Description taken from
Matsumoto, H., I. Nagano, R. R. Anderson, H. Kojima, K. Hashimoto, M. Tsutsui, T.
Okada, I. Kimura, Y. Omura, and M. Okada, Plasma Wave Observations with GEOTAIL Spacecraft,
J. Geomag. Geoelectr., 46, 59-95, 1994.
The Geotail spacecraft was launched from Kennedy SPace Center at 14:26 UT on July
24, 1992. The primary scientific objectives of the PWI are to characterize wave-particle
interaction processes and instabilities associated with the dynamics in the different
regions of the magnetosphere.
The PWI consists of three different sets of receivers: (1) the Sweep Frequency Analyzer(SFA),
(2) the Multi-Channel Analyzer (MCA) and (3) the Wave-Form Capture(WFC). The first
two sets of receivers are devoted to measuring wave spectra, while the last one is
designed to capture wave forms from two electric and three magnetic field components
of the measured wave emissions, simultaneously. The frequency ranges of the electric
and the magnetic fields which can be measured range from 5.6 Hz to 800 kHz and from
5.6 Hz to 12.5 kHz, respectively. The frequency range of wave forms which can be captured
by the WFC is from 10 Hz to 4 kHz.
PWI observation data are available in three types of telemetry formats called Format
1, 2, and 3. Format 1 includes the SFA and MCA data. These data are recorded onboard
magnetic data recorder and played back to NASA Deep Space Network.
Format 2 is a real time observation mode supported by Usuda Deep Space Center (UDSC)
in Japan. It includes all of PWI observation data with the highest resolution.
Format 3 is an unusual observation mode. We mainly use this format, when we need to
monitor US SIs' data in real time, because Format 2 does not include any US SIs' data.
Format 3 includes all of PWI observation data, however, WFC data quanities are coarse
relative to those in Format 2.
Sensors
In order to measure the weak electric and magnetic fields from plasma waves in the
geomagnetic tail region, PWI uses two kinds of electric sensors and tri-axial search
coils, having taken into account both the most suitable condition for field detection
and reduction of spacecraft noise. Two sets of long dipole antennas with a length
of 100 m tip-to-tip are dedicated to measuring the electric field. They are wire and
probe antennas termed `WANT' and `PANT', respectively.
Two masts of 6 m in length are used for mounting the magnetic field sensors well away
from the spacecraft in an attempt to the reduce the spacecraft noise. One of these
masts, called `MST-F', mounts two sets of the flux-gate magnetometers which are used
by the MGF team. They are dedicated primarily to the measurement of the DC magnetic
field. Two sets of search coil and their pre-amplifiers which are used by both the
PWI and MGF teams to measure wave magnetic fields or magnetic field fluctuations are
mounted on the other mast called `MST-S'.
The antenna elements of the WANT and PANT are radially deployed from the spacecraft
and orthogonal to each other. Their axes, U and V, deviate by 15 degrees clockwise
from the spacecraft X and Y coordinate directions, respectively. The assembly of PWI
tri-axial search coils (which is hereafter called PWI-SC) and their pre-amplifiers
is mounted on the top of the MST-S. The tri-axial search coils associated with the
MGF (MGF-SC) are mounted on the same mast but 2 m inside of the PWI-SC. The PWI-SC
is normally connected to the PWI measurement system. However, we can select the MGF-SC
by a telemetry command if necessary. The direction of the MST-S (and MST--F) deviates
by 45° clockwise from the spacecraft Y (and -Y) axis. The three axes of the PWI search
coils are defined in a cylindrical coordinate system as follows: \alpha is in the
tangential direction, \beta measures in the radial direction, and \gamma is parallel
to the spin Z axis of the spacecraft.
Sweep Frequency Analyzer (SFA)
The SFA provides spectral information on plasma wave amplitudes over the frequency
range from 24 Hz to 800 kHz for the electric field and 24 Hz to 12.5 kHz for the magnetic
field. The SFA consists of eight independent receivers covering 5 frequency bands
for the electric fields and three frequency bands for the magnetic fields. The receiver
specifications are listed in Table 1.
