An Introduction to Space Instrumentation,
Edited by K. Oyama and C. Z. Cheng, 227–237.
Plasma wave receivers for scientific satellites
H. Kojima
Research Institute for Sustainable Humanosphere, Kyoto University, Gokasho, Uji, Kyoto 611-0011, Japan
The design of plasma wave receivers for use onboard scientific satellites is introduced. Since space plasmas
are collisionless, kinetic energy is transferred via plasma waves. The role of plasma wave receivers is to reveal
wave-particle interactions in space plasma. Conventional plasma wave receivers are classified into three types:
multichannel analyzers, sweep frequency analyzers, and waveform capture. Each one has their own advantages
and disadvantages and thus they complement each other. The present paper provides guidelines for designing
plasma wave receivers by referring to examples from representative science missions. Furthermore, the prospects
of using plasma wave receivers in future missions are discussed. The mass and size requirements for onboard
instruments can be satisfied by developing small receivers. Extremely small plasma wave receivers can be
achieved using analog application-specific integrated circuit(ASIC) technology. We also introduce our recent
progress in developing a chip that implements the necessary analog components of plasma wave receivers.
Key words: Plasma wave, collisionless plasma, plasma wave receiver, analog ASIC.
1.
Introduction
servations as follows:
Since space plasmas are collisionless, their kinetic energies are exchanged and transferred via plasma waves. It is
thus essential to perform measurements on plasma waves to
understand the physical processes that occur in space plasmas. Consequently, many scientific satellites carry plasma
wave receivers.
Figure 1 shows a typical frequency-time spectrogram of
plasma waves observed by the plasma wave receiver onboard the Geotail spacecraft in the geomagnetic tail region.
The upper and lower panels respectively show the electric and magnetic field components of the observed plasma
waves. The color scale indicates the spectral intensity. Representative spectra are labeled in this figure, namely Broadband Electrostatic Noise (BEN), Auroral Kilometric Radiation (AKR), and Magnetic Noise Burst (MNB). Discussion of the detailed features of the plasma waves in Fig. 1
and their generation mechanisms is beyond the scope of the
present paper. Matsumoto et al. (1998) have reviewed the
plasma waves observed by the Geotail spacecraft.
Historically, the first plasma wave observations from
satellites were conducted by Vanguard 3 (Cain et al., 1961),
Lofti 1 (Leiphart, 1962) and Alouette 1 (Barrington and
Belrose, 1963). Since these satellites focused on characterizing so-called ‘whistlers’, they employed coil or loop antennas as plasma wave sensors. Plasma wave observations
were initiated just after the successful launch of Sputnik 1
in 1958. This means that the importance of plasma wave
measurements was generally understood from the dawn of
space physics.
Gurnett and O’Brien (1964) pointed out that satellite
plasma wave observations have several advantages over
ground-based observations in the view point of observing
whistler waves. We additionally describe several objectives
and several advantages of satellite-based plasma wave obc TERRAPUB, 2013.
Copyright 1) In situ observations of plasma waves reveal local
wave-particle interactions. In particular, in situ observations are crucial since electrostatic waves do not
propagate.
2) Plasma waves are sensitive to changes in the plasma
environment. A specific wave-particle interaction
corresponds to the excitation of a particular mode;
this permits physical processes to be identified from
plasma wave signatures.
3) Plasma wave receivers generally have higher time resolutions than other instruments, permitting transient
phenomena to be detected.
A plasma wave receiver is a sophisticated radio detector that can be used to observe electrostatic/electromagnetic
waves excited in space plasmas with high sensitivity. The
wave receiver employs electric and magnetic field sensors
with low-noise preamplifiers. It is critical to use both electric and magnetic sensors for the following reasons. Since
the refractive index (i.e., the ratio of the electric and magnetic field strengths) depends on plasma dispersions, information about the refractive index is critical for identifying
plasma wave modes. Moreover, electrostatic waves, which
cannot exist in vacuum, can be confirmed by the fact that the
electric field sensor detects waves without any fluctuations
in the magnetic field sensor.
