Anish Roshi & Frank Ghigo (Jan 23, 2002)
This note describes Radio Frequency Interference (RFI) measurements in the 680--920 MHz frequency band of the GBT to determine which frequency ranges are relatively free of RFI. The survey was done with the PF1 800 MHz receiver system of the GBT. The time variability of the RFI was also examined by repeating observations over a 8 hour period. Compared to other PF1 bands of the GBT, the 680--920 MHz band has relatively less RFI. The frequency range above 850 MHz contain considerably more RFI compared to the other frequency range in this band. Much of the `strong' RFI was present almost all the time during the survey. The feed arm servo does not produce much `strong' RFI in this band. The 680--920 MHz band can very well be used for many astronomical observations. Continuum observations in spectroscopic mode could use more than 30 % of any 40 MHz frequency band within the PF1 800 MHz band. 9 to 10 out of 21 galactic recombination lines are located at frequency ranges relatively free of RFI.
A data set for monitoring RFI in the 680 to 920 MHz was obtained on Jan 22-23, 2002 using the PF1 800 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 800 MHz system using the spectral processor, the center frequencies were set to 695, 730, 765, 800, 835, 870 and 905 MHz for successive data acquisitions. The reference frame was set to topocentric for these tests. Spectra from two orthogonal 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 15 secs integrations thus forming a total integration of 60 secs. The data was collected at an interval of about 1.0 hr starting at Jan 23 00:30 hrs and ending at Jan 23 8:00 hrs. The antenna elevation was about 70 deg during the data acqusition. The feed arm servo was ``on'' for most of the observations.
Fig. 1a and Fig. 1b show the 120 secs averaged raw spectrum (top) from one of the linear 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. 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 an RMS from the band shape corrected spectrum after eliminating all `strong' RFI. All spectral values above this RMS are considered as RFI. The frequencies of the RFI components thus picked up by this routine are written into the file rfi800jan22.freq .
Fig. 2 shows 6 minutes averaged band-shape corrected spectra from one linear polarization. 300 KHz (50 KHz below the rest frequency of hydrogen and 250 KHz above it) spectral windows near the rest frequencies of hydrogen recombination line transitions are marked in blue. All the data taken for the RFI survey is averaged to get these plots. Note that frequency range above 850 MHz has relatively more RFI. Also more than 30 % of any 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 avrg800jan22.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 60 sec integrated band-shape corrected spectra from one linear polarization when the feed arm servo system is turned ``on'' (light blue) and ``off'' (yellow). No `strong' RFI from feed arm servo is observed.
Fig. 4 shows the gray scale display of the 6 spectra on each 40 MHz band taken over 8 hrs. Much of the `strong' RFI is present through out the observations.
Compared to other PF1 bands of the GBT, the 680--920 MHz band has relatively less RFI. The frequency range above 850 MHz contain considerably more RFI compared to the other frequency range in this band. Much of the `strong' RFI was present almost all the time during the survey. The feed arm servo does not produce much `strong' RFI in this band. The 680--920 MHz band can very well be used for many astronomical observations. Continuum observations in spectroscopic mode could use more than 30 % of any 40 MHz frequency band within the PF1 800 MHz band. 9 to 10 out of 21 galactic recombination lines are located at frequency ranges relatively free of RFI.