Plot of all data from Jan 30-31 run averaged. (See Results section below for more details.)
Very strong and variable RFI from a satellite is seen between 2320 and 2346 MHz. This part of the band is essentially unusable. The 2487-2500 MHz RFI may also be due to a satellite. A summary of the strongest RFI components is given in Table 1. (click on the first column to see a plot of that feature.)
|Table 1. Strong RFI, 1.7-2.6 GHz|
|F(MHz)||Peak power (db)||HP width (MHz)||Comments|
|2320-2345||15.3||6.0||Satellite Digital Audio Radio Services|
|2487-2498||4.0||5.0||Non Geostationary Mobile Satellite Service?|
A table of all significant narrow band components is listed in Table 2. (see Results section for more details.) This table excludes the components from the satellite bands listed in Table 1.
We note signals that may be coming from the direction of the Jansky Lab at frequencies of about 1815, 2200, 2227, and 2244 MHz.
We experimented with the azimuth positioning of the GBT to find the azimuth that pointed directly towards the Control Building. This was done by visual sighting using the binoculars on the Control Room Deck. The azimuth of the control room turned out to be 96 degrees (+/- 0.5 deg).
Observations were done at 4 azimuths, 96, 186, 276, and 6 degrees, with the elevation set to 55 degrees in all cases. Table 3 summarizes the observations: the hour-long sequence of 28 scans was done at the 4 azimuths, then the whole series was repeated.
|Table 3: Observations 31 Jan 2002, project: rfiS_jan30|
|Scan #s||Start Time (UT)||Azimuth (deg)|
The 26 process-able spectra were pasted together to make a spectrum for the whole band. Since the spectra were spaced 35 MHz apart, the central 35 MHz of each spectrum was selected and the rest discarded. The result was a raw spectrum which showed the Spectral Processor bandpass repeated 26 times. This raw spectrum is useful for identifying the strongest RFI components.
A filtered version of the the spectrum was produced by doing a median filter of 7 channels in width on each individual spectrum, then dividing this filtered spectrum into the raw data. This produced a spectrum with a flattened baseline showing the narrow RFI features. These were pasted together as described in the last paragraph to produce a filtered spectrum for the whole band. The spectral processor has artifacts that occur in its bandpass at 0.25, 0.5, and 0.75 of the bandpass. For the 40 MHz band width this means that the artifacts occur at -10, 0, and +10 MHz with respect to the band center. These features were edited out by the filtering software.
It is useful to look at the raw spectrum as well as the filtered version, because wide band features are eliminated by the filtering process.
A plot of the averaged spectrum is shown in Figure 1.
The spectrum is dominated by the satellite RFI in the range 2320 to 2345 MHz.
To look for weaker features, the vertical scale is expanded and shown in Figure 2.
In Figure 2, the periodic appearance is due to the bandpass shape of the spectral processor repeated 26 times and exxagerated by the expanded scale. In this plot, one can pick out several strong RFI spikes that cannot be seen in Figure 1. Close-up views of these RFI spikes are displayed in plots that can be viewed by clicking on the frequency column in Table 1.
Another thing to note is that the noise level is much higher in X polarization (red plot) that in Y (green) at frequencies above 2200 MHz. Also the noise level is particularly high in X between 2250 and 2400 MHz. Perhaps there is some broadband effect of the satellite RFI, or the spectral processor is driven non-linear in the presence of these strong signals.
In Figure 3 we show the filtered spectrum for the X polarization, averaged over all scans.
Figure 4 shows the same information for the Y polarization. Note that the spectrum is cleaner for frequencies less than 2200 MHz in the Y polarization.
The list of RFI peaks is given in Table 2. The spectra shown in Figures 3 and 4 are normalized to a baseline of 1.0. Any peaks exceeding 1.002 are listed in Table 2. We have removed all the peaks in the list between frequencies of 2486-2500 and 2317-2346 MHz, all presumably due to satellite RFI. The power (P) in Table 2 is the y-coordinate from the plot (Figures 3 and 4) transformed to 0.001 units above 1.0; i.e., P = 1000(Y-1). The maximum of the 2 polarizations is listed.
In these spectra it is probably best to discount any of the peaks between about 2300 and 2500 MHz as having something to do with one of the satellite RFI signals. Aside from these, we can suspect signals that are large and positive as likely coming from a particular direction.
Looking North (az=6 degrees), there are apparently no significant signals.
Looking East (az=96 degrees), towards the Jansky lab, there are four significant signals, at frequencies of about 1815, 2200, 2227, and 2244 MHz. The signal at 2200 seems quite significant.
Looking South (az=186 degrees), there are no obvious signals.
Looking West (az=276 degrees), which might include RFI from Snowshow or Cass, there is perhaps one significant signal at 2226 MHz.