Unique features of plasma waves are mainly observed at
frequencies below the electron plasma frequency or the upper hybrid frequency in the case that the electron plasma
frequency ( f pe ) is larger than the electron cyclotron frequency ( f ce ) (in the case of f ce > f pe , features of plasma
waves are shown clearly below f ce ). In situ wave phenomena occur at frequencies below these frequencies, which are
determined by the local plasma.
Space plasmas are not uniform; their density varies in
different regions. Electromagnetic waves can propagate far
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Fig. 1. Frequency-time spectrogram of plasma waves observed in the geomagnetic tail region.
from their sources. Therefore, even when a satellite is in
a region of low-density plasmas, the onboard plasma wave
receiver must cover frequencies higher than local electron
plasma frequencies, because propagating electromagnetic
plasma waves generated in high-density regions can be observed in such a low density region. For example, the electron plasma frequency in the solar wind at a distance of
1 AU from the sun is typically of the order of a few tens
of kilohertz. However, a plasma wave receiver onboard a
satellite in the solar wind should cover frequencies up to a
few megahertz because waves are radiated from the sun and
terrestrial polar regions, which have much higher plasma
densities than that of the local solar wind. Therefore, the
frequency coverage of plasma wave receivers is determined
not only by local plasma densities but also by those of the
target regions for remote sensing.
On the other hand, the lowest frequency covered by a
typical plasma wave receiver is a few hertz. This lower
cutoff frequency is determined by considering the saturation effects of the sensitive receiver due to large DC elec-
tric or magnetic fields. In particular, for a spin-stabilized
satellite, DC fields can be observed as waves with the
frequency corresponding to the satellite spin frequency.
Very low frequency waves and DC fields can also be observed by instruments other than plasma wave receivers (see
Ishisaka (2012) for electric-field detectors and Matsuoka et
al. (2012) for magnetic-field detectors in the current issue).
The sensor preamplifier outputs are fed to the receiver
components. Plasma wave receivers are classified as spectrum or waveform capture receivers. The present paper describes these two types of receivers as well as their onboard
calibration.
2.
Plasma Wave Observation System
A plasma wave observation system consists of sensors,
different types of plasma wave receivers, and a digital processing unit. Figure 2 shows a block diagram of the Plasma
Wave Instrument (PWI) onboard the Geotail spacecraft,
which was launched in 1992 (Matsumoto et al., 1994a). The
Geotail PWI is a typical plasma wave observation system.
H. KOJIMA: PLASMA WAVE RECEIVER
229
Fig. 2. Block diagram of the PWI onboard the Geotail spacecraft.
The Geotail PWI makes use of two different types of electric field sensors for observing the electric field components
of plasma waves. They are tip-to-tip lengths of 100 m and
are addressed as WANT and PANT (abbreviations for WireANTenna and Probe-ANTenna, respectively). The WANT
is essentially a wire-dipole antenna. It has the advantage
of being able to observe plasma waves. On the other hand,
the PANT is suitable for observing very low frequencies or
DC electric fields. The detailed structure and frequency response of these sensors have been described by Matsumoto
et al. (1994a). The magnetic field component is detected by
a triaxial search coil magnetometer mounted at the top of
the mast. As shown in Fig. 2, the Geotail is not equipped
with a sensor for the component of the electric field along
the spin axis because deployment along the spin axis is
more difficult than deployment in the spin plane. However,
a spin-axis electric field sensor has usually been installed
in recent terrestrial magnetosphere missions based on the
technological advance.
The signals picked up by these electric and magnetic field
sensors are fed to the plasma wave receiver. The Geotail
PWI has three different types of receivers:
1) Sweep Frequency Analyzer (SFA),
2) Multichannel Analyzer (MCA), and
3) WaveForm Capture receiver (WFC).
