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skCUBE very-low-frequency radio waves detector and whistlers

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skCUBE Very-Low-Frequency Radio Waves Detector and Whistlers
Michaela Musilová
Slovak Organisation for Space Activities (SOSA)
Faculty of Electrical Engineering and Information
Technology of the Slovak University of Technology in
Bratislava
Bratislava, Slovakia
e-mail: musilova@sosa.sk
Miroslav Šmelko
Department of Aviation Technical Studies
Faculty of Aeronautics, Technical University of Košice
Košice, Slovakia
e-mail: miroslav.smelko@tuke.sk
Pavol Lipovský
Department of Aviation Technical Studies
Faculty of Aeronautics, Technical University of Košice
Košice, Slovakia
e-mail: pavol.lipovsky@tuke.sk
Jakub Kapuš
Slovak Organization for Space Activities (SOSA)
Bratislava, Slovakia
e-mail: jakub.kapus@sosa.sk
Ondrej Závodský
Department of Telecommunications and Multimedia,
Faculty of Electrical Engineering, University of Žilina,
Žilina, Slovakia
e-mail: zawin@svetelektro.com
Rudolf Slošiar
Slovak Organization for Space Activities (SOSA)
Bratislava, Slovakia
e-mail: rstrudy@gmail.com
AbstractskCUBE is 1U cubesat designed and developed in
Slovakia over a period of years from 2012-2015 with expected
launch in to the low-Earth orbit in 2017. It carries an
experimental very-low-frequency (VLF) radio waves detector,
which will attempt to record whistlers radio signals from
dispersed radio frequency components of lightning's. A lot has
been found about these radio waves caused by classical
lightning in past already, but there is not much known about
VLF radio from rarer terrestrial luminous events (TLE) like
gigantic jets. Gigantic jet is a type of lightning directed
upwards between storm clouds and lower ionosphere. We will
attempt to use VLF data from this detector to learn more
about these phenomena. However, in essence we want to study
peculiarities in all detected VLF spectrums regardless of type
of lightning together with properties of Earth’s ionosphere.
Due to design limitation of skCUBE, we placed the VLF
detector inside the satellite. The VLF detector consists of air-
core loop antenna, trans-impedance amplifier and micro-
controller capable of signal processing directly on-board. It
works in two modes, slow and fast. Slow mode will detect
gradual changes in VLF radio on timescales comparable to the
whole orbit of satellite (< 90 minutes), whereas the fast mode
will be capable of fast sampling of short events like whistlers (<
0.1 seconds) based on excess in pre-determined power flux
density threshold.
Keywords: cubesat, detectors, VLF, whistlers
I. INTRODUCTION
Radio waves of very low frequencies (VLF) have
intrinsically very long wavelength in the order of kilometers.
For a detection of such signals, one can therefore utilize an
antenna sensitive to magnetic component of electromagnetic
wave known as the magnetic loop. This type of antenna is
together with further electronics tailored for VLF reception
installed on-board of skCUBE [1] as well. Similar receivers
typically placed on satellites are devices composed from
coils equipped with high permeability core and placed along
three axis perpendicular to each other called search coil
magnetometers [2]. We could find them for example on
satellites DEMETER [3] and THEMIS [4]. One of their tasks
was to listen to radio signals from electric discharges in
Earth’s atmosphere. Such signals are interesting because they
reflect properties of their source and characteristics of
environment through which they propagate. Their origin
stems in storm activity causing classical lightning between
clouds or clouds and ground. However, a storm occasionally
does create another type of lightning called transient
luminous event or TLE, which is an electric phenomenon
appearing above the clouds [5]. Data from detectors like the
one on-board of skCUBE can potentially enrich our
understanding of this phenomenon for example together with
project ASIM - Atmosphere-Space Interaction monitor [6].
Its task will be monitoring of TLEs from ISS (International
Space Station) using cameras. Expected launch of ASIM is
scheduled for 2017. However, lightning is not the only
source of VLF in near-Earth environment. Among other
sources, the Earth’s magnetosphere is also a VLF source
due to its interaction with charged particles of solar wind.
Human activity is also strong source of VLF. There are
many transmitters Earth-wide transmitting in this radio band
constantly as we speak.
