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INTRODUCTION

In view of the International Heliospheric Year, the Inter-national Year of Astronomy, and the International Space Weather Initiative, and with the intention for future up-grade of the Metsähovi Radio Observatory (MRO) regard-ing new radio astronomical instruments, a measurement campaign was planned and organized between MRO and ETH (Eidgenössisch Technische Hochschule) Zurich. The measurements documented here took place from Wednes-day until Friday, September 1st–3rd, 2010, at Aalto Uni-versity Metsähovi Radio Observatory, Finland, after the installation and configuration of a new Callisto radio spec-trometer.

CALLISTO NETWORK

The idea of the Callisto project is to produce a simple and low-cost instrument, which can be sited in places of low infrastructure or remote locations. The e-Callisto network at the moment consists of sixteen different stations all around the world using Callisto spectrometers (see Fig-ure 1). The stations observe independently at frequencies, which are the most suitable for each location, for example depending on such features as local radio frequency in-terference (RFI) levels. The measurement results can be combined with each other and thus it is possible to get data from the whole frequency range (45–870 MHz) of the Callisto spectrometer because different stations use

different frequency ranges. Metsähovi is the northern most site of the network with a latitude of 60° 13’ 2.9”, thus long observations are possible especially at summer months.

STATION DESCRIPTION

The MRO is a separate research institute of the Aalto University School of Science and Technology under the Faculty of Electronics, Communications, and Automa-tion. It operates a 14 m diameter radio telescope (observ-ing frequency range 2–150 GHz) at Metsähovi, Kylmälä, Finland (see Table 1, about 35 km west of the the Aalto University School of Science and Technology campus in Espoo. The institute also has premises in the Electron-ics, Communications, and Automation Faculty building, Otakaari 5, Espoo. In the same area, near MRO, there are also the buildings of the Metsähovi Observatory (Uni-

Callisto Radio Spectrometer for Observing the Sun—Metsähovi Radio Observatory Joins the Worldwide Observing NetworkJuha Kallunki, Minttu Uunila Aalto University, Metsähovi Radio Observatory

Christian Monstein Eidgenössisch Technische Hochschule (ETH), Institute of Astronomy

Authors’ current addresses: J. Kallunki and M. Uunila, Aalto University, Metsähovi Radio Observatory, Metsähovintie 114, 02540 Kylmälä, Finland, e-mail: kallunki@kurp.hut.fi. C. Mon-stein, ETH, Institute of Astronomy, Wolfgang-Pauli-Strasse 27, CH-8093 Zürich, Switzerland. Manuscript SYSAES-201000074 received October 6, 2010; revised March 6, 2013, and ready for publication March 8, 2013. Review handled by M. De Sanctis. 0885/8985/13/ $26.00 © 2013 IEEE

Figure 1. Coverage of e-Callisto network (1.9.2010). Altogether e-Callisto consists of sixteen different stations located all over the world. This coverage enables 24 hours of observations.

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Figure 3. The Callisto receiver and PC in the receiver room at Metsähovi Radio Observatory. The power supply for the Callisto is behind the Callisto spectrometer.

versity of Helsinki, optical astronomy) and the Metsäho-vi Space Geodetic Station (Geodetic Institute, geodesy). The MRO has been operational since 1974. The research topics at MRO are: solar millimeter and microwave ra-diation, variable quasars, active galaxies, molecular line radiation, and very long baseline interferometry (VLBI). MRO is internationally known for developing VLBI in-strumentation and related software and is participating in the European Space Agency’s (ESA’s) Planck Satellite.

MEASUREMENT INSTRUMENTATION

At MRO we use the commercial logarithmic periodic digi-tal video broadcast terrestrial (DVB-T) antenna (see Figure 2) connected via a low noise pre-amplifier from mini cir-cuits (type: ZX60-33LN-S+, typical noise figure 1.1 dB, gain ~20 dB) and a low loss coaxial cable (RG214) to the mea-suring instrument, see Figure 3. The distance between the

pre-amplifier and the Callisto spectrometer is approximate 20 m. The commercial broadband antenna “Log periodic combination aerial for very high frequency (VHF)/ultra-high frequency (UHF)” covers the range from 175 MHz up to 858 MHz in one band covering VHF channels 5–12 and UHF channels 21–69. The logarithmic periodic antenna and the pre-amplifier were mounted to the antenna, which has been used for observing at centimeter lengths with a center frequency of 11.2 GHz solar bursts (see Figure 4) since 2001 [3], [4]. So, the tracking system was already in existence before the installation of the Callisto system. This speeded up the installation of the Callisto system. Because of the in-stallation of the system at MRO, it is now possible to get si-multaneous solar burst observations from both centimeter

Table 1.