Table 1. Specification of SFA
| Band |
Frequency Range |
Freq. Step |
Bandwidth |
Source |
Sweep |
| 1 |
24 Hz - 200 Hz |
1.3 Hz |
2.6 Hz |
B and E |
64sec |
| 2 |
200 Hz - 1600Hz |
10.7 Hz |
10 Hz |
B and E |
64sec |
| 3 |
1.6 kHz - 12.5 kHz |
85.4 Hz |
85 Hz |
B and E |
8 sec |
| 4 |
12.5 kHz - 100 kHz |
683 Hz |
680 Hz |
E only |
8 sec |
| 5 |
100 kHz - 800 kHz |
5.47kHz |
5.4 kHz |
E only |
8 sec |
Each receiver has a very good frequency resolution of 1/128 of the receiver frequency
band although their time resolution (64 sec for Bands 1 and 2, and 8 sec for Bands
3 - 5) is somewhat coarse.
The SFA measures one wave electric field component of either the E_U or E_V component
and one wave magnetic field component of either the B_\alpha or B_\gamma component.
The selection of which field component is measured is carried out by a telemetry command.
Each receiver has an effective dynamic range of ~90 dB. The electric field receivers
are placed in the High gain mode by stepping up the level in the Low gain mode by
30 dB.
The SFA has another mode of operation which can be used. Despite its operation as
the SFA, we can also fix the measured frequency to one preset value in each frequency
band. This mode provides very high time resolution for the observed signals at an
arbitrarily selected frequency. The time resolutions in this mode are 0.5 sec for
Bands 1 and 2, and 62.5 msec for Bands 3, 4, and 5, respectively. This mode change
is also carried out by a telemetry command.
Multi Channel Analyzer (MCA)
This subsystem is provided by the University of Iowa. The MCA contains two spectrum
analyzers with fixed frequency channel filters. It provides high time resolution data
to complement the coarser time resolution data of the SFA. However, their frequency
resolution is coarse because they have only four frequency channels per decade in
frequency.
The input signal to the MCA is provided from the output of the common front-end circuit
of the PWI system as well as to SFA and WFC receivers. One multi-channel spectrum
analyzer is used to measure the electric field and is composed of 20 channels covering
the frequency range from 5.62 Hz to 311 kHz. The other spectrum analyzer is used for
magnetic field measurements and has 14 channels covering the frequency range from
5.62 Hz to 10 kHz. The bandwidths of the filters are +/- 15% of the channel center
frequency in the frequency range below 10 kHz and +/- 7.5% of the center frequency
for frequency above 10 kHz. The MCA instrument measures the wave electric field of
either the E_U or E_V component (depending on which antenna is used) with a dynamic
range of ~110 dB and the wave magnetic field of either the B_\alpha or B_\gamma component
with a dynamic range of ~100 dB.
Signals from all channels are sampled simultaneously so that the ratio of the electric
to magnetic field strength may be calculated accurately. The signals are sampled once
(data acquisition time is 1.037 msec) every 250 msec in Format 2 (and every 500 msec
in Format 1). Each channel of the spectrum analyzer has a suitable integration time
in the linear detection of signal ranging from 500 msec in the lower frequency channels
and decreasing to a few milliseconds for those channel above 3 kHz.
Wave-Form Capture (WFC)
The WFC data are used for the detailed analysis of the wave characteristics, such
as determination of the wave vector, polarization, Poynting flux, and antenna sheath
impedance. The system has five (two electric and three magnetic) receivers each connected
to a sensor through each front end circuit. The WFC has two different operation modes:
Memory mode and Direct mode.
