PF1 800 MHz RFI

Observations

On July 7-8, 2004 GBT receiver checkout observations were performed for the PF1 800 MHz band receiver. Serveral scans to identify RFI in the nominal receiver bandpass were made as part of these observations. The spectrometer was used for these observations. The spectrometer was setup to provide 200 MHz instantaneous bandwidths in two separate bandpasses. The two bands had center frequencies of 780 MHz and 820 MHz allowing the whole 800 MHz receiver bandpass to be cover instantaneously.

Horizon scans were taken where the GBT was moved from 0 degrees azimuth to 360 degrees azimuth at a constant elevation of 15 degrees. The telescope was slewed at 30 degrees per minute in azimuth and a 3 second integration time was used.

I have not attempted to calibrate the data. Only raw data from the "cal off" phase is shown.

Pointed Scans

Table 1.Table of Figures.

Figure	Azimuth	Elevation	Frequency Range (MHz)		Figure	Azimuth	Elevation	Frequency Range (MHz)		Figure	Azimuth	Elevation	Frequency Range (MHz)
1	130	90	680 - 730		30	90	15	680 - 730		59	330	15	680 - 730
2	130	90	730 - 780		31	90	15	730 - 780		60	330	15	730 - 780
3	130	90	780 - 830		32	90	15	780 - 830		61	330	15	780 - 830
4	130	90	830 - 880		33	90	15	830 - 880		62	330	15	830 - 880
5	130	90	880 - 920		34	90	15	880 - 920		63	330	15	880 - 920
6	130	90	680 - 690		35	90	15	680 - 690		64	330	15	680 - 690
7	130	90	690 - 700		36	90	15	690 - 700		65	330	15	690 - 700
8	130	90	700 - 710		37	90	15	700 - 710		66	330	15	700 - 710
9	130	90	710 - 720		38	90	15	710 - 720		67	330	15	710 - 720
10	130	90	720 - 730		39	90	15	720 - 730		68	330	15	720 - 730
11	130	90	730 - 740		40	90	15	730 - 740		69	330	15	730 - 740
12	130	90	740 - 750		41	90	15	740 - 750		70	330	15	740 - 750
13	130	90	750 - 760		42	90	15	750 - 760		71	330	15	750 - 760
14	130	90	760 - 770		43	90	15	760 - 770		72	330	15	760 - 770
15	130	90	770 - 780		44	90	15	770 - 780		73	330	15	770 - 780
16	130	90	780 - 790		45	90	15	780 - 790		74	330	15	780 - 790
17	130	90	790 - 800		46	90	15	790 - 800		75	330	15	790 - 800
18	130	90	800 - 810		47	90	15	800 - 810		76	330	15	800 - 810
19	130	90	810 - 820		48	90	15	810 - 820		77	330	15	810 - 820
20	130	90	820 - 830		49	90	15	820 - 830		78	330	15	820 - 830
21	130	90	830 - 840		50	90	15	830 - 840		79	330	15	830 - 840
22	130	90	840 - 850		51	90	15	840 - 850		80	330	15	840 - 850
23	130	90	850 - 860		52	90	15	850 - 860		81	330	15	850 - 860
24	130	90	860 - 870		53	90	15	860 - 870		82	330	15	860 - 870
25	130	90	870 - 880		54	90	15	870 - 880		83	330	15	870 - 880
26	130	90	880 - 890		55	90	15	880 - 890		84	330	15	880 - 890
27	130	90	890 - 900		56	90	15	890 - 900		85	330	15	890 - 900
28	130	90	900 - 910		57	90	15	900 - 910		86	330	15	900 - 910
29	130	90	910 - 920		58	90	15	910 - 920		87	330	15	910 - 920

50 MHz Bandwidth Plots at Az=130, El=90

Figure 1. Click on image for postscript version. 130 degrees Azimuth, 90 degrees Elevation. 680 MHz to 730 MHz.