II. DETECTOR
The detector consists of three main parts, an air-core loop
antenna, trans-impedance amplifier (TIA) and
microcontroller unit (MCU). The antenna serves as a
container for electric signal induction due to changing
magnetic component of electromagnetic waves going
through it. The TIA amplifies the signal and passes it to the
MCU, which does its sampling and Fourier transform
directly on-board. Advantage of this detector is weight of its
antenna, which is only 26 grams. For example, search coil
magnetometer placed on-board of DEMETER satellite is
almost 0.5 kg [7]. However, we found that noise of our
detector is dominated by the MCU. Therefore, it had to be
intentionally set to 14 MHz only in order to balance required
performance and the noise level. This MCU clock speed is
enough to achieve 200 kHz sampling frequency and
sufficient computing capacity to execute the spectral analysis.
We estimate whistler magnetic induction amplitudes in
range from 2 to 60 pT by calculation of the magnetic
induction amplitude of whistlers detected by the satellite
DEMETER [7], based on reported amplitudes of their
electric field intensities from 0.01 to 0.3 mV/m [9]. These
values were calculated from equation (1), which can be
derived from Maxwell equations in dielectrics [e.g. 9].
E
c
n
B
(1)
The B is the amplitude of magnetic induction (in Tesla),
E is the amplitude of electric field intensity (in V/m), c is the
speed of light (3 x 108 m/s) and n is the local phase refractive
index, which we averaged for plasma at DEMETER’s orbit
to 60.
Figure 1. Printed circuit board with detector’s MCU.
A. Antenna Coil
Antenna coil has no core and consists from 1000 turns of
copper wire with area of 40 cm2. The width of winding is 5
mm. Diameter of the wire is 0.12 mm. The measured total
inductance H is 149 mH and resonance frequency fR = 189
kHz, which from equation (2) gave us self-capacitance C
around 4.7 pT. DC resistance is 392 .
LC
fR1
2
1
(2)
Theoretical voltage induction in the coil can be estimated
from Faraday’s law expressed by equation (3)

cos2 0rms
HNAV
(3)
V is induced voltage in Volts, µ
0 is the permeability of
vacuum equal to 4 x 10-7 H/m, N is number of coil turns, A is
the area of each winding in m2, ν is frequency in Hz, Hrms is
the RMS value of the magnetic field strength, in A/m, θ is an
angle between magnetic field vector and the coil frame
normal.
B. Transimpedance Amplifier
Transimpedance amplifier (TIA) was chosen due to the
low input impedance. Coil in short-circuit connection acts as
a current source and thanks to the low impedance the input is
resistant for electro-static noise. The low pass anti-aliasing
filter is appending the amplifier with corner frequency 28
kHz. Schematics of TIA with filters is presented on Figure 2
Figure 2. Schematic of transimpedace amplifier with low-pass filters.
Low noise JFET input operational amplifier TL072 was
used for both stages of TIA.
Figure 3. Printed circuit board and white plastic container with antenna coil.
C. Microcontroller
Microcontroller STM32F746 was used for signal
sampling and data processing. The controller is based on the
high-performace ARM® Cortex®-M7 32 bit RISC core. The
core features a single floating point unit with implemented
full set of DSP instructions. The microcontroller is capable to
sample and analyze the VLF signal gathered by the sensing
coil.
D. Signal Processing
The amplified signal detected by the coil antenna goes
further to anti-aliasing filter with threshold frequency of 25
kHz. Internal analog-to-digital converter of a microprocessor
samples the signal at frequency of 200 kHz. When samples
reach buffer limits, FIR decimation down samples input
signal, so the output data rate is reduced by factor of 4. These
output samples are inputs to the spectral Fourier analysis
using 256 frequency bins.
E. Sensitivity
We measured sensitivity of the detector by putting it at
distance of z = 17 cm from one loop coil. It was fed by
controlled variable electric current I from a PC sound card
and measured by multimeter Fluke 287. This coil had a
radius R = 12.5 cm and was a source of the VLF signal used
for sensitivity measurement. We could calculate the
magnetic field induction amplitude B generated by the loop
along its central normal from Biot-Savart law expressed by
equation (4)
)( 2
422
2
0Rz IR
B
(4)
Then, by using program HDSDR, we watched when the
generated signal steps over the receiver's noise. We find, that
minimum amplitude of variable magnetic induction
fluctuations for signal detection above the MCU noise is B =
5.3 x 10-11 T (equivalent to intensity of magnetic field 4.2 x
10-5 A/m) [10, 11].
