Anish Roshi & Glen Langston (Jan 19, 2002)
This note describes Radio Frequency Interference (RFI) measurements in the 290--395 MHz frequency band of the GBT to determine which frequency ranges are relatively free of RFI. The survey was done with the PF1 342.5 MHz receiver system of the GBT. The time variability of the RFI was also examined by repeating observations over a 14 hour period. Much of the RFI in this band is time variable. At any instant more than 15 to 20% of the band will be RFI free at 120 sec integration. The feed arm servo and laser metrology systems contribute significantly to the RFI in this band and therefore be turned `off' during observations. Although the RFI situation at the PF1 342.5 MHz band is bad, some frequency ranges within this band can be used for astronomical observations. Particulary: (a) continuum observations in spectroscopic mode could use more than 15 to 20 % of a 40 MHz frequency band within the range; (b) 8--10 out of 28 galactic recombination lines are located at frequency ranges relatively free of RFI.
A data set for monitoring RFI in the 290 to 395 MHz was obtained on Dec 27-28, 2001 using the PF1 342.5 MHz receiver of the GBT in dual polarization mode. The backend used was the spectral processor in 1024 channel, 40 MHz bandwidth mode. To scan the full frequency band of the 342.5 MHz system using the spectral processor, the center frequencies were set to 310, 340, and 375 MHz for successive data acquisitions. The reference frame was set to topocentric for these tests. Spectra from two orthogonal circular polarizations (except the first 3 spectra, which were obtained by using linear polarizations) were collected using 2 FFT banks of the spectral processor in the above described mode. A data scan (ie spectrum with a center frequency) consists of four 30 secs integrations thus forming a total integration of 120 secs. The data was collected at an interval of about 1.0 hr starting at Dec 27, 23:00 hours and ending at Dec 28, 8:00 hours. The antenna was ``tracking'' the ``access'' position during the survey.
Fig. 1 shows the 120 secs averaged raw spectrum (top) from one of the polarizations and band-shape corrected spectrum (bottom) at the time (in UT) indicated on the title. A running five point median filtered raw spectrum was used as an estimate of the band-shape. The raw spectrum is useful to identify any broad RFI feature which will not be present in the band-shape corrected spectrum. An example of such a broad RFI feature is seen near 299 MHz in the spectrum at Dec 27 23:10:26 (UT). The yellow line in the bottom figure can be used as a guide to identify the RFI components picked up by an automatic RFI detection glish routine. The glish routine computes a spectral RMS after eliminating most of the `strong' RFI from the band-shape corrected spectrum using an iterative method. All spectral values above this RMS are considered as RFI. Note that this RMS is still larger than the RMS in the spectrum with no RFI. The frequencies of the RFI components thus picked are written into the file rfi300Dec27.freq . The solid vertical lines in the bottom figure show 300 KHz spectral windows (50 KHz below the rest frequency of hydrogen and 250 KHz above it) near the rest frequencies of hydrogen recombination line transition.
Fig. 2 shows 30 minutes averaged band-shape corrected spectra from one polarization. The 300 KHz spectral windows near hydrogen recombination lines are shown (light-blue line) in this plot as in Fig. 1. All the spectra shown in Fig. 1 corresponding to each frequency range are averaged to get this plot. From Fig. 2, it is clear that about 15 to 20 % of a 40 MHz frequency range could be used for continuum observations. The frequencies and relative amplitudes of the RFI components picked up by the automatic RFI detection glish routine are written into the file avrg300dec27.freq . As in Fig.1, the yellow line can be used as a guide to identify the RFI components picked up by the glish routine.
Fig. 3 shows 120 sec integrated band-shape corrected spectra from one linear polarization when the feed arm servo system is turned ``on'' (light-blue) and ``off'' (yellow). Significant `strong' RFI from feed arm servo is observed.
Fig. 4 shows 120 sec integrated spectra from one linear polarization when the laser metrology system is turned ``on'' (light-blue) and ``off'' (yellow). Note the `broad' RFI features near 299 and (presumably) 332 MHz. These features were present in all the spectra taken before the laser metrology system was turned ``off''. However, we haven't confirmed whether these features are due to metrology system by turning it ``on'' later.
Fig. 5 shows the gray scale display of the 15 spectra on each 40 MHz band taken over 14 hours. Much of the RFI are time variable.
Much of the RFI in this band is time variable. At any instant more than 15 to 20% of the band will be RFI free at 120 sec integration. The feed arm servo and laser metrology systems contribute significantly to the RFI in this band and therefore be turned `off' during observations. Although the RFI situation at the PF1 342.5 MHz band is bad, some frequency ranges within this band can be used for astronomical observations. Particularly: (a) continuum observations in spectroscopic mode could use more than 15 to 20 % of a 40 MHz frequency band within the range; (b) 8--10 out of 28 galactic recombination lines are located at frequency ranges relatively free of RFI.