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Session 40 - Adverse Environmental Impacts on Astronomy.
Display session, Tuesday, January 16
North Banquet Hall, Convention Center

[40.02] Increased RFI Requires Monitoring at VLA and VLBA

M. Claussen, G. Taylor, C. Janes, W. Brundage (NRAO)

Radio Frequency Interference (RFI) adversely impacts radio astronomy observations at the National Radio Astronomy Observatory's (NRAO) Very Large Array (VLA) and Very Long Baseline Array (VLBA) radio observatories. Surveillance radar, UHF (Ultra High Frequency) TV stations, weather balloons, military activity, satellite traffic, and microwave radio communication are frequent sources of interference, though the effect of several has been mitigated. Adding to the problem is allocation of frequencies to various satellite systems in and near radio astronomy frequency bands. The effect of known interfering sources such as the proposed constellations of Low Earth Orbit (LEO) satellite systems can sometimes be mitigated through planning and negotiations, but interference must first be identified before mitigation can be considered. An RFI monitoring system at the VLA documented over 50 cases of strong intermittent interference within the radio astronomy frequency bands at 1330 MHz to 1427 MHz during the first 6 months of 1995. To identify these and other interfering sources, various monitoring systems are being planned and put in place at the VLA and VLBA sites. Monitoring results are being made accessible via the World Wide Web as the information becomes available. Summaries of interference problems are given in the observational status reports via the NRAO home page,, and actual plots of interference are updated periodically. A record of downtime as a result of RFI, especially where the downtime occurred while using a frequency band allocated to radio astronomy, would be a powerful tool in high-level spectrum management negotiations. NRAO has established a preliminary downtime report and is considering various improvements to it. Though RFI is by no means a "show-stopper" for terrestrial-based radio astronomy, the increased use of the radio frequency spectrum means that radio observatories should consider monitoring the frequency bands they use, identifying RFI sources, and pursuing mitigation wherever possible.

Program listing for Tuesday