F. Transfer Function
Due to delivery deadline of skCUBE, we did not manage
to get measured all characteristics of this experiment like the
transfer function on time. Hence, we measured it later on a
mockup VLF detector constructed almost in the same way as
the original but with a bit different operation amplifier
AD8033BR. Our measurement was performed in a similar
way than the sensitivity measurement. We fed one loop
signal source coil with variable current of pre-determined
amplitude and frequency by AGILENT 33521A signal
generator and put the detector’s air core loop antenna in its
middle. Again, using simplified Biot-Savart law shown by
equation (5), we were able to estimate amplitude of
generated magnetic field induction.
R
I
B20
(5)
Afterwards, we measured amplitude of voltage at the
output from our mockup amplifier using HANTEK
DSO1062B oscilloscope.
III. THEORY
We expect, that strongest and most frequent VLF radio
signals to be detected will come from electric discharges in
Earth’s atmosphere (lightning and terrestrial luminous
events). In order to get to satellites in orbit, such signals have
to pass through ionosphere, which is essentially magnetized
plasma [12]. Such environment can change radio signals
original characteristics and turn it into a whistler [13].
A. Whistlers
Whistlers are VLF signals from dispersed radio
frequency components of electromagnetic impulse caused by
lightnings called “atmospheric” or “sferic” [13] and which
can enter ionosphere along Earth’s magnetic fields [8].
B. Ray-tracing in PYTHON
For whistler data analysis, we wrote a ray-tracing code in
programming language PYTHON, which we call PYRAT
(PYthon RAy Tracing) [15]. It allows us to simulate radio
waves propagation through various ionospheric models
including IRI, the International Reference Ionosphere [18].
Some of our whistler simulations are shown in Figure 4.
Figure 4. Simulated whistlers with the PYRAT code. Group delay time is
the time it takes for whistler to reach skCUBE orbit at 600 km (assumed
ionospheric entry is at 120 km). IRI stands for International Reference
Ionosphere [18]. Differences in plotted whistlers reflect properties of used
IRI models as function of day/night and various latitudes. Two whistlers
were calculated based on simple models for electrons and protons
stratification according to Yabroff [19].
By comparison of detected whistlers with these
simulations, we will be able to test accuracy of ionospheric
models and search for pecularities in energy distribution of
whistlers with focus on TLEs like the gigantic jets. TLEs are
however much rarer then classical lightning [16].
IV. SUMMARY
skCUBE is Slovak 1U cubesat equipped with radio
waves detector for VLF band from 3 to 30 kHz. It consists
from air-core loop antenna (weight only 26 grams), TIA
amplifier, anti-aliasing filter and MCU performing signal
processing directly on-board. With such hardware
configuration we are hoping to reach a of sensitivity 53 pT of
magnetic fluctuations. This sensitivity should be enough to
detect TLEs because of lower orbit of skCUBE (approx. 500
km) in comparison with DEMETER (710 km). Measured
data will be recorded, sent to the ground station and stored
for future use. Based on the data from detected whistlers, we
expect the possibility to study pecularities in ionospheric
models and energy distributed in electromagnetic impulses
from lightnings. Data gathered by this mission will be open
access and available for use by other scientists.
ACKNOWLEDGMENT
Authors would like to thank Ondřej Santolík and Ivana
Kolmašová from the Institute of Atmospheric Physics of the
Czech Academy of Sciences in Prague for valuable
discussion and input to topics described in this paper. We
would like to thank INGMETAL Moulds & Tools, Prešov,
Slovakia; the RISE Association, Prague, Czech Republic;
and TOMARK, s.r.o. division TomarkAero, Prešov,
Slovakia for supporting this paper and its presentation, and
all partners supporting project of the first Slovak satellite
skCube mentioned on its official website www.skcube.sk.
REFERENCES
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[15] https://github.com/protoplanet/raytracing
[16] Chen, Alfred B., et al. "Global distributions and occurrence rates of
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[17] http://www.linear.com/designtools/software/
[18] https://omniweb.gsfc.nasa.gov/vitmo/iri2012_vitmo.html
[19] Yabroff, Irving. Computation of whistler ray paths. Stanford
Research Institute, 1959.
... Especially the onboard computer, power supply, stabilizing electromagnetic actuators and sun position sensors can be considered as the unique technologies. Neural networks were used to calibrate onboard magnetometers [4] [5]. The satellite has an onboard computer with its own realtime operating system. ...
... For this power source, RMC s.r.o. was awarded the Unique of the Year award at the international exhibition EloSys in 2015. There was also a scientific experiment on the satellite to observe the annular lightning in the Earth's ionosphere [5]. skCUBE satellite ended its activities on 12.1.2019 ...
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