Development of Very Low Frequency (VLF) Data Acquisition System Using Raspberry Pi 1
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Y.L. Soon , K.B. Gan & M. Abdullah Department of Electrical, Electronic & Systems Engineering, UKM, Bangi, Malaysia 2 Space Science Center, Institute of Climate Change, UKM, Bangi, Malaysia
[email protected]
Abstract — Space Space
weather is an important area of research. The Space Weather Monitor program was introduced by Stanford SOLAR Center with the aim of building and distributing inexpensive ionospheric monitors (SID). These are used to detect changes to the Earth’s ionosphere which are caused by solar flares and other ionospheric disturbances that might affect very low frequency (VLF) (VLF) radio propagation. SuperSID that has been developed by Stanford SOLAR Center for SID detection is a low cost but sensitive space weather monitor which is currently being used around the world. Since 2010, UKM has participated in the “International Space Weather Initiative (ISWI)” and this program has given positive impact and triggered Malaysian students’ interest in space science at the secondary school level. In 2012, researchers at University Kebangsaan Malaysia (UKM) succeeded in building their own VLF receiver system, known as the UKM-SuperSID for SID detection, effectiveness determination and development of a teaching kit for SuperSID Introductory Project. However, for both systems, a desktop PC is needed in the existing system to run the SID program and to save the data for further analysis. Thus, power consumption, cost and size of the computer are becoming limiting factors for educators in Malaysia who wish to attract the interest of the young students in Science, Technology, Engineering & Mathematics (STEM) education. This is mainly because the UKM-SID system currently comprises of a loop antenna, preamplifier, sound card and a computer where the computer is required to capture, analyze and save the data, making it difficult to bring it to schools and the community. To overcome this problem, a portable VLF data acquisition system using Raspberry Pi was proposed to detect SID in this study. This system consists of a VLF receiver, preamplifier, analog to digital converter (ADC) and Raspberry Pi. The acquisition software was compiled with Python and run in the Linux environment. As a result, a portable VLF acquisition system using Raspberry Pi has been successfully developed which is able to detect and monitor frequencies transmitted from FTA (16.8 kHz), France and NWC (19.8 kHz), Australia.
I.
Free electron in the ionosphere reflects very low frequency (VLF) radio signal to allow radio communication over the horizon and around our curved Earth. The strength of the received radio signal changes according to how much ionization has occurred at the ionosphere where the VLF waves reflect from [1]. Hence, field strength monitoring of VLF radio wave transmission via the ionosphere of the Earth is an important ground base to study solar flares and their effects on VLF wave propagation within the range of 3-30 kHz. At VLF, both ground and ionosphere are good electrical conductors and forms a spherical earth-ionosphere waveguide [2]. The unique VLF propagation is used over the globe for stationary naval vessel to communicate with the divers as the wave can penetrate into the oceans. Besides, scientists have used the frequencies of VLF to study the natural phenomena that occur on land and in space [3,4]. Sudden Ionospheric Disturbance (SID) occurs in association with the ionospheric electron density variations and has long-lasting effects on the ionosphere [3]. Each layer of the ionosphere has a c haracteristic critical frequency that relates to the number of electrons in the layer. The electron density in the ionospheric layers varies mainly with the X-ray radiation created during the solar flares. During the high solar activity periods, large amount of X-ray and EUV flux strikes the D-region heights of the ionosphere (from 50 km to 90 km) and increase the ionization rate hence electron density [4]. These occurred of disturbances have an influence on the propagation of VLF radio wave. Stanford SOLAR Center has introduced a Space Weather Monitor program with an aim to build and distribute an inexpensive ionospheric monitors (SID) that used to detect changes to the Earth’s ionospheric caused by solar flares and other ionospheric disturbances [5]. Currently, most people around the world are using the low cost but sensitive space weather monitor, named as SuperSID that has been developed by the Stanford SOLAR Center for SID detection [5]. In 2012, researchers University Kebangsaan Malaysia (UKM) had succeeded to build out their own VLF receiver system, known as UKM-SuperSID for SID detection, effectiveness determination and development of teaching kit for SuperSID Introductory Project [6,7]. For both systems, a desktop PC is needed to run the SID program and save the
Keywords-SuperSID, VLF acquisition system, Raspberry Pi, Python, ionosphere
T
I NTRODUCTION
data for further analysis. As such, it is diffic ult to bring it to the school and the community to promote pace science to our school children due to its size, c st and power consumption. In order to increase our you ng generation’s interest in Science, Technology, E ngineering & Mathematics (STEM), an easier way to s etup a portable space weather monitoring system is developed and promoted to them.
