SENTINEL 2

A new system for RFI monitoring

J. Monari1, S. Montebugnoli1, C. Bortolotti1, M. Cecchi1, M. Roma1, S. Mariotti1 and K. Jiricka2

1 CNR IRA, Bologna (ITALY)

2 Astronomical Institute AVCR, Ondøejov (CZECH REP)

Abstract

The SENTINEL 2 is conceived as a fully automatic system for continuous RFI monitoring activities in the 0.3 – 2 GHz frequency band. In this article we describe the prototype of the system and to discuss the possibilities of its future upgrade.

Introduction

Nowadays the very crucial problem for radioastronomy is represented by the continuous growth of electromagnetic pollution in the frequency bands traditionally reserved for radioastronomical observations (ranging from tens of MHz up to hundreds of GHz). The high sensitivity of the radiotelescope plays in this case a negative role making them very vulnerable to the man made electromagnetic pollution. Therefore one of the most important activity at the radiotelescope sites is continuous monitoring, handling and, when possible, mitigating of the RFI (Radio Frequency Interference). This difficult task must be faced from different points of view: technological, organizational and political.

The Medicina RFI monitoring systems

At the Medicina Radioastronomical Observatory we use, at present, two different systems devoted to RFI monitoring:

  1. The fixed RFI monitoring system is composed a 25 m high antenna tower with a broadband antenna system, a RF amplifiers and antennas pointing control system, a broadband communication receiver ICOM IC-R9000, a broadband IFR-A8000 Spectrum Analyzer, and systems for data storage and plotting.
  1. The mobile RFI monitoring system is composed of a FIAT Ducato TDI van equipped with a broadband antenna and RF amplifiers system, an ICOM IC-R9000 receiver, a HP8562A 26.5 GHz spectrum analyzer, and other systems for the mobile unit logistic operation.

The SENTINEL 2 system for RFI monitoring in the 0.3 – 2 GHz frequency band is now under construction. This system is not intended as a measurement system but only as a “watchdog” that continuously informs the observer about the RFI situation in the bands of interest and, at the same time, keeps a RFIs log files updated

While the first two systems, periodically used in order to both detect and determine the position of the interfering source, require a crew of two operators, the SENTINEL 2 system will be developed for fully automatic round the clock RFI monitoring and RFIs log files updating.

The Sentinel 2 system

Considering a normal scanning receiver with 80 kHz of frequency resolution, it takes 21.250 steps to sweep the 0.3 – 2 GHz band. Since each step takes some time, the obtained RFI scenario at the end of the sweeping cycle may be different from the real situation. Scanning the same band with a 20 MHz window (downconverted into the video band and digitized at about 40 Msamples/s) it is possible to compute a power spectrum over a programmable number of channels (e.g. 256 in order to obtain a frequency resolution ~80 kHz of the previous case). Since the usable bandwidth is only 15 MHz wide (2-17 MHz), we have to compute overlapped spectra. In this case the number of steps requested to sweep the overall band isreduced by a factorof about 190 for each direction. If the FFT is computed fast enough, a good RFI scenario, much closer to the real one than that in the previous case, isobtained. In order to be able to scan all the frequencies in the 0.3 – 2 GHz range distributed in all the 360 degrees of azimuth, we plan to use 6 antennas of 60o of beamwidth looking in directions N-E, E, S-E, S, S-W, W, and N-W with E-planes tilted by 45o to the horizon (in order to be sensible to both polarization). Each antenna is connected to the system via a low insertion loss fast solid state switch remotely controlled by the main 500 MHz PC. The RF signal is directly supplied to the input of a HP 8562A. The second IF output (310.7 MHz) is down converted to 0-20MHz. The PC samples the video band signal via a fast 12 bit A/D converter (72 dB of dynamic range, 40 Msamples/s) and performs the FFT to get the power spectra. The frequency resolution can be changed by changing the number of channels. The data are stored on a large SCSI 2 HD array. All the blocks (antennas, switches, and the receiver) are controlled by the main PC via IEEE488 or RS232 ports.

Future upgrade

In order to start the on-line RFI mitigation activity we plan to use the SENTINEL 2 system as the reference receiver by scanning the six antennas in order to cover all directions (on the horizontal plane). In this way we are expecting to observe the same RFI scenario observed by the dish antenna (in this case through the secondary lobes). A FFT (or KLT) transform on the reference receiver output is computed and then “detracted” from a similar transform performed on the same bandwidth coming from the radioastronomic receiver. The result will be then transformed back to the time domain and reduced in number of bits in order to be compatible with the MK IV formatter. This could be implemented for each of the video converters of the MKIV At present this is only an idea that we consider worth to be seriously investigated. Since we need to clean very wide bands, the main system bottleneck is represented by the huge amount of digital data processed in the on line operations. At the Medicina laboratories a very fast modular Mercury ® ALTIVEC vectorial CPUs cluster, using a RACE way ® wide band CPUs interconnection, is under evaluation for this purpose. Because it is a modular system, it can be tailored (by changing the number of computing nodes) for the requested bandwidth.


Principle block diagram of the SENTINEL 2 system