In the Memory mode, wave signals are measured simultaneously as E_U, E_V , B_\alpha,
B_\beta, and B_\gamma. These are fed to a gain-controller followed by an anti-aliasing
filter (LPF) with an upper frequency cutoff of 4 kHz, then through high pass filters
(HPF) which have a low frequency cutoff of 10 Hz. An alternate HPF with a low frequency
cutoff of 100 Hz can be used with the electric field measurements. Wave forms of the
analog signal are sampled then converted into 12 bit data by an A/D converter with
a frequency of 12 kHz. The 12 bit digital data are compressed into 8 bit by a quasi-logarithmic
compression method. The compressed 8 bit data are then stored into onboard memory
with a storage of 512 kBytes for a period of 8.7sec. The stored wave form data are
read out of memory and telemetered to the ground using the PCM telemetry during a
uninterrupted period of 275 seconds in telemetry Format 2 and 375 seconds in telemetry
Format 3. The start timing for initiating wave form capture is controlled by either
free running timing pulse (INTER. mode) or when the instrument receives one of eight
different sources for triggering signals. The instrument is triggered when one of
the following measurements exceeds a preset level:
1. The intensity of Z axis component of the DC magnetic field as measured by the outboard
fluxgate magnetometer of the MGF.
2. The amplitude of the plasma turbulence detected by a single probe of the EFD.
3. The magnitude of ion and/or electron moments as measured by the electrostatic analyzers
of the LEP-EA.
4. The amplitude of the electric field measured on the 100 kHz channel of the MCA.
5. The amplitude of the electric field measured on the 316 Hz channel of the MCA.
6. The amplitude of the magnetic field measured on the 10 Hz channel of the MCA.
7. The integral amplitude of the electric field over the entire frequency range of
Band 1 (frequencies less than 250 Hz) of the SFA.
8. The integral amplitude of the magnetic field over the entire frequency range of
both Bands 1 and 2 (frequency less than 1.875 kHz) of the SFA.
The selection among the possible eight triggering sources and its preset level or
amplitude is made via telemetry commands.
When the WFC instrument is operated in the Memory mode the following three modes of
operation are available for storing and reading the data:
1. AFT: Storage of 512 kByte of data after instrument triggering.
2. MID: Storage of two successive 256 kByte sequences of data, one before and the
other after instrument triggering.
3. BEF: Storage of 512 kByte of data before instrument triggering.
In the last two modes, a continuous sequence of signal sampling and storage into the
memory is repeated until a triggering signal is detected by the instrument.
A common problem of noise interference in the plasma wave instruments on spinning
occurs as each group of solar cell circuits cycles on/off when the solar cell surface
of the group faces to the sun-lit side then to the shadow side of the spacecraft.
This repetition of the circuit from on to off to on again causes radiation of electromagnetic
noises. To mitigate this problem, a high pass filter (HPF) with a cutoff frequency
of 10 Hz is inserted in the receivers in order to suppress the detection of low frequency
noise from the solar cell circuits. For the electric field receivers, we prepared
an option for changing the HPF cutoff frequency from 10 Hz to 100 Hz in case the noise
extended to higher frequencies then expected. The selection of the lower frequency
cutoff is based upon the observed in situ noise level. These noise suppression measures
limit frequency range of the observed wave forms to a band from 10 Hz (or 100 Hz)
to 4 kHz.
Besides the Memory Mode, the WFC can be operated in a Direct mode, with two possible
variations: one is the Single channel mode in which only one field component is measured
continuously and telemetered to ground on a real-time basis. In this operation mode
the instrument can measure continuous wave forms of one electric or one magnetic component
with an upper limit in frequency of 640 kHz in telemetry Format 2 (or 470 Hz in telemetry
Format 3). The other method of instrument operation is in the Dual channel mode where
two field components are measured and telemetered alternately. In this mode the upper
limit in frequency is 320 Hz in telemetry Format 2 (or 235 Hz in telemetry Format
3) for each component. For both of these mode, the data sampling frequency is three
times the upper limit in frequency.
The WFC system can measure the wave amplitude with a dynamic range of 66 dB. The receiver
gain keeping its dynamic range can be stepped up in the gain controller "H/L" by 40
dB and 20 dB in electric and magnetic channels to amplify the weak signal, respectively.