The first two types of receivers measure wave spectra,
whereas the last one is designed to simultaneously capture
waveforms of two electric and three magnetic field components. Detailed descriptions of each type of receiver are
given in the following sections.
3.
Sensors
Figure 3 shows an artist’s sketch of the Geotail spacecraft. It has two sets of electric field sensors and one set of
magnetic field sensors for plasma wave measurements. A
dipole antenna is used for electric field measurements. Two
pairs of dipole antennas are deployed in the spacecraft with
their spin planes perpendicular to the spacecraft spin axis
(see Fig. 3).
Search-coil magnetometers and loop antennas are used
for measuring the magnetic field components of plasma
waves. Typically, the search-coil magnetometers are used
for low-frequency waves below 100 kHz and the loop antennas are used at frequencies above a few tens of kilohertz.
To avoid interference from the electronics inside the spacecraft, the magnetic field sensors are often installed away
from the main body. For example, the triaxial search coil
magnetometer of the Geotail is mounted at the end of a 6m-long mast (see Fig. 3).
The principles and designs of the electric and magnetic
sensors are described in detail by Gurnett (1998). Suffice it
to say here that the design of the electric field sensor is critical. The interaction with the surrounding plasma should be
considered when designing the sensor. The sensor is used
for both plasma wave and static electric field measurements.
In terms of observing the electric fields of plasma waves, a
long dipole antenna in space can be regarded as a short antenna, because the wavelengths of the plasma waves are in
much longer than the antennas. The antenna impedance in
vacuum is capacitive under this condition. Consequently, it
is important to estimate the capacitance of the electric field
antenna in vacuum when designing a plasma wave receiver.
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H. KOJIMA: PLASMA WAVE RECEIVER
Fig. 3. Artist’s sketch of the Geotail spacecraft.
In practice, the impedance depends on plasma conditions
3) The total amount of data can be made much smaller
such as electron temperatures, densities, and the presence
than that of a waveform receiver by optimizing the
of photoelectrons. It also depends on the type of electric
time and frequency resolutions.
field antenna. Many theoretical and numerical studies have
4) Continuous data collection is possible.
estimated the antenna impedance and the effective antenna
Disadvantages:
length in plasmas (e.g., Meyer-Vernet and Perche, 1989;
Miyake et al., 2008). Detailed analysis of electric field
1) Its electronic circuit is more complex than that of a
wave observations requires a precise determination of the
waveform receiver and it thus requires more resources
antenna impedance. However, the capacitive impedance
from the mass and power budgets.
of the antenna in vacuum is generally used in the design
2) The receiver is not able to detect instantaneous phephase. Furthermore, since plasmas are dispersive media,
nomena.
the antenna impedance varies with frequency, which makes
In contrast, waveform receivers have the following adit impossible to maintain impedance matching at the input
vantages and disadvantages:
of the preamplifier. Consequently, the preamplifier is deAdvantages:
signed with an impedance of over 100 M, which is much
larger than the expected antenna impedance.
1) Waveform receivers can detect phase. This means
they can measure all wave properties except wavenumber. The information on the phase of a plasma wave
4. Plasma Wave Receiver
is important for studying nonlinear phenomena, espeWhile a spectrum receiver provides information on wave
cially since waveforms are not always sinusoidal. Geoamplitude and frequency, a waveform receiver measures the
tail plasma wave observations revealed Electrostatic
phase as well.
Solitary Waves (ESW) in the BEN (Matsumotoet al.,
A spectrum receiver has the following advantages and
1994b). The discovery of the ESW is a representative
disadvantages.
achievement of the waveform receiver.
Advantages:
2) Waveform receivers can observe instantaneous phe1) Narrowband spectra can be detected with a high
nomena.
signal-to-noise ratio.
3) Their electronic circuits have simple structures.
2) The observed frequency range is divided into sevDisadvantages:
eral bands that are covered by independent receivers.