A. Loop Antenna The loop antenna as shown in Figure 2 (built by PERMATA PINTAR stude ts, UKM) was used to capture the VLF signal from variou s VLF transmitter stations. The antenna was made up of low cost PVC frame and 20 turns of #26 AWG wire, with the size of 1 meter square. RG58 coaxial cable was used to c nnect the antenna output to the UKM-SuperSID preamplifie r.
Thus, the purpose of this project is to de elop a portable VLF data acquisition system using Raspber y Pi. Raspberry Pi with small form factor is used to replac e the huge size desktop PC. The system consists of VLF receiver, preamplifier, ADC, Raspberry Pi and LCD display. Peripheral devices can be accessed through t he Raspberry Pi with the Python. SuperSID is an open source software which written in Python. It is available from the Gi tHub website to acquire data from connected devices a d display the collected data. Several steps in setting the c onfiguration file of SuperSID have been carried out, so that he software can run properly [8]. II.
METHODOLOGY
The project divided into two main part , hardware and software development. The hardware part consists of an antenna, preamplifier, a USB sound card, R aspberry Pi and LCD display. For software part, the Sup erSID software written in the Python has downloaded in aspberry Pi to capture the signal from the desire station . The Supersid software is open source and available down loaded from the website https://github.com/ericgibert/supers id [9]. Fig. 1 shows the block diagram of the VLF data ac quisition system which consists of a power supply, loop antenna, preamplifier, ADC, Raspberry Pi and LC D display. The distant VLF signal is received by the loop a ntenna and very low induced output voltage is amplified by t he preamplifier. A 9V adapter was used to supply the power to the preamplifier. The amplified VLF signal was sampled at 44.1 kHz using a USB external sound card ( Creative Sound Blaster X-Fi Go! Pro). Thus, the data acqui sition system is able to detect signals with maximum freque cy up to half of the sampling rate, 22.05 kHz. The received VLF signal was processed and stored in Raspberry Pi usin g the SuperSID software.
Figure 2. Loop Antenna
B. UKM- SuperSIDPi Prea plifier The UKM-SuperSID Pi reamplifier was built based on the SuperSID preamplifie developed by the Stanford SOLAR Center [6]. The U M-SuperSID Pi preamplifier is redesigned into a small d uble layer PCB (32.55x47.01 mm2 ) and which is able t connect to the USB external sound card and Raspberry Pi easily [5]. In this work, the surface mount devices (SM D) components were chosen to solder on the preamplifier ue to small form factor of the preamplifier. The schematic diagram (Fig. 3) and PCB layout (Fig. 4) of the prea plifier were redrawn using the electronic CAD software.
Figure 1. Block diagram of VLF monitoring system Figure 3. Schematic diagra
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of UKM-SuperSID Pi preamplier
The USB sound card was added in the configuration file using command “Card = Pro ”. III.
R ESUL AND DISCUSSION
A. UKM-SuperSIDPi Prea plifier PCB Board The newly designed PCB board has a smaller form factor (32.55×47.01 mm) compar ed to the existing ones from Stanford SOLAR Center as shown in Fig. 5. The original SuperSID preamplifier has gain around 1000 [5]. The performance of the UKM -SuperSID Pi preamplifier was carried out using a function generator with the frequencies swept from 2-200 kHz wi th amplitude of 20 mV. The amplifier gain can be determ ined using Eq. 2.
Figure 4. PCB layout of UKM-SuperSID Pi reamplier
C. USB Sound Card Configuration The sound card is needed to convert the signal from analog to digital. Creative Sound Blaster X- i Go! Pro sound card was used to capture and sample the mplified signal from the preamplifier. The USB sound ca d is capable to sample the amplified signal at the samplin g rate of 44100 kHz. The sound card setting was done in th LXTerminal in order to make sure that the USB sound card was detected by Raspberry Pi [10, 11].
(2)
where preamplifier ou tput voltage.
