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RFI Scans and Known Sources

To enable observer planning for RFI avoidance, here is an archive of the most recent RFI scans to give the observer an idea of spectral occupancy as seen by the GBT receivers. Four zoom levels are provided starting at 100 Jy of the average RFI seen during the scan taken by the GBT.

Prime Focus 1 (342 MHz) – 02/18/2020

Prime Focus 1 (800 MHz) – 05/27/2021

L-Band – 07/04/2021

S-Band – 01/24/2021

C-Band – 07/09/2021

X-Band – 01/17/2021

Ku-Band – 11/18/2020

K-Band Focal Plane Array – 01/19/2021

Ka-Band – 09/20/2020

Q-Band – 03/03/2020

RFI scans are performed routinely by the operators during gaps between astronomical observations. The aim of the technique is to do the best job of monitoring narrow-band RFI coming from the horizon (which comes at the cost of monitoring changes in the RFI from satellites, nearby planes, etc.). The GBT, which can’t point below an elevation of 5 deg (typically many beamwidths), has very little sensitivity to horizon-based RFI in its forward direction. The sidelobes in the forward direction are also not uniform. The telescope is much more sensitive to radiation that comes from the horizon and that enters the sidelobes of the feeds. To make the sensitivity of the feed patterns uniform around the horizon we position the elevation of the antenna so that the flange of the feed is parallel to the horizon. The feed sidelobes also have uniform sensitivity as they cover a very large solid angle. Gregorian receivers require a different elevation than PF receivers to put the feeds into this orientation. However, the single telescope feed arm will introduce azimuthal diffraction patterns on top of the feed sidelobe patterns. To smooth out this azimuthal dependence, the telescope moves at near its top speed from Az=0 to 180 (or Az=180 to 0 if that’s a more efficient route). With this tactic, one can’t expect the monitoring of RFI that comes from the forward direction (satellites, etc.) to be anything more than hit and miss.

The data reduction uses the average of the raw bandpass data across the full slew and the average of the two polarizations (if the receiver has dual polarization). The raw bandpasses are put through a high-pass filter (with an upper frequency of 0.1 channels-1), which removes the overall bandpass shape. The use of a high-pass filter does the best job of depicting narrow-band RFI (our primary aim), but which comes at the cost of compromising the detection of wide-band RFI if it is significantly wider than about 10 channels.

Since the noise diode flickers throughout the observing, the bandpass power is converted into units of antenna temperatures using the ratio of the detected total power to the change in power when the diode is on. Since observers want to know the level at which the RFI would contaminate their observations, the signal strength observers would see is mimicked by converting Ta to Jy using the antenna’s main-beam gain.

The most recent GBT (raw, unfiltered) RFI data is stored as .fits files at:/home/gbtdata/TRFI_MMDDYY_RN where MM is month, DD is the day, YY is the last two digits of the year, R is the receiver letter designation (see below) and N is the session number for that particular day (1,2, 3 etc.) GBTIDL may be used to examine these plots dynamically and in more detail.

Further plots and files can be found at https://science.nrao.edu/facilities/gbt/interference-protection/ipg/rfi-scans . Please direct any questions to bgregory@nrao.edu