Therefore, we can optimize the dynamic range of a re1) Since waveform receivers cover a wide frequency
ceiver by assigning a different gain to each receiver
range, the gain must be reduced for the most intense
considering the expected intensities of plasma waves
waves in the whole observable frequency range.
in each frequency band.
H. KOJIMA: PLASMA WAVE RECEIVER
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2) To avoid aliasing, high-frequency sampling is neces- Table 1. Center frequencies and bandwidths of the MCA onboard the
Geotail spacecraft.
sary at the A/D converter. Such sampling leads to an
increase in the power consumption.
Channel
Center frequency
Bandwidth
3) Waveform receivers generate a huge amount of data,
Ch 1
5.62 Hz
1.69 Hz
which means that not all observational data can be
Ch 2
10 Hz
3 Hz
transferred to the ground by telemetry. Therefore, only
Ch 3
17.8 Hz
5.34 Hz
intermittent observations can be performed. Waveform
Ch 4
31.1 Hz
9.33 Hz
receivers need to stop measuring during transmission
Ch 5
56.2 Hz
16.9 Hz
Ch 6
100 Hz
30 Hz
of data to the ground.
Spectrum and waveform receivers complement each
other. However, since space missions have limited resources, some missions do not carry spectrum receivers.
Instead, an onboard digital processing unit with a CPU or
digital signal processor (DSP) is used to generate spectra
from the observed waveforms by taking the Fast Fourier
Transform(FFT). However, note that this type of spectrum
receiver suffers from the same disadvantages as waveform
receivers.
4.1 Spectrum analyzers
There are two typical types of spectrum receivers: MCAs
and SFAs.
4.1.1 Multichannel Analyzer Figure 4 shows a
block diagram of a typical MCA. The MCA consists of
many Band Pass Filters (BPFs) with different center frequencies that are equally spaced on a logarithmic scale. Table 1 shows the BPF center frequencies and bandwidths for
the MCA (electric field component) on the Geotail spacecraft. Each band has a bandwidth of ±15% of the center
frequency for channels below 10 kHz and of ±7.5% for
channels above 10 kHz.
The sensor preamplifier outputs are fed to each BPF
through differential amplifiers. The gain of the main amplifier in each band is adjusted for the intensities of the expected plasma waves. Therefore, the spectrum analyzer has
a wider dynamic range than that of the waveform receiver
(the gain of the waveform receiver should be adjusted for
the most intense plasma waves expected over a wide frequency range). A logarithmic amplifier is often used in a
MCA to realize a wide dynamic range.
The output of the main amplifier is rectified and
smoothed before the A/D. The output analog data from each
channel is simultaneously sampled and held. The frequency
resolution of the MCA is determined by the number of installed BPFs and their bandwidths, which makes it difficult
to obtain a high frequency resolution. However, the time
resolution is higher than SFAs (see the following section)
due to the ability of MCAs to simultaneously sample and
hold data.
4.1.2 Sweep Frequency Analyzer The SFA provides
a narrow-band frequency spectrum by incrementally varying the observation frequency. Consequently, it has an excellent frequency resolution. Table 2 shows the specifications of the SFA onboard the Geotail spacecraft. It consists of eight independent receivers covering five frequency
bands for electric fields and three frequency bands for magnetic fields. The frequency measurement period for bands 1
and 2 is 64 s and that for bands 3, 4, and 5 is 8 s. These
sweep periods are determined by considering the filter response, the phase locking accuracy of the first local oscilla-
Ch 7
Ch 8
Ch 9
Ch 10
Ch 11
Ch 12
Ch 13
Ch 14
Ch 15
Ch 16
Ch 17
Ch 18
Ch 19
Ch 20
178 Hz
311 Hz
562 Hz
1 kHz
1.78 kHz
3.11 kHz
5.62 kHz
10 kHz
17.8 kHz
31.1 kHz
56.2 kHz
100 kHz
178 kHz
311 kHz
53.4 Hz
93.3 Hz
168.6 Hz
0.3 kHz
0.534 kHz
0.933 kHz
1.686 kHz
1.5 kHz
2.67 kHz
4.67 kHz
8.43 kHz
15 kHz
26.7 kHz
46.65 kHz
tor, the telemetry capacity, and the required time resolution
from a scientific viewpoint.
