Radio-Frequency Interference

The bands within the tuning range of the EVLA which are allocated exclusively to radio astronomy are 1400-1427 MHz, 1660-1670 MHz, 2690-2700 MHz, 4990-5000 MHz, 10.68-10.7 GHz, 15.35-15.4 GHz, 22.21-22.5 GHz, 23.6-24.0 GHz, 31.3-31.8 GHz, and 42.5-43.5 GHz. No external interference should occur within these bands. RFI is primarily a problem within the low frequency bands, and is most serious to the D configuration, as the fringe rates in other configurations are often sufficient to reduce interference to tolerable levels.

Radio frequency interference (RFI) at the EVLA will be an increasing problem to astronomical observations. Table 10 lists some of the sources of external RFI at the VLA site that might be observed within the EVLA tuning range. Figure 3 shows a raw power spectrum at L-band. Figure 4 shows a similar plot for the new S-band system. Table 10 gives a summary of the origins (where known) of the prominent features shown in the two figures.

Figure 3	Figure 3: Spectrum of L-band RFI. This shows the major interfering signals seen across the full 1 GHz bandwidth available to the L-band receivers. Each of the eight "spectral windows" displays 128 MHz from a separate sub-band. These are raw data, uncalibrated for the bandpass of either the digital filter or the receiver. The high linearity of the EVLA's electronics and correlator will permit astronomical observing within any frequencies not containing external interference. Note that the y-axis is in logarithmic units (dB).
Figure 4	Figure 4: Spectrum of S-band RFI. This shows the raw spectrum of the lower half of S-Band - 2.0 to 3.0 GHz. No significant RFI is seen in the upper half. The major interference at 2.35 GHz is from satellite radio. The y-axis is in logarithmic units (dB).

: Figure 3

Figure 3: Spectrum of L-band RFI. This shows the major interfering signals seen across the full 1 GHz bandwidth available to the L-band receivers. Each of the eight "spectral windows" displays 128 MHz from a separate sub-band. These are raw data, uncalibrated for the bandpass of either the digital filter or the receiver. The high linearity of the EVLA's electronics and correlator will permit astronomical observing within any frequencies not containing external interference. Note that the y-axis is in logarithmic units (dB).

: Figure 4

Figure 4: Spectrum of S-band RFI. This shows the raw spectrum of the lower half of S-Band - 2.0 to 3.0 GHz. No significant RFI is seen in the upper half. The major interference at 2.35 GHz is from satellite radio. The y-axis is in logarithmic units (dB).

**Table 10: Identified VLA RFI Between 1 and 4 GHz**
Frequency (MHz)	Source	Comments
1025-1150	Aircraft navigation	Very strong
1200.0	VLA modem
1217-1237	GPS L2	Very strong
1243-1251	GLONASS L2
1254	Aeronautical radar
1263	Aeronautical radar
1268	COMPASS E6
1310	Aeronautical radar
1317	Aeronautical radar
1330	Aeronautical radar
1337	Aeronautical radar
1376-1386	GPS L3	Intermittent
1525-1564	INMARSAT satellites
1564-1584	GPS L1	Very strong
1598-1609	GLONASS L1
1618-1627	IRIDIUM satellites
1642	2nd harmonic VLA radios	Sporadic
1683-1687	GOES weather satellite
1689-1693	GOES weather satellite
1700-1702	NOAA weather satellite
1705-1709	NOAA weather satellite
1930-1990	PCS cell phone base stations
2178-2195	???
2320-2350	Satellite radio

Plots and tables of known RFI are available online, at http://science.nrao.edu/facilities/evla/observing/RFI/; plots of all RFI observations from 1993 onwards are available online, at http://www.vla.nrao.edu/cgi-bin/rfi.cgi. For general information about the RFI environment, contact the head of the IPG (Interference Protection Group) by sending e-mail to nrao-rfi@nrao.edu.

The impact of RFI on astronomical observing depends on the configuration and wave- length. It is worst for the D configuration, but RFI effects, as seen in an image, will be appreciably attenuated in the extended configurations and at higher frequencies due to fringe phase winding.

The EVLA electronics (including the WIDAR correlator) have been designed to minimize gain compression due to very strong RFI signals, so that in general it will be possible to observe in spectral regions containing RFI, provided the spectra are well sampled to minimize Gibbs ringing, and spectral smoothing (such as Hanning) is applied. We fully expect useful astronomical data to be extracted even if extremely strong interfering signals are located a few MHz away.

Extracting astonomy data from frequency channels in which the RFI is present is much more difficult. Testing of algorithms which can distinguish, and subtract RFI signals from interferometer data is ongoing.