Geographical coordinates of the Metsähovi Radio Observatory site in Finland.

Coordinate Value

Latitude 60° 13′ 2.9″ North

Longitude 24° 23′ 43.1″ East

Height 60.28 m above sea level

- (FIN2000 geoid model)

Local time GMT + 03 h

Figure 2. Linear, horizontally polarized, logarithmic periodic antenna is mounted on the lower rim of a microwave dish (11 GHz). The preamplifier is located directly underneath the dish antenna. The Callisto receiver is located in the nearby office of the labo-ratory, see Figure 3.

Figure 4. Observed solar burst at 11,2 GHz (August 1, 2010). The mea-surement system at MRO is called Sunant and its operating frequency is 11,2 GHz and the dish diameter is 1.8 m.

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and meter wavelengths. An overview of the whole Callisto measurement system is illustrated in Figure 5.

The Callisto spectrometer (see Figure 6) e-C34 has a detec-tor sensitivity of 25.4 mV/dB including control cables and radio frequency (rf) adapters supplied by ETH Zurich. The frequency range of Callisto goes from 45 MHz to 870 MHz in three subbands. The channel resolution is 62.5 kHz, while the radiometric bandwidth is about 300 kHz. The sampling time is exactly 1.25 ms per frequency-pixel while the integra-tion time is about 1 ms. The frequency in the output data is expressed in MHz and the detector output is expressed in mV. Both are stored in a simple ASCII file that can be ana-lyzed with any scientific data analysis program like Interac-tive Data Language, Mathematica, Matlab, or EXCEL. Every 15 min a new FIT file will be generated, which is saved auto-matically to local web server (nova). It is possible to save data for example for plotting light curves in an ASCII file contain-ing five time series of certain frequencies. Callisto uses auto-matically local sunrise and sunset times for the data saving.

It is also possible to set observa-tion times manually. Callisto is locked to an external 1 MHz ref-erence signal that is generated from a 5 MHz hydrogen maser reference signal. Therefore it is possible to get systematically sampled data files.

At MRO, an additional mea-surement was taken by con-necting a 50 Ω resistor. This measurement is a reference sig-nal used to evaluate the power level in dB above this broad-band load (a passive system at ambient temperature in the or-der of 290 Kelvins). In addition two different levels of a noise sources were applied for cali-bration purposes.

RESULTS

SPECTRAL OVERVIEW AT METSÄHOVI RADIO

OBSERVATORY, FINLAND

The total measured spectrum is shown in Figure 7. It is composed of 13,200 channels each 62.5 kHz apart. In Figure 7, −120 dB is referenced to the background noise level given by a 50 Ω resistor (termination) at 300 Kelvin. Beside ter-restrial transmitters like frequency modulation (FM)-radio and DVB-T we can recognize a lot of noise caused by local electronic systems. Also some local transmitter (data links of Schengen police communication) can be identified. The green color shows signal from sky and environment, the blue color shows the cold noise source at 290 Kelvin, the orange color “warm” noise source at +5.3 dB ENR, and the red color ’hot’ noise source at +15.3 dB ENR as reference levels.

Figure 5. Block diagram of the Callisto measurement system at MRO.

Figure 6. A closer look at the Callisto spectrometer, a compound low-cost spectrometer based on commercial TV tuner.

Figure 7. The spectral overview measured at Metsähovi Radio Observa-tory, September 3, 2010.

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Cal l isto Radio Spectrometer

Figure 9. The first strong observed solar radio burst (September 24th, 2010, 14:10:42 UT) at Metsähovi Radio Observatory. The ob-served radio burst was a type I solar flare.

