RFI
Mitigation Project at the Italian Radio Telescopes
Abstract
Radio astronomy has been plagued by undesired interference
since its earliest days. Nowadays, the frequency bands allocated by the
International Telecommunication Union (ITU) to passive science, even if they are still
fundamental for radio astronomy, are no more sufficient to perform cutting-edge
science. Astronomers require to observe the Universe in more and more parts of the
radio spectrum, and increasing effort is being dedicated to methods for RFI
detection and excision.
In recent years the impact of RFI has increased enormously
at the Italian telescope sites, Medicina, Noto and SRT, not only due to its
abundance and increasing appearance at higher frequencies, but also because of
the more frequent usage of our antennas for single-dish wide band radio continuum
observations. In contrast to interferometric techniques, single-dish observations
are in fact more vulnerable to RFI as astronomical and RFI signals are
coherently added. Moreover, the higher sensitivity of the SRT makes this
antenna even more vulnerable to undesired artificial emission. Various methods for
the mitigation of these signals can be utilized at different levels during the
acquisition of data. These range from anticipatory methods to change the local
RFI environment by means of regulating manners up to the excision or
cancellation of RFI signals in the post-detection stage.
In 2013 the Italian National Institute for Astrophysics
funded a project of national interest for RFI mitigation at the Italian radio
telescope sites.
In this paper we present
the first results of a project aiming at the mitigation of one of the most
pressing problems for observational radio astronomy in Italy and the
cm-wavelength telescopes world-wide: the ever-deteriorating situation of Radio
Frequency Interference (RFI). We illustrate the campaigns conducted at the Noto
32m radio telescope and the Sardinia Radio Telescope (SRT) observing sites to
monitor the evolution of RFI at these locations in the frequency range 0.05-40
GHz. A new FPGA-based spectrometer and an offline software tool for RFI
detection and excision are presented and their performances are summarized.
2. RFI monitoring at the Noto and
Sardinia Radio Telescopes
The locations of
the Italian radio telescopes are different in terms of their RFI environment. The
specific topography at the Medicina site leads to a full exposure of the
telescope within one of the most densely populated areas in Europe. The
monitoring of the radio astronomical frequency bands in the range 0.3-40 GHz
has been a continuous activity at Medicina since the telescope construction (see
f.i. [2], [3]) by means of a fully steerable receiving system on a 22m-high
control tower, plus a mobile laboratory. As sources of interference continue to
proliferate, and as the requirements of astronomical observations become
increasingly demanding in terms of frequency range and bandwidth, accurate and
continuous RFI monitoring is mandatory for new telescopes like the SRT or even
for future observations with existing facilities like the Medicina or Noto 32m
antennas.
Three intensive
RFI measurement campaigns were carried on at the Noto radio telescope (December
2014 [4] and May 2016) and at the SRT (November 2016) by using the SRT RFI mobile
laboratory [5]. The RFI van is a state-of-the-art laboratory equipped with an
antenna set, microwave components and a spectrum analyzer (Agilent E4464A) able
to measure frequency spectra from 50 MHz to 40 GHz and in both linear (vertical
and horizontal) polarizations.
An accurate
monitoring of the RFI environment around both the Noto and SRT sites was
performed with the same modus operandi. First, a suitable location on the top of
a hill close to the telescope, was chosen for a free-space 360°-azimuth
measurement. Two frequency spectra for each polarization were taken after a
360°-azimuth scan (one minute long) by setting the spectrum analyzer first in a
broad (3 MHz resolution) and then in a narrow (100 KHz resolution) bandwidth configuration
and both in max-hold acquisition mode. The broad band configuration allows the
acquisition of different kinds of signal (including impulsive and fast
transient ones) with a system sensitivity of about -85 dBm. The narrow band
configuration is used to improve the system sensitivity (up to -100 dBm) to
better measure the bandwidth of each signal. Therefore, four spectra were taken
for the bands listed in Table 1 and 2 for the SRT and Noto telescopes respectively.
An accurate characterization of the signals has been performed for all the
spectra. Transmission type (continuous wave or impulsive), polarization,
carrier and bandwidth, peak level direction and periodicity (for impulsive
type) are the main signal features used to depict the RFI environment around
both telescopes.
A second acquisition has been done with the
spectrum analyzer in the aforementioned configuration, this time at a distance
of 100 m or less from each telescope site. In this way it has been possible to
compare homologous spectra of the same RFI environment acquired at two
different locations and to find out the actual signals received by the radio
telescopes, including the site self-generated RFI from power lines, electronic
apparatus, radio frequency sensors, etc. In such a way we could also compare
the RFI environment at the two telescopes in terms of both frequency occupancy percentage
(with respect to the whole bandwidth of each receiver) and type of service sharing
the frequency band with the radio astronomical receivers (see third and fourth
columns in Tables 1 and 2 for SRT and Noto respectively).