The SFA is a heterodyne receiver. Frequency downconversion is performed by multiplying the measured and
local oscillator waves. Figure 5(a) shows a block diagram
of a typical SFA that has two frequency down-conversion
stages (thus, it is a double superheterodyne receiver).
The role of the input BPF is to eliminate waves with frequencies outside the observation range. It does not need to
have sharp reduction features since its purpose is to prevent
the main amplifier from being saturated by intense waves
outside the target frequency range.
The first local oscillator sweeps from f 1 = f 1L to f 1 =
f 1H . As it sweeps, the SFA incrementally measures wave
spectra at frequencies. Since frequency stability of the first
local oscillator in each step is important, it requires the
Phase-Locked Loop (PLL).
The output of the mixer at the first stage is the result of
frequency down-conversion
f in(obs) = f 1 − f 1C
(1)
f in(image) = f 1 + f 1C .
(2)
and
Here, f 1 and f 1C respectively denote the frequency of the
first local oscillator and the center passband frequency of
the first BPF. It is impossible to distinguish these two waves
with f in(obs) and f in(image) at the output of the first stage
mixer. While the f in(obs) component is desired, the f in(image)
component (known as the image spectrum) needs to be
eliminated. The purpose of the LPF (Low Pass Filter)
shown in Fig. 5(a) is to dampen the image spectrum component.
The same consideration is necessary in the design of the
first BPF before the second stage. The real and image
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Fig. 4. Typical block diagram of a MCA (for one component of sensors).
Fig. 5. (a): Block diagram of a typical SFA (for one frequency band), (b) and (c): Concept of the image spectrum and design of LPFs.
spectra, f 1−out(obs) and f 1−out(image) , are
f 1−out(obs) = f 2 − f 2C
(3)
f 1−out(image) = f 2 + f 2C .
(4)
and
Here, f 2 and f 2C respectively denote the frequency of the
second local oscillator and the center frequency of the second BPF.
Figures 5(b) and (c) depict the concepts used to eliminate
the image frequency component. The characteristics of the
LPF and first BPF are crucial in the design of the SFA because insufficient reduction by these filters can lead to misinterpretation of the measured data due to the observation
of the image spectrum. They require sharp reduction features to eliminate the image frequency spectra. Therefore,
high-order filtering is necessary. The maximum reduction
level should match the dynamic range of the receiver.
After the second down-conversion, the output of the
mixer is fed to the second BPF. The second BPF limits the
frequency bandwidth of each frequency sweep step. A logarithmic amplifier is located after the second BPF to realize a wide dynamic range. Figure 6 plots the characteristics of the quasi-logarithmic amplifier of the SFA onboard
the Geotail spacecraft. Table 3 shows the characteristic frequencies of the SFA onboard Geotail.
4.2 Waveform capture
The waveforms picked up by the sensors are directly
sampled in the waveform receiver, so-called ‘WaveForm
Capture (WFC).’ The most important advantage of waveform capture is that it provides phase information; spectrum
H. KOJIMA: PLASMA WAVE RECEIVER
233
Table 2. Specifications of the SFA onboard the Geotail spacecraft.
Band
1
2
3
4
5
Frequency range
24–200 Hz
200–1600 Hz
1.6–12.5 kHz
12.5–100 kHz
100–800 kHz
Freq. step
1.3 Hz
10.7 Hz
85.4 Hz
683 Hz
5.47 kHz
Bandwidth
2.6 Hz
10 Hz
85 Hz
680 Hz
5.4 kHz
Source
B and E
B and E
B and E
E only
E only
Sweep period
64 s
64 s
8s
8s
8s
Table 3. Characteristic frequencies of the SFA onboard the Geotail spacecraft.
