Protection Group 9/8/98 Meeting

In attendance: Fisher, Ford, Lacasse, McKinnon, Niday, Weadon

Minutes

     Approved.  Lacasse commented that action items recorded in the
minutes ought to be reviewed in the ensuing meeting.

****Action item: Lacasse to send out revised minutes from 8/25 meeting
ASAP.  Fisher will put them on the web in 2 days if there are no
comments.

Call Outs

     In his observations at the 140', Murphy reported seeing the
G.P.S. L3 signal at 1383.3 MHz on three separate occasions.  There
exists a CORF agreement that this tone is to used very rarely.

****Action Item: McKinnon to follow up with the Air Force on the above.

     Murphy also saw interference at around 1380 MHz.  We had previously
speculated that this could be the third harmonic of the ComSat radios
used in the GBT construction.  However Murphy was able to detect one of
the signals and hear half of a conversation which sounded more like a
cell phone.  Niday pointed out that some air to ground radios operate at
around 460 MHz.  The newer ones operate at 895 MHz.  So most probably
the signal originated from "airline phones."

GBT Receiver Room Shielding Effectiveness Test
     
     Work on the GBT receiver room, to get it ready for shielding
effectiveness testing is almost complete.  The only outstanding item is
that beryllium-copper fingers need to be added to the turret
gasket. Testing could commence next week.  Electronic equipment is
available for the test.  Barkley is coordinating with Hargreave for the
use of a cherry picker.

PC based Continuum Detector

     The PC is installed in the anechoic chamber control room.

****Action Item: Weadon to produce a "cheat sheet" for the system.

     A little progress has been made on the filter/detector system for
the continuum detector.  Specs on a filter chip, and an associated
program to select components to go around it have been obtained.

Green Bank Elementary/Middle School

     Ford wrote a letter to the architect thanking him for the
opportunity to review the drawings, and requesting that NRAO be kept in
the loop on relevant items in the design.

****Action item: McKinnon to check to see how fluorescent fixtures were
handled in the Jansky lab addition.
     
Interference Reporting System

     Modifications to the interference reporting system suggested by the
IPG have been incorporated into the software.  Balser has begun training
operators.  He also intends to train Jerry Lawrence, so that Jerry would
be in a better position to train future operators.

Filters for the L-band Receiver

     Superconducting filters are being developed for Arecibo.  These
filters would notch out the Iridium band.  Fisher has spoken to Norrod
about these.  Norrod in turn will speak to Stennes about them.

     McKinnon noted that Motorola provided a 'Radio Astronomy Special
Equipment" or RASE box for McKinnon's recent test.  This box could be
used to provide blanking signal for radio astronomy backends.  The
signal it provides is a TTL compatible square wave.  The phasing of the
square wave is set by signals the box receives from passing Iridium
satellites.  Motorola would probably give these boxes to whichever
observatories need them. The GBT spectrometer is equipped to handle such
a blanking signal.  The spectral processor could be.  Ford remembered
some negative comments about these boxes (to the effect that they would
be useless with more than one satellite overhead) made during the recent
Quiet Zone review.  McKinnon estimated that up to 8 Iridium satellites
would be above the horizon at any given time.

****Action Item: Lacasse to coordinate the addition of electronics in
the spectral processor to "OR" RFI blanking signals from the IF part of
the spectral processor with the RASE signal.

****Action Item: Ford to look into the possibility of adding a "Blanking
Signal Combiner" to the GBT suite of equipment.

     For Minter's recent observation, filters that would not pass the
Glonass band and would pass his band of interest were not available.

****Action Item: Fisher to push for the availability of appropriate
filters.

Monitor Station

     Fisher presented some strawman designs for the monitor station.  He
had consulted with Grammer and White on the phase locked oscillators and
fiber modems respectively.  He requested volunteers to work on various
parts of the system.  It was suggested that Grammer and White might be
able to contribute.  Weadon also volunteered to work on the digital
parts of the system, and possibly on the RF parts if he could get some
help.

     Fiber modems are incorporated in the design so that monitoring can
eventually be done from several locations to facilitate direction
finding.  A location in the vicinity of the OVLBI antenna, one in the
140' service tower and one on the GBT were proposed.  A reasonable goal
would be to have one monitoring station on the air next summer.  A quick
and dirty station to monitor the 1300 to 1700 MHz band is also proposed
to get something on the air quickly.  Comments on the proposed design
were requested

Green Bank Industrial Site

     The Pocahontas County Development Authority has periodically talked
about developing the site they own in Green Bank.  There was general
agreement that NRAO need to keep up with this to avoid introducing a
significant source of RFI close to the site.

Next Meeting

Sep. 22, 1998, at 0800