In general the
SRT receiver bands turn out to be more polluted than the Noto telescope ones,
with the SRT coaxial P-L receiver being the most affected one. With some
differences between the two telescopes, the P- and L bands are affected mainly
by aeronautic digital links, self-generated RFI, Digital Terrestrial Television
(DTT), cellular phone network, radar and satellite signal. Sporadic wide-band radio
emissions from power lines can be received in these bands, however they have
not been taken into account in the frequency occupancy calculation in Table 1
and 2. The other frequency bands are essentially clean except for some RFI due
to HiperLAN and digital links (SRT C-band and Noto S- and Low C-band), some
cellular phone network digital links in the SRT K- and X-band respectively and also
surveillance radar services. Finally, it is worth noting that, thanks to these RFI
monitoring campaigns, unauthorized HyperLAN stations were found out at a
village close to Noto. After reporting them to the local administration, they
were switched off and hence not included in Table 2.
3. On-line mitigation: the FPGA-based WBLGB spectrometer
In the last years there has been a great rise of
different real-time digital backends, generally enabled by the adoption of
field programmable gate array (FPGA) technology as the main computing engine.
These architectures are suitable for a simple pipelined elaboration of a stream
of digital data and adapt very well to the case of radio astronomy. In this
evolving digital domain, the Medicina radio telescope staff has developed great
experience in the usage of FPGA-based systems as developed by the CASPER consortium
[6]. The adoption of these instruments has enabled the research of new
solutions in the fields of beamforming and digital FX correlation [7] and can
easily be applied to single-dish data elaboration for our purposes.
A complete
hardware system for our RFI processing chain must be composed of three
elements: analog to digital conversion; real-time elaboration in the digital
domain; transmission of resulting data to the telescope control system for
further elaboration and storage. All of this can be accomplished by means of a
single ROACH board (Reconfigurable Open Architecture Computing Hardware, [8]),
equipped with two iADC converter boards, providing us with a cost- and power-effective
solution, easily scalable and upgradable to the next hardware generations with
really little effort.
In Medicina, a
new spectrometer named WBLGB has been developed as a firmware operating on a
ROACH board in order to identify RFI signals. A lot of effort has been
dedicated to make this implementation well suited for RFI investigation. First,
the spectrometer adopts Polyphase Filter Banks which help to isolate the
interfering signal in terms of frequency without artificially polluting the
adjacent regions of the spectrum. In addition, the spectrometer implements logics which help to control any eventual
digital overflow caused by extremely powerful signals being processed: digital
gains can be applied along the elaboration chain making it robust in front of
strong RFIs. Lastly, the time domain is carefully taken into account: data are
timestamped in real-time by FPGA logics directly connected with the laboratory’s
standard frequency clock and Pulse Per Second reference. The spectrometer can thus
operate with a time resolution up to one millisecond per spectrum (see Figure 1
for an example). With this instrument we carried on different campaigns at the Medicina and SRT radio telescopes
observing well known RFI sources and trying to characterize those directly
using the radio telescope.
4. Off-line mitigation: the Dish Washer software tool
Dish Washer (DW) is a Python-based software tool aimed
at the offline detection and flagging of RFI in data collected from single-dish
radio telescopes. DW is composed by 3 Python sub-packages in charge of
different functionalities and responsibilities, and a C-programming library for
the implementation of efficient RFI detection algorithms. The dw.gui sub-package implements the Graphical User Interface (GUI)
allowing visualization, inspection and manual flagging of data. The dw.core sub-package implements the core functionalities
of the package, like the definition of data structure and I/O, manual and
automatic flagging, etc. The dw.flag sub-package provides a framework for the implementation of RFI detection
algorithms in Python or, for the sake of computational efficiency, in C
language. This last possibility is offered via the libdw library which provides initialization functions and some
basic flagging algorithms.
Particular attention has been devoted to the development
of a user-friendly GUI to allow for data inspection by means of cross sections,
contrast adjustment, etc. The GUI handles and summarizes observational metadata
useful for the flagging process, like observing date and time and the antenna
pointing coordinates. The flagged regions may be manipulated by means of GUI
tools for their creation/deletion, visualization, merging.
Modern
software tools must cope with the complexity and large amounts of data
delivered in each observation by state-of-the-art radio astronomical
instrumentation. Therefore DW has been designed to make the flagging process as
efficient as possible by including dedicated functionalities for the
propagation of the flagging matrices among different selected datasets, both for
multi-feed data
and multiple spectral sections. Finally, in order to exploit
modern multi-core/multi-thread processors and efficiently run extremely
expensive RFI detection algorithms on large amounts of data, the DW code
supports parallelization through the use of OpenMP [9] directives.
DW has been designed to be as flexible, re-usable and
expandable as possible in terms of support to different input data formats and
implementation of new RFI excision methods.
Currently DW handles the standard FITS data format common
to the three single-dish Italian radio telescopes and offers manual interactive
flagging plus basic automatic RFI detection through dedicated algorithms. Support
to other data formats can be easily provided by adding appropriate software
modules to DW. In principle, I/O classes can be implemented for any kind of
storage format, including text files, sql DBMS, HDF5 etc. Easy integration of
new RFI detection algorithms, developed both in Python and C programming
languages is possible as well. The first public release of Dish Washer is
foreseen as free software under the GNU General Public License version 3 (or
later).