Band
1
2
3
4
5
LPF
250 Hz
1.875 kHz
12.8 kHz
110 kHz
880 kHz
First local
415.2–584.8 Hz
3.322–4.678 kHz
26.57–37.43 kHz
212.6–299.4 kHz
1.701–2.395 MHz
receivers such as MCA and SFA do not observe phase. Digital waveform capture was used in the Geotail spacecraft,
which led to the discovery of ESWs in the plasma sheet
boundary layer as causing the BEN. The Geotail waveform
capture revealed that the broadband spectrum is generated
by a series of isolated pulses, which may have broadband
spectra.
Figure 7(a) shows a block diagram of a typical WFC.
The role of the first LPF is to limit the observed frequency
range. Since there is a wide frequency range below the cutoff frequency of the first LPF, the main amplifier must be
adjusted to the dynamic range of the received signals over
the entire observational frequency range determined by the
first LPF. Plasma waves at lower frequencies are generally
more intense than those at higher frequencies. Therefore,
the gain needs to be adjusted to low-frequency waves in
general. In addition, a logarithmic amplifier cannot be used
for the WFC since it distorts the waveforms. It is thus difficult to design a high-sensitivity receiver for the WFC. These
are the reasons why the WFC frequently has an automatic
gain control (AGC) or a gain control system adjustable by
telemetry commands.
The second LPF is an anti-aliasing filter. The input
signals are sampled at a sampling frequency of f s . According to sampling theory, waves can be measured up to
the Nyquist frequency f N , which is half the sampling frequency. Aliasing occurs for signals with frequencies above
f N . Figure 7(b) shows the relationship between the aliasing
signal f a and f N . A signal with a frequency f higher than
the Nyquist frequency appears as an aliased signal at a frequency f lower than the Nyquist frequency. If this signal
lies in the observed frequency range, it can be mistaken for
a real signal. The purpose of the second LPF is to reduce
aliased signals in the observation band.
The dotted lines in Fig. 7 indicate the required frequency
responses of the second LPF for two different cases, A and
B. Here, f 1 denotes the cutoff frequency of the second LPF.
To analyze waveforms at frequencies above f 1 , it is necessary to sufficiently reduce f N . The reduction should match
the dynamic range of the A/D converter (response A). On
the other hand, if wave phenomena above f 1 are completely
First IF
390.6 Hz
3.125 kHz
25 kHz
200 kHz
1.6 MHz
Second local
—
2.84 kHz
20 kHz
210 kHz
1.61 MHz
Second IF
—
289 Hz
5 kHz
10 kHz
10 kHz
Fig. 6. Characteristics of the quasi-logarithmic amplifier of the SFA
onboard the Geotail spacecraft (Matsumoto et al., 1994a).
ignored in data analysis, frequency response B can be selected. In this case, the reduction feature of the second
LPF is more relaxed so that a reduction equivalent to the
A/D dynamic range should be ensured at a frequency of
f2 = 2 fN − f1.
To relax the design of the second LPF, a sampling frequency f s was used that is more than three times higher
than f 1 . However, since oversampling increases the power
consumption of the A/D, we meet the limit of an oversampling. Therefore, a high-order Chebyshev filter or elliptical
filter is often used for the second LPF.
Figure 8 shows a plot of the frequency response (without
sensors and preamplifiers) of the WFC onboard the Geotail.
Figures 8(a) and (b) represent the characteristics of the gain
and phase for frequencies up to 4 kHz in the WFC low gain
mode, respectively. Both the phase and the gain need to be
calibrated during ground tests.
5.
Digital Processing
Rcent progress in digital technology has enabled sophisticated lightweight receivers to be developed. In particular, high-speed CPUs, digital signal processors (DSPs),
and field-programmable gate arrays (FPGAs) are useful.
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Fig. 7. (a) Block diagram of a typical WFC. (b) Concept of aliasing effect and design of the anti-aliasing filter.
