RF Interference Monitoring for the Onsala Space Observatory

Abstract: With the continuous and rapid developments in wireless services and allocation of radio frequency spectrum of theses services, huge interferences have been observed in the field of radio astronomy. According to the international regulations, parts of the spectra are reserved for radio-astronomical observations. Man-made signals entering the receiver chain of a radio telescope have much higher power compared to natural or passive signals received at the radio telescopes. Passive signals received at radio telescopes are normally 60 dB below the receiver noise level. Active signals generated by man-made wireless services pollute the natural emissions by completely masking them due to high signal strength. The cosmic radiation is determined by the fundamental laws of physics, thus the frequencies are fixed and cannot be changed. So interferences created by active services lead to wrong interpretations of the astronomical data.The present thesis deals RF interference monitoring system for the Onsala Space Observatory. As part of the thesis, a software application has been developed, which communicates with different types of digital reveivers (spectrum analyzers) attached with antenna controlling hardware to control omnidirectional and steerable antennas. A steerable antenna is used to find the direction of interference source by moving the antenna in azimuth and elevation direction. The detection of interference signals is done using the recently proposed GSK estimator which is state-of-the-art methodology for detecting interferences at observatories. The interference finding criteria is based on the decision thresholds of the variance of the received power levels. If the variance exceeds a threshold value on a particular frequency, interference is marked on that frequency.The RFI monitoring system developed in this thesis work reads the spectra of a number of different desired frequency spans associated with particular antennas. This application supports four channels and each channel can be associated with any of the three omnidirectional antennas and a steerable antenna. When the measurements are completed, a file containing frequency points, power levels and the indication of any interference is saved and plots are generated. These plots include received power levels (in dBm and Jansky units) and the interferences at particular frequency. These plots are also replicated to a web server and can be viewed on HTML page for remote monitoring. The RFI monitoring system can also be used to analyze the offline data by generating different plots for a particular antenna associated with a particular channel.