Comparative measurements of radio frequency interference in the 10–250 MHz

Highlights

• Measurements of the radio spectrum in the range of 10–250 MHz in three Polish radio astronomical sites.

• Long wave radio interferences.

• Radio purity in the surroundings of the LOFAR system station located in Bałdy.

• The possible impact of IT equipment on the sensitivity of the LOFAR system.

Abstract

Mitigation of radio frequency interferences (RFI) is crucial for improving the quality of collected radio observations. For the instruments working in the decimeter and meter ranges – LOFAR (LOw Frequency ARray) is a good example – RFI is particularly vexing thus the widest possible knowledge of man-made emissions in the vicinity of radio telescopes is highly desirable. The results of the measurements of RFI performed at the LOFAR PL612 station in Bałdy as well as the Radio Astronomical Observatory in Piwnice and the Astronomical Observatory and Astronomical Education Centre in Białków are shown and a comparative analysis has been carried out. Owing to this, solutions either to suppress RFI sources or to filter out undesirable signals in the spectra have been proposed. The results will be particularly important for the development of the LOFAR system and the extension of its usable waveband to the 10–30 MHz range. The presented measurement results also suggest the necessity to introduce a continuous RFI monitoring at LOFAR sites.

Introduction

Radio astronomy is a domain of astronomy in which, from the very beginning [1], the characteristics of the astrophysical objects studied are inferred from the radio waves they emit. In the group of the most important objects and phenomena where radio waves are generated – apart from the radiation of the Milky Way observed by Karl Jansky [1] – the following can be mentioned:

• radio galaxies and quasars (see eg. [2]);

• pulsars [3];

• cosmic masers [4].

From the point of view of radio astronomical studies focused on extremely faint signals of extraterrestrial origin, it is important to note that radio waves in various frequency ranges have been the only medium used in telecommunications since the beginning of the 20th century. Although the data receiving and processing systems in modern radio telescopes are equipped to filter out unwanted signals, they are still significant obstacles. The advancement of both radio astronomy and telecommunications has resulted in the division of radio bands into domains reserved for various areas of activity [5], [6]. Nevertheless, the characteristics of radio signals mean that even when they are outside the allocated range, they can affect the receiving systems through the side lobes and radio frequency interference (RFI). All of these effects are more significant at longer radio wavelengths.

The issue of eliminating RFI from radio astronomical data is common and many ways to automate it have been developed. It is particularly important in interferometric observations [7], [8]. However, for many observations made with single instruments (both single-dish radio telescopes and phase array), data smoothing takes place in off-line processes that can be automated. There are several ways to eliminate RFI. The first is a simple identification of the corrupted part of the data received by the radio telescope and removing it individually or systematically. This is the case, for example, when observing pulsars with LOw Frequency ARray (LOFAR) stations operating in single-mode [9]. This approach applies to disturbances that occur accidentally. The second way to eliminate RFI is based on a thorough understanding of the RFI distribution and elimination of influence at the level of the initial signal processing. For the LOFAR radio telescope, the very structure of the receiving system and the division of the band eliminate, for example, VHF man-made signals [10].

Radio telescopes used today not only require a good evaluation of the construction site with special regard to the measurement of RFI [11] but also detailed studies related to the impact of RFI on the scientific results acquired from the observations [12]. This paper focuses on the LOFAR radio telescope — it is described in paragraph 3.1. RFI measurements were performed in each planned location of the LOFAR system stations to ensure that there would be no burdensome obstacles in low-frequency observations [13].

In recent years, skillful RFI analysis of radio astronomical observations can be essential for certain specific observations. An example is SETI, which requires a very broad knowledge of the RFI environment at the instrument [14]. New techniques related to the statistical treatment of RFI during observation are also implemented and applied, an example of which is the system implemented at the RATAN-600 radio telescope [15]. To eliminate RFI, they must first be well recognized and flagged, and this can be accomplished by algorithms in information systems based on machine learning techniques [16], [17].

In the case of the LOFAR radio telescope, the ionosphere is also an important source of signal distortion and it is crucial to model/include its influence during instrument calibration [18], [19], although the current study only deals with unwanted RFI signals recorded in the spectrum during control measurements.

Although a location distant from technical and telecommunication installations was selected for the construction of the PL612 station of the LOFAR system [20], many RFIs can be found in the data. In the case of interferometric observations, local RFI does not play a significant role, as the data analysis involves the correlation of signals received at different sites (and the uncorrelated interference is simply neglected), whereas in observations in single-station mode, they are very undesirable and may distort the scientific results.

Two examples are also presented of the RFI influence on the results of the observations made with the LOFAR PL612 station in Bałdy. In 1 the results of the observations of the bright pulsar PSR J0332+5434 are shown. The panels on the left show the data that has not been purged from RFI. The procedures used in PL612 for pulsar observations are typical and have been described previously [9]. In brief, pulsar observations are processed using a 3-dimensional median filter. The filter uses a moving window that calculates the median of the values contained in the window. In the current study, a three-cell-long window was applied to remove most of the noise. The RFI cleaning procedure is essential and affects the final result, which is related to the signal-to-noise (S/N) ratio. This is very clearly seen in the bottom (e and f) panels of 2. It is worth mentioning in this point that in panels e and f, the Y axis represents a relative value related to the system characteristics, which did not reflect the real flux density. For the observation of pulsars, the key value is the S/N value, which is significantly influenced by the RFI.

The LOFAR system is extremely sensitive to the small changes in the signal level caused by ionospheric scintillations. This is used in a special type of observation where changes in the dynamic spectra of bright, compact galactic and extragalactic sources of radio emission are studied [21]. When studying the subtle effects of the ionosphere’s influence on the signal flux density received by antennas, it is very important to separate the RFI, which can have a significant impact on the measurement results. The scintillation cleaning process includes three steps. In the first step, a median filter limited to every single channel is applied. The observations are then flattened by the 10th percentile and the remaining spikes are removed by a standard deviation check across a pass-band. In the final step, the dataset is detrended. This is performed by applying the moving average in a three-minute-long window with a step, equal to the measurement interval (20 ms) 3.

It should be added that using the LOFAR PL612 station, measurements of the influence of unevenness and dynamics of the ionosphere on the pulsar signal are also conducted [22]. In this case, the S/N ratio (which influences the measured signal of the pulsar) must be maximized by a thorough RFI reduction.

Main goals of the work” has been added: “Our work has two main goals. The first one scopes to check the RFI conditions in the 10–250 MHz frequency range at selected locations. Radio telescopes are located in Piwnice and Bałdy. In the case of Baiłków there are plans to build one in the near future. The appropriate measuring equipment has been assembled and software has been developed for this purpose. The second goal is to obtain the answer: whether the RFI conditions of the LOFAR station environment require the implementation of the RFI continuous monitoring and the installation of special equipment (antennas and analyzers) (see Fig. 4, Fig. 5).