In attendance: Acree, Beaudet, Fisher, Ford, Ghigo, Lacasse, McCullough, Niday, Sizemore
Minutes of Previous Meeting
The group was asked to think about how to make various IPG resources (monitor station, etc.) easily accessible on the LAN.Status of actions on local sources of interference
There will soon be competition for the anechoic chamber. The antenna group will be building a high frequency antenna range in the chamber. It is important to coordinate with this group so that the needs of both groups are best met. Carla, Chuck, Randy and Jeff will meet later to discuss how best to interface with the antenna group.
Both the antenna group and the IPG have a need for a turntable in the anechoic chamber. One of the first tasks for the IPG's liaison with the antenna group will be to try to work out common specifications.
The agenda listed three items on which RFI suppression work is either on-going or imminent: Feed Arm Servo, Lasers, and Main GBT Drives. During the meeting a few additional items were added to the list. The first of these is a lightning detector. This is a device which the operations group wants to install by April 1, 2002. The second is the 43 MHz emitted by the GBT HVAC system; it interferes with the site radio system. The third item is the DGS data acquisition system which the laser group is incorporating into the laser measurement system. Work on this item will be done by the laser group. The final item is the GBT motion warning lights. Replacement lights which are RFI quiet have been received by telescope operations. One of these will be examined by McCullough to make sure they have not been "improved" in such a way that they will now cause interference.Harmful Levels Document
There was some discussion on how to get engineers to take advantage of the RFI expertise in the IPG group both for fixing old designs and for designing new equipment. Niday cautioned against reverting to the attitude that was prevalent in the early days of the IPG. Basically the attitude was that it was the IPG's not the design engineer's responsibility to fix anything that emitted RFI. Lacasse mentioned that Ford, due to his management responsibilities was in a good position to steer engineers towards that IPG for help when required. Beaudet mentioned that RFI compliance needs to be addressed as part of design reviews.
Fisher will distribute a current version of this document to all members of the group.Short-term strategy for response to interference reports
Acree will discuss testing limits with various members of the IPG off-line.
When the 140-ft. and 300-ft. telescopes were in operation, response to RFI was primarily reactive. The exception to this was that for some observations below 1 GHz, efforts were made before the observations to quell power line noise. The hope is that we can now be more proactive. However, the reality is that a proactive approach won't catch all the interference and that some call outs will be necessary. The response to these call outs must be quick to keep the observer from losing much time.
A discussion of possible items to be included in the strategy for dealing with RFI ensued. Following are some of the items mentioned:Next Meeting: 2 - 19 - 02, 0800There needs to be a quick way of distinguishing if the RFI is caused by the receiving system or is external.Acree and Ford will draft a call-out procedure. Fisher will draft a requirements document for an interference database. These will be discussed in future meetings.
There should be some type of calling tree. The tree should include just a few people who are really on top of the RFI situation.
As receivers are commissioned, scans of the bands for RFI should be made, and the interference identified and catalogued.
The monitor stations should be used to characterize bands before observing sessions start.
The responsibilities of operators with regard to RFI should be better defined. Perhaps, with a little training, less observing time would be lost.
A comprehensive, easy to use database should be compiled and made available to the operators and IPG group. With possible restrictions, it should be available to the observers as well. The database should include both known transmitters and local sources of interference. This database needs to be well maintained to be useful. Government catalogs are available for ground based transmitters, but no such catalog is available for satellites. FCC compliance tests of satellites may be useful in characterizing emissions from satellites. It may be necessary to observe them to get an accurate operational picture.
As an example of a quick way of compiling a list of interfering sources in a band, Ghigo presented some work recently done by Anish. A median filter was used to identify peaks in the band. The peaks were listed by frequency in a file.
A list of systems which can be shut down under various conditions would be useful.
Although it appears that that it would be useful to standardize, NRAO-wide, on a database program, similar things have been attempted in the past and not worked well.