D. Raspberry Pi and Display Raspberry Pi is a credit-card sized comp uter that can be used in electronics projects and works as the desktop PC, for example playing high-definition video [8] . The Raspbian Wheezy operating system was installed int the Raspberry Pi using a 32 GB Class 10 SD card. Cl ss 10 SD card supports 10 MB/s non-fragmented sequen ial write speed which enables the high speed bus mode. LCD monitor was used as the display for Raspberry Pi thr ough the HDMI interface. E. SuperSID Software on Raspberry Pi Figure 5. UKM-SuperSID Pi PCB Board
SuperSID software is an open source so ftware available from GitHub (https://github.com/ericgibert/ supersid). Since the SuperSID software was written in Pyth n software and maintained by Eric Gibert [9]. There are ext a modules need to be installed on Raspberry Pi in order to r n the SuperSID software. These modules are matplot lib, wxPython (wxgtk2.8), numpy and alsaaudio. Matplot lib was used to plot the SID data and Advanced Linux Sou nd Architecture (alsaaudio) has full support for capturing a d playing back the audio signal. Numpy is a general purpose array processing package to manipulate large m lti-dimensional arrays of arbitrary records. The wxPython i s a GUI toolkit that allows users to create programs with h ighly functional graphical user interface. Before execute the Supersid software, t e configuration files (supersid.cfg) needs to be properly configured to monitor VLF signals from the desire sta ion. Command “viewer=wx” is used to enable the graphic l user interface (GUI) mode. The GUI mode requires more resources from Raspberry PI compared to the text mode tha can be enabled via command “viewer = text”. The command line “audio_sampling_rate” in the sampler.py nd supersid.cfg files has to be changed to 48000 Hz de pending on the sampling rate supported by the sound car . The acquired data will be saved in the pre-configured dat _path directory.
The gain versus frequenc y plot is shown in Fig. 6. Based on Fig. 6, the UKM-SuperS ID Pi preamplifier has low pass filter characteristic with a cutoff frequency at around 25 kHz.
Figure 6. Graph of Gain versus Frequency for the UKM-SuperSID Pi pre mplifier
B. VLF Acquisition System est The VLF data acquisitio n system consists of Raspberry Pi, UKM-SuperSID Pi pre mplifier, USB sound card is shown in Fig. 7. It was i nstalled in Level 6, Research Complex, UKM to monit or VLF signals from various stations.
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IV.
CONCLUSION
A portable VLF data acquisition system using Raspberry Pi has been developed in this project. The VLF monitoring system has successfully detected the frequencies transmitted by NWC (19.8 kHz) from Australia. This tool could be a very useful teaching aid for promoting science, technology, mathematics & space science education to the students in secondary school. The educators can easily operate the system without the need of expensive computer. Besides that the developed acquisition system can be implemented for diurnal variation, sudden ionosphere disturbance and solar flare detection. For future work, 96 kHz USB sound card namely, Wolfson audio card is suggested to be implemented to capture signals from the VLF transmitter station that has frequency more than 22 kHz.
Raspberry Pi USB Sound Card
Pream lifier Figure 7. The overall system setup
The system was connected with loop antenna and the data was logged hourly into the SD card for further analysis. The system was configured earlier to monitor VLF at 19.8 kHz transmitted by the transmitter station at NWC, Australia [5]. A distinct peak 19.8 kHz has been detected by the VLF acquisition system (Fig. 8).
ACKNOWLEDGEMENT The authors would like to thank the Yayasan Sime Darby (YSD) for sponsoring this work under the Kursi Perubahan Iklim UKM-YSD Grant: ZF-2014-019. R EFERENCES
) z H / B d ( y t i s n e D l a r t c e p S r e w o P
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[5]
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Abdullah, M., Bais, B., Hasbi, A.M., Majid, R.A., Yatim, B., Ali, M.A.M., Bahari, S.A., Daud, N.M., Mokhtar, M.H., Zain, A.F.M., & Asillam, M.F., “Development of UKM-SID teaching module for space science education”, International Forum on Engineering Education 2012 (IFEE 2012), pp. 80-85, 2012.
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Michalak, S., “Raspberry Pi as a measurement system control unit”, Signals and Electronic Systems (ICSES), 2014 International Conference, pp. 1 – 4, 2014
[9]
Gibert, E., SuperSID, https://github.com/ericgibert/supersid, 2013 [25 Ogos 2013].
Frequency
Figure 8. Clear peak 19.8 kHz detected by the VLF monitoring system
C. Comparison between UKM-SuperSID and UKMSuperSIDPi Table 1 shows the comparison between UKM-SuperSID and UKM-SuperSIDPi system. The UKM-SuperSIDPi system has lesser power consumption (10W) and cheaper (RM 120) compared to the UKM-SuperSID. It does not require any desktop which enables it for educational purpose without extra investment in the computer system. TABLE 1 Comparison between UKM-SuperSID and UKM-SuperSIDPi
UKM-SuperSID Dekstop / Laptop PC Windows OS 500 Watt RM 1 500.00 Any Soundcard (Maximum sampling rate: 96 kHz)
UKM-SuperSID Pi Raspberry Pi model B Linux OS 10 Watt (2 A @ 5 V) RM 120.00 USB External Soundcard (Maximum sampling rate: 48 kHz)
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