They permit onboard signal processing such as FFT and
wavelet analysis. Combining onboard signal processing
with a waveform receiver leads to the realization of a spectrum receiver. Waveform data digitized by an A/D are fed
to a digital unit that generates FFT spectra. Such a spectrum
receiver is both simple and flexible. Its time and frequency
resolutions can be controlled by software. However, it has
several disadvantages. For example, a high-speed A/D is required for high-frequency bands, which increases the power
consumption. Furthermore, the dynamic range of the receiver has to be adjusted to the frequency band in which the
most intense plasma waves are expected.
6.
Onboard Calibration
An onboard calibration system is used in both pre-flight
tests and in flight. The output from an onboard oscillator is
fed to the input of the preamplifier of the electric-field antennas and the calibration coil attached to the magnetic-field
sensor. For a waveform receiver, the onboard calibration
system needs to provide both phase and gain information
about the circuit. For this purpose, a synchronous detection
method will be employed on the BepiColombo Mercury
Magnetospheric Orbiter, which will be launched in 2014
(Kasaba et al., 2008).
Figure 9 shows a block diagram of the onboard calibration system for the electric-field sensors using the synchronous detection method. It includes a calibration signal
source, whose output has undergone digital–analog conversion from waveform data stored in memory.
A local signal is input to the preamplifier through the low
resistance R1. When R1 is small relative to the antenna
impedance |Z A |, the impedance does not affect the output
waveforms from the WFC. The waveforms of the calibration source are directly sampled at the A/D after the receiver
circuit. The digitized waveforms are then processed according to synchronous detection theory. The multiplier in the
digital processor calculates the product of a digitized waveform with the signal source and with one shifted in phase
by π/2, producing the signals
V (t) · S(t) =
|G WFC (ω)| V 2
{cos(2ωt + θ) + cos θ } , (5)
2
and
V (t)·S π2 (t) =
|G WFC (ω)| V 2
{sin(2ωt + θ) − sin θ } . (6)
2
Here, S(t), S π2 (t), and V (t) represent the waveforms of the
signal, π/2 shifted signal, and receiver output, respectively;
G WFC (ω) denotes the transfer function from the front end
of the preamplifier to the analog output of the receiver; and
θ is the phase shift in the circuit,
θ = G WFC (ω).
(7)
By averaging the results, we obtain
V (t) · S(t) =
|G WFC (ω)| V 2
cos θ,
2
(8)
and
V (t) · S π2 (t) = −
|G WFC (ω)| V 2
sin θ.
2
(9)
H. KOJIMA: PLASMA WAVE RECEIVER
Fig. 8. Frequency responses of (a) gain and (b) phase in the low gain mode
of the WFC onboard the Geotail spacecraft (Matsumoto et al., 1994a).
This gives the absolute value and phase of G WFC (ω).
Using this method, the frequency response (gain and
phase) of the entire receiver circuit can be determined, including the preamplifier. Furthermore, this system can be
used to determine the antenna impedance of the electricfield sensors, which strongly depends on the density and
temperature of the surrounding plasma. In the system
shown in Fig. 9, the input resistance R2 is comparable to
the antenna impedance |Z A |, so that the WFC output includes the effect of that impedance. Thus, gain and phase
information of G i (ω) · G WFC can be obtained using the
above-mentioned synchronous detection method. Here, G i
denotes the transfer function of the electric-field sensor
that includes the effect of the antenna impedance. Since
G WFC (ω) is obtained in the low resistance mode of R1 independently, we can calculate the G i from the G i (ω)·G WFC .
7.
Summary and Perspectives on Plasma Wave
Receivers for Future Missions
The present paper reviewed the properties of conventional plasma wave receivers. They are based on mature
technologies that have advanced since the earliest days of
satellite observations. Plasma wave receivers do not contain mechanically engineered parts except for the deployment mechanism of the sensors. Instead, they contain many
electronic components.
