GBT S-band RFI survey, part 2: Feb 7-8, 2002

by F. Ghigo (Feb.12, 2002)


On Feburary 7-8, 2002, a survey for interference was done with the GBT 2_3 GHz receiver over the frequency range 1.7 to 2.6 GHz using the Spectral Processor. Data were acquired with the GBT positioned at four different azimuths (az = 6, 96, 186, and 276 degrees), all at an elevation of 55 degrees. Linear polarization was used. The laser power and computer systems were off for these measurements. The results are compared with data from the previous week in which the laser systems were on.

Several RFI signals appear on this recent data that did not happen in the previous week, when the laser systems were on. Signals at some of the same frequencies as the previous week appear at drastically different power levels. In view of the apparent variable nature of many of the sources of RFI, we cannot definitely associate any signals with the laser systems. A better approach to identifying RFI from the laser systems might be to switch the lasers on or off every hour, doing a spectrum sweep in between. Another approach would be for the IPG to track down the various RFI peaks that have been identified in this survey.

Figure 1. Plot of all data from Feb 7-8, averaged.
Figure 2. Zoomed plot.
Figure 3. Filtered and normalized plot. (See Results section below for more details.)

The major RFI features are the same as in the previous survey. Refer to the previous writeup for plots. The directional nature of the signals is also similar.

To compare the results from the Jan 30 data with the present data, a plot of the difference in the filtered data is shown in Figure 4 and Figure 5. See further discussion below in the results section.


The observing used the same setup and sequence of scans at successive sky frequencies as in the previous run (Jan 30-31). The journal of observations is summarized in Table 1. All observations were done with the GBT at elevation 55 degrees.
Table 1: Observations 08 Feb 2002, project: rfi_S_feb07
Scan #s Start Time (UT) Azimuth (deg)
21-48 02:24 96
49-77 03:30 186
78-105 04:44 276
106-133 05:49 6
134-161 07:01 96
162-189 08:06 186
190-217 09:12 276
218-245 10:18 6
246-273 11:32 96


Processing was again done using the uni2 package of aips++. The same procedures described in the writeup of the Jan 30-31 survey were used here.


Raw Spectra

The spectra averaged over all 4 azimuths is shown in Figure 1.
The last spectral sweep (scans 246-273) was not included in this average, in order to exactly match the average that was done in the previous run, i.e., 8 spectral sweeps with 2 sweeps at each azimuth.

The spectrum is again dominated by the satellite RFI in the range 2320 to 2345 MHz. Weaker features can be seen in Figure 2, plotted with an expanded scale.

Again, we note that the noise level is much higher in X polarization (red plot) that in Y (green) at frequencies where the strong satellite RFI is in or near the band.

Filtered Spectra

The filtered, baseline-flattened spectra were produced as described before by dividing each spectrum by the median-filtered version of itself to brings out narrow band features.

Figure 3 shows the filtered averaged over all scans: the X polarization is shown in red, Y in green. Again, the spectrum is cleaner for frequencies less than 2200 MHz in the Y polarization.

The list of RFI peaks based on the data plotted in Figure 3 is given in Table 2. All peaks exceeding 1.002 are listed in Table 2, omitting those between frequencies of 2485-2500 and 2315-2347 MHz. The power (P) in Table 2 is the y-coordinate transformed to 0.001 units above 1.0; i.e., P = 1000(Y-1). i.e., the power is in parts per thousand of the system temperature. The maximum of the 2 polarizations is listed.

Note particularly strong RFI around 2460-2470 MHz, and at 2200 MHz, that was not very notable in the Jan 30-31 data. Close-up plots of these features in the raw data are shown in Figure 6A and Figure 6B.

Laser dependant RFI?

To look for differences between the two observing runs, we present the difference between averaged filtered spectra for the Jan 30-31 run and the Feb 7-8 run.
Figure 4 shows this "on-off" spectrum, and
Figure 5 shows the same data on an expanded scale.
In these plots, the positive-going spikes are signals that were stronger in the Jan 30-31 data, i.e., with the laser systems on. Likewise, the negative-going spikes are the signals that are stronger in the Feb 7-8 data, i.e., with the laser systems off.

Inspection of these plots suggests there was more and stronger RFI on the latter date than the former.

We have listed tables of RFI components from the difference spectra. Table 3 are the components that were stronger on Jan 30-31, and Table 4 lists those stronger on Feb 7-8. Amplitudes are listed in units of 0.001 of the system temp. The tables list all components exceeding 0.002 of the system temp.

Directional dependences

To look for RFI that may be coming from a particular direction, we have averaged the 2 spectra that were taken at each azimuth pointing and from it subtracted the spectrum averaged over all directions. The difference spectra are given in Figures 7-10.
Figure 7: difference spectrum for az=6 (North)
Figure 8: difference spectrum for az=96 (East: toward the Jansky Lab)
Figure 9: difference spectrum for az=186 (South)
Figure 10: difference spectrum for az=276 (West)

Again, most of the RFI seems associated with the az=96 spectrum, i.e., in the general direction of the Jansky Lab. In particular the very strong features at 2200 MHz and 2467 MHz seem associated with this direction.