NATIONAL RADIO ASTRONOMY OBSERVATORY Green Bank, WV MEMORANDUM January 13, 1998 TO: R. Fisher, R. Lacasse, R. Norrod, M. McKinnon, C. Niday, T. Weadon FROM: R. Lacasse SUBJ: Minutes of Interference Protection Group 1/13/98 Meeting In attendance: Fisher, Lacasse, McKinnon, Niday, Weadon RFI Call-outs Again, there were no call outs, but a few instances of interference were noted. The birdie on the C-band receiver at 4860MHz was seen again on 1/8/98 during a VLBI set up. Grammer said that he is scheduled to take the receiver off the telescope soon, and will investigate whether the birdie is somehow internally generated. Observations by Lockman at 1612 MHz were affected by two types of interference; his memo describing these is appended. **** Action item for McKinnon to come up with an interference reporting system for the 140'. Approval of Last Meeting's Minutes Minutes approved. Fisher does not think he has copies of all the minutes. **** Action item for Lacasse to supply Fisher with text versions of minutes that are not on the web. Interference from Laser System The 1500MHz reference oscillator for the GBT Laser Ranging System has been tested a few times by the IPG. Improvements have been made, and the last test showed that the interference at 1500MHz had been reduced to about 50db above the CCIR limit. Parker recommended turning the system off if/when it interferes on the GBT. The IPG feels that this is not an acceptable option. It was felt that further progress will probably require a group effort: Chuck and Rich to work with Radcliff, Fisher to help with the Spectral Processor if required. PC Data Acquisition System Weadon has received both the hardware and software. So far, it has not been possible to properly configure the system. Weadon will continue to work with the factory. Schiebel has offered to help with the programming once the unit is properly configured. Fisher stated that, from his perspective, the simplest system would continuously sample at 100 to 1000 Hz with some type of marker to show switching times (sig/ref etc.). A record with 1000 switching cycles would be adequate. The TestPoint software has the mathematical capability of doing FFT's and folding data. The latter will work best if the switching cycle period can be made an integer number of A/D samples. Monitor Station Amplifiers have been received. Three 380' runs of RG213 or equivalent coax, plus a control cable are still needed to get the system running. The market cost is expected to be 1 to 2K. Weadon suggested looking at surplus sources for cable. Additional Items to Be Tested Improvements to the 1500MHz Laser Ranging Reference Oscillator and the Feed Defroster are still being pursued. Weadon recalled that improvements to other items that had been measured and found wanting had been deferred, in October 1997, pending the completion of items critical to the operation of the GBT. Further discussion led to the agreement that the list should be revisited. The priority of fixing potential "show-stoppers" and production item may need to be adjusted. **** Action item for Weadon and McKinnon to review list of tested equipment and make recommendations for further testing. Fisher pointed out that the shielding effectiveness of the GBT receiver room will remain unknown, probably for a long time. Equipment that will go in this room needs to be designed appropriately. Access to Shielded Rooms in Jansky Addition The addition to the Jansky Lab contains several shielded doors. To prolong the life of the doors operations people have decided to leave certain doors open, without compromising the shielding of certain connected rooms. Weadon recommended that the IPG test this approach at some convenient time. Status of New Equipment The new spectrum analyzer should be shipped very soon. Hard- line coax cables were installed in the trough in the anechoic chamber. Attenuators ordered in late 1997 have been received. Also the biconical antenna, with continuous coverage from 20 to 300 MHz has been received. Spectral Processor GUI Work on a GUI for operating the Spectral Processor from the Anechoic Chamber is complete. **** Action item for Fisher to check the 140' schedule and schedule a test/demo. Next Meeting January 27, at 0800. ******************************************************************* Appended Memo by Lockman describing Interference at 1612MHz To: NRAO Green Bank Interference Protection Group From: F.J. Lockman Subject: Report on OH/IR Star Monitoring at 1612 MHz Date: Sept 28, 1997 Spectroscopic observations of OH emission in the 1612 MHz band were made toward 22 known OH/IR stars On September 13-14 between UT 19.9 and UT 7.6 using the NRAO 140 Foot Telescope at Green Bank, WV. The observations were made by integrating for 6 min intervals, first while the telescope was pointed at a reference position, then again while the telescope was pointed toward the star. Telescope motion and equipment setup overhead leads to about 1 min dead time between each observation. Two spectra were taken simultaneously in orthogonal circular polarization.The individual spectra were centered at a frequency that ranged between 1611.6 and 1612.5 MHz due to correction for the velocity of an individual star, and Doppler correction for the earth's motions. Each spectrum covered 1.25 MHz in 512 channels. In total about 9 hours of data were obtained. The stars were distributed widely across the sky -- the position of the telescope varied in azimuth from 155 to 322 degrees and in elevation from 13 to 82 degrees during the observations. Two types of RFI were detected. The first, attributable to GLONASS, was seen in two spectra. The data were completely ruined. The total time that this interference was present was in the range 2-13 minutes, where the uncertainty is due to the 6 min quantization of the observations. This represents between 0.4% and 2.5% of the entire observing time lost to GLONASS. A second type of RFI showed up halfway through the observations. It appeared to be less than 2.5 kHz wide and was isolated at two sky frequencies: 1612.17 and 1612.73 MHz. It was present in about half of the spectra, though almost always at a level that did not pose a threat to most of the data. This emission was often highly circularly polarized. Usually only one of the two frequencies was present, but on several occasions both were seen.