Technological Research Lines

RFI Monitoring

RFI monitoring

It is well known that radio astronomical observations are extremely vulnerable to man-made radio frequency interferences (RFI). For this reason, a lot of effort is usually given to the selection of the site suitable to host a new radio astronomical facility. A radio quiet location generally implies: a reasonable shielding offered by mountains surrounding the radio telescope, a rather large distance from heavily populated centers, a good relationship with local telecommunication authorities to avoid the installation of electrically noisy sources too close by.

Once the site has been chosen, a continuous monitoring of the RFI environment must be performed to assure a radio spectrum as clean as possible. Therefore, designed hardware has been built to monitor both the frequency bands allocated by ITU to RAS, and the non-RAS bands, now used by our receivers or potentially usable in the future if their level of RF quietness is acceptable with a significant statistical credibility.

In the special case when man-made interferences are detected, either due to intentional or spurious effects, an official report is transmitted to the Territorial Authority of the National Administration. However, such kind of broadband surveys also produce other benefits. For example, the “robustness” design of the receivers can be greatly improved, by knowing in advance the actual levels and spectral distribution of the interfering signals present at that particular site. Also dynamic scheduling of the radioastronomical observations can make use of the statistically significant “lack” of interference at particular times (at nights, weekends). Finally, future mitigation techniques may be optimized by adapting them to the characteristics of the RFI to be excised or flagged. Great attention is also devoted to the generation of auto-interference by all types of devices operating at the telescope site.

The scheme of the RFI monitoring systems adopted at SRT consists of two different stations: (i)a fixed station with antennas placed on a tower close to the radio telescope, and (ii)a mobile laboratoryto move closer to the RFI transmitter. The combination of those facilities allows to have the highest probability in detecting and identifying all kinds of possible RFI (wide-band or narrow-band emission, continuous wave or pulsed signal) as well as their characteristics like amplitude, central frequency, bandwidth, type and content of their modulation, azimuth direction, polarization, and finally the exact location of the transmitting source (by using triangulation with the mobile laboratory). The main features of the monitoring systems installed in the two stations are: high versatility, amplitude sensitivity close to the ITU-RA769 levels, high dynamic range by appropriate filtering, and an almost continuous frequency coverage of the radio spectrum.

While the mobile van allows multiple triangulation measurements, the fixed station can operate automatically unattended in order to guarantee continuous data acquisition and RFI monitoring 24 h/day. This feature can produce a database statistically significant for all research that may follow.