FURThER READING: RELEvANT INTERNET ADDRESSES

C MRO – http://www.metsahovi.fi/en/

C MRO—Solar Research – http://www.metsahovi.fi/en/sun/

C Local data depository – http://www.metsahovi.fi/callisto/

C CALLISTO – http://soleil.i4ds.ch/solarradio/

C IHY – http://ihy2007.org/

LONG TIME OBSERVATION WITH CALLISTO

Although the Sun was not active at the time of installation and configuration, we observed the sky for several hours. Four 15 min FIT files combined to two hours of observa-tion, representative of a series of similar observation files, is shown in Figure 8. This was done to allow the identification of possible cross modulation by strong transmitters in the FM band and nearby DVB-T transmitters. Actually no cross modulations due to nearby civilian and military transmit-ters were detected during the time of observation. All data is stored in local depository on an http server of Metsähovi. The data containing flares will also be transferred automati-cally to the Callisto data archive at ETH in Zurich, Switzer-land [5].

FIRST LIGHT—SOLAR RADIO BURST

The very first radio solar bursts were observed September 18, 2010. The first detected bursts were really weak type III solar flares, which were visible at the frequency range of 175–225 MHz. The first stronger radio burst was detected September 24, 2010 (Figure 9). This was a type I solar flare, which was visible at the frequency range of 270–330 MHz. The bright-est pixel at 300 MHz is 9.3 dB above background noise level. The same flare was observed simultaneously with Callisto in Bleien/Switzerland (BLEN), Ondrejov/Czech Republic (OSRA), Humain/Belgium (ROB), and Mexico (UNAM). By September 31, 2010, a total of four solar radio bursts were observed.

CONCLUSIONS

The radio spectrum at MRO suffers from local electron-ics, broadband television, DBV-T, FM-radio, and some lo-cal data-links of nearby Schengen police communication and hence Metsähovi in the present situation is not ideal as a host site for a solar frequency agile or even to have an FFT spectrometer used at low frequencies. The gen-

eral situation will be notably improved by installing the whole system at a more remote location or by improving the shielding of nearby electronic equipment. Also during the weekends, RFI-levels seem to be notably lower than during the office hours. Nevertheless it will be possible to detect strong flares with more than 50 sfu. A larger an-tenna with more gain would improve the situation dras-tically due to the fact that the signal-to-noise ratio of the solar flares would improve. Also a pre-amplifier with bet-ter noise figure and larger gain could improve the whole system’s noise temperature. The most harmful interference frequencies are generated by the local electronics. Howev-er, during the observations the antenna is pointed away from the laboratory buildings. Thus the spectra is reason-able during observations, especially at higher frequencies (>300 MHz). The first results have shown that the measure-ment system is operating reliably and it can detect some weak solar radio bursts.

ACKNOWLEDGMENTS

We especially want to thank the Metsähovi Radio Obser-vatory crew for providing electronic and mechanical com-ponents as well as for their workmanship in preparing the antenna and pre-amplifier. We thank the MRO for travel support during the campaign. We also thank Andreas James for the manufacturing and testing of Callisto eC34 spectrom-eter.

Figure 8. Two hours of observation at Metsähovi Radio Observatory on September 3, 2010. The x axis denotes time, expressed in UT, and the y axis denotes frequency in reversed order.

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REFERENCES

[1] Metsähovi Radio Observatory, http://www.metsahovi.fi.[2] Benz, A. O., Monstein, C., and Meyer, H. CALLISTO, A New

Concept for Solar Radio Spectrometers, Kluwer Academic Publish-ers, The Netherlands, 2004.

[3] Puhakka, P. Total Power Radiometer in Solar Microwave Research, Master’s Thesis, Helsinki University, Department of Physical Sciences, 2002.

[4] Kallunki, J. Possibilities of the Metsähovi Radiotelescopes for Solar

Observations, Licenciate Thesis, Helsinki University of Technol-ogy, Department of Applied Physics, 2009.

[5] ETH Zurich, http://soleil.i4ds.ch/solarradio/.

PIONEER AWARDSThe AESS Pioneer Award announcements and awardee contributions with de-tails of their award have been transferred to this title from our sister publication, the IEEE Transactions on Aerospace & Electronic Systems – TAES, starting with the 2012 award, which was contained in this SYSTEMS Magazine, AESM 28, 5, May 2013.

The 2011 Awardee, James V. Leonard, was announced on the cover of the Oc-tober 2011 issue of AESM; his contributions concerning details of the award, although prepared, have not yet been received for publication; they will appear when released.

A history of the Pioneer Award will be found on page 41 of the May 2013 issue of SYSTEMS together with a listing of all prior awards.