Green Bank, WV


Sept. 18, 2001
TO: R. Fisher, J. Ford, F. Ghigo, R. Lacasse,  C. Niday, T. Weadon
FROM: R. Lacasse
SUBJ.: Minutes of Interference Protection Group 09-18-01 Meeting

In attendance: Fisher,  Ford,  Lacasse, Niday

Minutes of Previous Meeting

Interference Reports
The first observations with the 600 MHz receiver were performed this week.  Using just a spectrum analyzer, many unwanted signals that were way above (20 to 30 db) the noise in the bandpass were detected.  Norrod tried turning off the active surface electronics and laser system and noticed no significant change.  A similar test will be performed with the feed arm servo.   Fisher will observe in the same band with the monitor station for comparison.  Ghigo will do an azimuth scan to isolate any azimuthal dependencies.  This band is allocated to television stations and it is possible that some of these signals are television.

The need to be proactive by characterizing the interfering signals in the bandpasses of all receivers was reiterated.  Fisher stated that the Spectral Processor, with its larger dynamic range, was a better tool than the GBT Spectrometer for this task.

Review of Action Items:

   1. Interference Manager position status (John)

A verbal offer has been extended and accepted.  The follow up letter is in the mail.
  2. End-of-year purchases (all)
Fisher has purchased two antenna positioners, a lightning detector, and two RS232 fiber modems.  There was no progress on buying a spectrometer with frequency coverage to 10 GHz to allow monitoring of any part of the GBT IF band from the control room.   Similarly, no progress on purchasing antennas to cover bands above 1 GHz or having the IPG spectrum analyzer calibrated.
  3. RFI considerations for new Education Center (John, Frank)
No news.  Ford stated that his conversations with the RFI consultant left him feeling that we were dealing with a competent engineer.
   4. Site-wide interference suppression policy - position  paper (John, Rick)
Comments from Jewell were reviewed by some members of the IPG.  Jewell wants to expand the scope of the IPG to include external interference as well.  Concern was expressed by the group that, even with the additional hire, there may not be enough time available to do the required job properly.  Ford will request additional manpower.
   5. RFI suppression of GBT equipment   (all)
Quite a bit of testing of the Laser Ranging System control boxes has been done.  A retrofit has been designed for these boxes.  Parts have been ordered to retrofit 20 units.  Work is presently on-going to suppress RFI from the laser-ranging instrument itself.  So far it appears that some of the RFI from this unit is due to improperly grounded shields.

After additional modifications, the ultrasonic anemometer meets our RFI limits, and it still appears to function as an anemometer!

Some progress was made on the servo system length transducers.  Plans for an additional enclosure have been submitted to the shop and additional filters have been ordered.  A plan to build an enclosure for filters in the receiver room was discussed with Egan.

  6. Use of the GBT warehouse by Operations (John)
The group reviewed a memo by Bignell detailing reasons for moving the telescope mechanics into the GBT Warehouse.  The memo makes many good points about efficiencies gained from having the mechanics close to the GBT.  It acknowledges the threat of additional interference, and makes a few suggestions on how to deal with it.  However, the general feeling of the group is that this move flies in the face of many things the IPG has tried to accomplish.  With the mechanics will come a variety of power tools, computers and other emitters of RFI.  A lot of work has gone into attempting to make equipment on the GBT compliant with ITU limits; a lot of work remains.  The additional equipment will make the job even more difficult.  The distance to the present mechanics facilities provides approximately 20 db of space loss attenuation.  Also, the memo emphasizes the advantages of having the mechanics close to the GBT but ignores the disadvantages of being remote from the central shop.  If the mechanics need additional space, perhaps the observatory could consider providing it close to the present facility.  Ford will draft  a reply to the memo and circulate it to group members for comment.

   7. Monitor station developments (Rick)

Fisher fixed up the software so that the computer can control the remote monitor station.  Software to control the spectrometer via GPIB is under development.  The PC controlling the station is uses the Linux operating system.  This will make it possible to set up a daemon in the PC through which the monitor station can be accessed by the Monitor and Control system.
   8. Measurements of PC/Workstation in a shielded box (Chuck)
The box is presently being used for the tests of the servo length transducers described in (5) above.  The need to work on shielding PCs was again emphasized.  Niday was asked to begin work in this direction by having the shop machine plates for various connectors associated with PC hardware.
 Next Meeting:  October 16, 2001