Figure 5. Click on image for postscript version. 130 degrees Azimuth, 90 degrees Elevation. 880 MHz to 920 MHz.

10 MHz Bandwidth Plots at Az=130, El=90

Figure 29. Click on image for postscript version. 130 degrees Azimuth, 90 degrees Elevation. 910 MHz to 920 MHz.

50 MHz Bandwidth Plots at Az=90, El=15

Figure 30. Click on image for postscript version. 90 degrees Azimuth, 15 degrees Elevation -- towards Jansky Lab. 680 MHz to 730 MHz.

10 MHz Bandwidth Plots at Az=90, El=15

Figure 58. Click on image for postscript version. 90 degrees Azimuth, 15 degrees Elevation -- towards Jansky Lab. 910 MHz to 920 MHz.

50 MHz Bandwidth Plots at Az=330, El=15

Figure 59. Click on image for postscript version. 330 degrees Azimuth, 15 degrees Elevation -- towards Cheat Mountain. 680 MHz to 730 MHz.

10 MHz Bandwidth Plots at Az=330, El=15

Horizon Scans

The images below (Figures 88 and 89) show the raw data for each polarization. These data still contain the receiver bandpass which shows up as the broad (greater than about 5 to 10 MHz) undulations in frequency. Only the brightest RFI show up in these plots.

The bright emission around an Azimuth of 90 degrees is from the Jansky Lab, surrounding buildings and Arbovale. The bright emission around 50 degrees Azimuth is from the area of the 140 Foot Telescope. (Could the 140 Foot area really be a reflection of the Jansky Lab area off of the 140 Foot and into the GBT?)

Figure 88 Click on image for postscript version. X (left) and Y (right) polarizations. 200 MHz bandwidth centered on 780 MHz.

Figure 89 Click on image for postscript version. X (left) and Y (right) polarizations. 200 MHz bandwidth centered on 820 MHz.

Figures 90 and 91 show the data for each polarization where the receiver bandpass has been subtracted. This was done by ordering the data within 25 frequency channels on each side of a point and taking the 10th lowest value as the baseline level for that point. This removes most of the baseline except for areas where the baseline has large gradients - such as the bandpass feature near 817 MHz.

Figure 90 Click on image for postscript version. X (left) and Y (right) polarizations. 200 MHz bandwidth centered on 780 MHz. Baseline has been removed.

Figure 91 Click on image for postscript version. X (left) and Y (right) polarizations. 200 MHz bandwidth centered on 820 MHz. Baseline has been removed.

Figures 92 and 93 show the data for each polarization where the first and last integrations (towards the North) have been linearly interpolated in time and subtracted from the data. These plots best show directions from which low-level, broad-band RFI is orginating.

Figure 92 Click on image for postscript version. X (left) and Y (right) polarizations. 200 MHz bandwidth centered on 780 MHz. Data has had all emission from the North subtracted from the whole data set.

Figure 93 Click on image for postscript version. X (left) and Y (right) polarizations. 200 MHz bandwidth centered on 820 MHz. Data has had all emission from the North subtracted from the whole data set.

Summary

Many RFI frequencies exist in the PF1 800 MHz bandpass. The best area for observations appears to be between 801 MHz and 850 MHz. RFI definitely increases towards the Jansky Lab and the surrounding residential areas.

The vast majority of the RFI in this scan is due to UHF TV stations (below 806 MHz), Airphones and cellular control towers in the 869-894 MHz range. These signals originate from outside the NRQZ. The favorable range of 801 MHz - 850 MHz is due primarily to the lack of cellular phone handsets (824-849 MHz) and the non-use of a couple other allocations in this range in the vicinity of the observatory. Note that a virtual absence of RFI in the plots 49 - 51, which were taken with the GBT pointed in the direction of the Jansky Lab, reinforces this theory. It is likely that the apparent directionality in some of the plots is due to gaps in our terrain shielding. Note that our horizon is actually below the horizontal in one direction and close to the horizontal in a few others.