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Archival Spectrum Monitoring



1. Need

2. Feasibility

Requirements

Probably shouldn't be done before each observing run. My estimate is that, with our typical 35 observing session per week, we'll loose 3 hrs per week of Astronomy to such measurements. Except for the PF receivers, RCO's don't happen often enough for our other receivers (once every 2 years). Instead I suggest that this be done during Tcal measurements, which are scheduled once to twice a month and cover 1 to 3 receivers per month. These are bandscans, by definition and the RFI data comes as a byproduct that, up to now, we have been throwing away. We are able to cycle through measuring all receivers once to twice a year. And Tcal measurements are often already part of a RCO. But, Tcal measurements are at whatever Azimuth and Elevation we find calibrators at the time of the observing. What we need to discuss is whether it is best for the scientific staff or RFI group to take the Tcal data and pull out the RFI information. Whether Tcal measurements need extra telescope time to do a bandscan at a single Az and El. How to get the RFI information out of IDL.

My back-of-the-enevlope estimate suggest that, in comparison to your monitoring station, the GBT will have about 10 db less sensitivity to locally generated RFI. The weak sidelobes of the GBT that fall on the horizon (-70 dB at best at 1 GHz) are probably no match to a small but directed antenna. Thus, using telesscope time for a repeated, directed RFI survey may not be as useful as a survey with the monitoring stations. Ron Maddalena

Ron has a good point that doing spectrum surveys with the GBT receivers will take up too much telescope time. At best, it might be done on maintenance days, which would be hit and miss and require quite a bit of attention from someone to find appropriate times and initiate the scripts. RickFisher

Rick's measurements (paper attached) using the dipole array antenna, which has a gain of ~7dBi shows a 9dB Signal-to-Noise advantage of the monitoring station over the L-band sidelobes. The Omidirectional antennas on the station have a gain of 0dBi, which mean measurements using these (which would not require motion) still give us about 2dB advantage, and if we used the directional antennas (which do require motion and may be weather-restricted), with gains around 13dBi, we've got a huge advantage (15dB) over the GBT sidelobes. So your back-of-the envelope calculations have bourne out experimentally already. Rick used his own data collection system (a dual-channel, 12-bit A/D PC-card) for this. I was thinking (as one possible approach) that we might want to send the input from the monitoring station into the spectral processor, and use the existing data-handling tools of the GBT to archive it. I don't know how to do this, however, and would need to make an official project of it if we decide to go that route. CarlaBeaudet

monit_gbt_comparison.pdf: Rick compares GBTMS and GBT sidelobe sensitivity

One of the projects on my TO DO list is a head-to-head comparison of sensitivities from the spectrum analyzer and the software FFT technique. I did a quick look at this, which looked encouraging, but we need something more quantitative on a variety of RFI spectrum types. We can also compare simultaneous spectrum analyzer results taken at the RFI Trailer Monitoring Station and the GBT RFI Monitoring Station. And we should also do some more GBT receiver to GBT RFI Monitoring Station comparisons on signals other than DME.

Before launching into routine spectrum monitoring, let's try a few things to see what's most useful and most sensitive. From this experience we can make some better judgments on how to proceed. If you like, I can sketch out some test scenarios, and, hopefully, we can find some time to execute them. RickFisher

Options

Front Ends

GBT RFI Monitoring Station

Operating range: 130 MHz - 3 GHz

Pros:

Since we can run the RFI monitor stations independently, I think that's a good place to start. After some experience, we can take stock and see if the products are useful. There are two approaches, each of which has its advantages and disadvantages: spectrum analyzer scans and FFT spectrum scans. Both of these can be automated without too much work. RickFisher

Cons:

RFI Trailer Monitoring Station

Operating range: 100 MHz to 2.0 GHz (can easily be extended to 4 GHz)

Pros:

Cons:

GBT RCO's

Operating range: By definition matched to GBT operating ranges

Pros:

Cons:

Back Ends

Spectrum analyzer
Spectrum analyzer scans cover the most spectrum in the shortest amount of time. They could be repeated fairly frequently (however long it takes to do a full spectrum sweep). Both the RFI Trailer Monitoring Station and the GBT RFI Monitoring Station have or can be outfitted with this capability. These would be "low sensitivity surveys" because the integration time on each spectral channel is typically less than a second. Scanning 2 GHz with 30 kHz resolution and one second per channel would take about 20 hours, not counting overhead. Because spectrum analyzers don't have square-law detectors there's not much to be gained with longer integration times. RickFisher

GBT Spectrometer

We can probably gain a factor of ten or more in sensitivity over a spectrum analyzer with A/D sampling of the monitor antenna output and doing the FFT's in software. This is currently available only on the GBT RFI Monitoring Station. Since software FFT's are considerably slower than real time, this covers less spectrum in a given time, but the spectrum noise goes down as the square root of the integration time for as long as you want. 100 seconds per spectrum is probably reasonable. The spectral resolution is also as good as you need with 1 kHz being a reasonable value. The A/D in the computer at the GBT RFI Monitor Station output in the GBT equipment room allows us to do about 6 MHz at a time. We could increase this to about 12 MHz, if warranted, with a new A/D. The A/D - FFT approach also lends itself to the detection of short pulses, like DME, radar, power line noise, and switch closures, which a spectrum analyzer doesn't handle well. RickFisher

Data Display

Requirements:

OASIS

Pros:

Cons:

IDL & Other astronomy display tools
Pros:

Cons:

Decisions

Two Options, outlined below, will be pursued in parallel:

GBT-Based RFI Monitoring

GBT RFI Monitoring Station

A Single Web Page, linked off the RFI page, will give access to the information. Carla has created a 1st draft of this page.

-- CarlaBeaudet - 27 Sep 2007

Attachment: sort Action: Size: Date: Who: Comment:
monit_gbt_comparison.pdf action 4030763 20 Nov 2007 - 16:17 CarlaBeaudet Rick compares GBTMS and GBT sidelobe sensitivity

Topic ArchivalMonitoring . { Edit | Attach | Ref-By | Printable | Diffs | r1.12 | > | r1.11 | > | r1.10 | More }
Revision r1.12 - 25 Jul 2008 - 12:42 GMT - CarlaBeaudet
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