In recent space missions such as formation flights and
planetary missions, the mass and size budgets for onboard
235
instruments have been severely restricted because of the increasing number of research goals of these missions. Miniaturization of onboard instruments is thus important in developing new instruments for future space missions. A reduction in the size of electronic components immediately leads
to miniaturization of plasma wave receivers.
Recent progress in FPGAs has led to high-performance
onboard digital processing. For example, a spectrum receiver can be realized based on a waveform capture receiver
using a FPGA or DSP. Another digital type of plasma wave
receiver has been used in the sounding rocket SS-520-2 and
Kaguya spacecraft for a lunar mission. A programmable
down-converter (PDC) has been installed in these receivers
(Hashimoto et al., 2003; Kasahara et al., 2008). In that digital device, wideband waveforms sampled at a high rate are
down-converted into waveforms with a narrow band. Taking the FFT of the output waveforms results in frequency
spectra in a limited spectral range. The converted frequency
range is controllable by onboard software. Thus, “digital”
sweep frequency analyzer can be realized by combining a
PDC and its control software. Such a digital sweep analyzer has a much smaller mass than an analog type sweep
frequency analyzer because it does not require mixers or a
local sweep oscillator with the PLL.
A plasma wave receiver cannot completely eliminate the
use of analog electrical circuits since analog circuits occupy
a large portion of the electrical board in the form of various amplifiers and filters. However, their size can be reduced by using analog Application Specific Integrated Circuit (ASIC) technology. ASIC could be implemented by
chips that are a few square millimeters in size. Kojima et
al. (2010) and Fukuhara et al. (2011) have developed such
a chip that stores essential analog components such as filters and amplifiers. Figure 10 shows a prototype chip developed by our group. It contains several types of low-pass
filters and amplifiers. Such work is expected to lead to extreme miniaturization of plasma wave receivers. Kojima et
al. (2010) and Fukuhara et al. (2011) demonstrated the feasibility of a waveform receiver that is the size of a business
card and that can observe six components of waveforms.
A single small package should eventually contain both
waveform and spectrum receivers, in contrast to recent missions in which only a waveform receiver is used because
the required resources for the analog spectrum receiver are
much larger. In those missions, the wave spectrum was obtained directly or after digital down-conversion by the onboard FFT calculation from the observed waveforms. However, this technique contains the disadvantages of a waveform receiver. It requires a high-speed A/D in observing
wave spectra at high frequencies. Furthermore, the gain
of the receiver has to be adjusted to the frequency range
in which the most intense waves are expected to be observed. Therefore, the digital spectrum receiver does not detect weak plasma waves in other frequency ranges. In contrast, using an analog spectrum receiver (such as a sweep
frequency receiver), the spectrum can be divided into several bands with individual frequency ranges such as those
listed in Table 2 and the gain of each band can be varied
independently. The analogue spectrum analyzer requires
much more resources such as weight and size than those
236
H. KOJIMA: PLASMA WAVE RECEIVER
Fig. 9. Onboard calibration system of the BepiColombo spacecraft.
Fig. 10. Analog chip for a plasma wave receiver: the package is shown on the left and the bare chip is shown on the right.
of the digital one. However, the weight and size can be
greatly reduced by using an analog ASIC. Consequently,
plasma wave receivers in future missions will ideally contain both waveform and spectrum receivers that are installed
on a small chip. The design of an analogue chip needs proficiency especially in developing a low noise circuit. However, the attempt to develop small plasma wave receivers is
essential for future space missions.
Acknowledgments. The author thanks S. Yagitani, Y. Kasahara,
and M. Ozaki of Kanazawa University, Japan for the discussion
in writing the present paper. The author thanks H. Fukuhara
and S. Okada of Kyoto University, Japan for their hard works
in developing the small plasma wave receiver using ASIC. The
study on the miniaturization of the plasma wave receiver was
supported by Grant-in-Aids for Scientific Research (A) 19204048
and 23244097.
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H. Kojima (e-mail: kojima@rish.kyoto-u.ac.jp)