RFI Mitigation in Radio Astronomy

Project overview:
A very small fraction of the radio spectrum below 30 GHz is allocated for radio astronomy research, but cosmic radiation is present at all frequencies. We are developing techniques for making astronomical observations at frequencies occupied by active users of the radio spectrum. Our recent efforts have concentrated on 1200-1350 MHz band into which the 1420.4 MHz spectral line of neutral hydrogen from distant galaxies is Doppler shifted. This frequency range is mainly occupied by aircraft traffic control radar.

Alok Singhal, Dr. Fisher and Dr. Hogg (NRAO) are investigating the influence of detailed galaxy dynamics on the absolute magnitude - HI line width relation that is extensively used to derive distances to galaxies and, hence, the distribution of galaxies in the universe. The physical connection between and galaxy's brightness and its rotational speed is not well understood, so the strong correlation between its absolute magnitude and its HI line width is remarkable and largely empirical. Alok is looking for clues in the data base of galaxy observations that may shed some light on the dynamics that produce this relationship.

Alok Singhal and Dr. Fisher have been analyzing data samples of signals from radars in Virginia and Puerto Rico, which are seen by the Green Bank, WV and Arecibo, PR radio telescopes. We have shown that very strong radar signals can be removed from the radio astronomy data (see here),and we are planning to extend this technique to integration times of an hour or more. A byproduct of these data is a map of the high terrain near the radio observatories which shows the many paths of propagation of RFI into the observatories. This information provides valuable guidance in the development of coherent RFI canceling methods for other types of interfering signals.

Jit Sarkar, an undergraduate engineering student, is working on a senior thesis to develop a hardware device for generating blanking signals that are synchronous with the radar pulses received by the GBT in Green Bank. These blanking signals will be used by existing spectrometers to avoid collecting data when the radar pulses are present. His device, based on a TI TMS360C6211 digital signal processing chip, generates the measured radar pulse sequence and will watch the radar pulse arrival times to maintain synchronization.

Qing Zhang, a graduate student in the Engineering and Computer Science Dept., and her advisers, Dr. Yibin Zheng and Dr. Steven Wilson, are collaborating with Drs. Fisher and Bradley on the development of signal processing algorithms for minimizing the data loss in radar pulse excision from radio astronomy data. Her techniques involve the use of digital filters matched to the radar signal, threshold detection, and automatic deletion of individual samples from the data before the Fourier transform process to generate the power spectrum.

Drs. Fisher and Bradley have a collaborative program of RFI mitigation research with Engineering an Computer Science faculty and students of Brigham Young University. This research is concentrating on coherent signal cancellation techniques as applied to GBT and VLA observations. This collaboration presents opportunities to UVa astronomy graduate students for the application of RFI mitigation to scientific projects which may be difficult or impossible without them.

For additional Information please e-mail Rick Fisher: rfisher@nrao.edu.

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