INSTRUMENT DESCRIPTION
INSTRUMENT DESCRIPTION
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The Galileo Plasma Wave Receiver is described by Gurnett et al., 1992 [GURNETTETAL1992]. Contents:
Scientific Objectives:The basic objective of this investigation is the study of plasma waves and radio emissions in the magnetosphere of Jupiter. The Voyager 1 and 2 flybys of Jupiter have now clearly shown that many complex types of plasma wave and radio-emission phenomena occur in the Jovian magnetosphere. These include electromagnetic whistler mode emissions called chorus and hiss, electromagnetic continuum radiation trapped in the magnetospheric cavity, electrostatic waves associated with harmonics of the electron cyclotron frequency, and a wide variety of escaping radio emissions. Some of these waves, such as the whistler mode emissions, are believed to play an important role in the dynamics of the magnetosphere by controlling the pitch-angle scattering and loss of energetic charged particles. In other cases plasma waves provide an important diagnostic tool by revealing various characteristic frequencies of the plasma, from which quantities such as the electron density can be computed. Since the Galileo spacecraft will be the first orbiter of Jupiter, this spacecraft will provide a much more comprehensive study of the Jovian magnetosphere than was possible with the previous Pioneer and Voyager flybys of Jupiter. Specifically, the orbit of Galileo will provide a survey of the magnetotail at distances of up to 150 RJ over a range of local times near local midnight, a region that has never previously been explored; repeated passes through the plasma sheet, the magnetopause, and the tail lobes; and numerous close flybys of the Galilean satellites. Of particular importance will be a very close pass by the satellite Io. The Voyager flybys showed that volcanic gases escaping from this moon are the main source of plasma in the Jovian magnetosphere. The primary energization of plasma in the Jovian magnetosphere is believed to occur in a dense plasma torus that surrounds Jupiter near Io's orbit. This energization is associated with many complex plasma wave phenomena, including the generation of intense kilometric and decametric radio emissions. In addition to exploring regions never previously investigated, Galileo, by virtue of its long lifetime in orbit around Jupiter, also provides a unique new capability for carrying out studies of temporal variations on time scales that cannot be investigated with a single flyby. For example, it is known that the kilometric and decametric radio emissions associated with Io and its plasma torus have temporal variations on time scales of weeks and longer. With Galileo these temporal variations can be monitored over periods of several years and compared with other remote sensing instruments. These measurements should be able to tell us, for example, whether the variations are associated with changes in the volcanos on Io. Considerable interest also exists in searching for evidence of magnetospheric substorm phenomena, possibly comparable to auroral substorms in the Earth's magnetosphere. With the Galileo plasma wave instrument, it should be possible to provide remote sensing of substorms in a manner comparable to the remote sensing of terrestrial auroral kilometric radiation, which is known to be closely associated with terrestrial substorms. To carry out comprehensive studies of plasma waves and radio emissions at Jupiter, the Galileo plasma wave instrument incorporates several new features that provide improvements over the previous Voyager 1 and 2 measurements. These improvements include (1) simultaneous electric and magnetic field measurements to distinguish electrostatic waves from electromagnetic waves, (2) direction finding measurements to determine source locations, and (3) better frequency and time resolution to resolve fine structure in the plasma wave and radio emission spectrum. The main instrument package and the electric dipole antenna system were designed and constructed at the University of Iowa, and the search coil magnetic antenna was provided by the Centre de Recherches en Physique de l'Environnement Terrestre et Planetaire (CRPE). Calibration:An extensive series of calibrations and performance checks were performed on the plasma wave instrument both on and off the spacecraft. Since the logarithmic compressors used in the spectrum analyzers do not give a true logarithmic response, the transfer function of the logarithmic compressors must be calibrated. Because of the large number of channels, it is not practical to calibrate each frequency channel separately. Instead, the transfer function is measured for each logarithmic compressor, and a frequency response calibration is performed at a fixed amplitude for all channels using that compressor. This procedure provides accurate calibrations for each frequency step because each band of the receiver uses a single filter and logarithmic compressor. A look-up table can be constructed which converts the telemetry data number to input signal strength. When combined with the overall frequency response across each band, these calibrations are sufficient to determine the signal strength in all channels served by this filter band and compressor. In addition to the amplitude response of the compressors, a frequency response is also performed for each frequency channel. All frequency channels are checked to confirm that the filter bands have the proper shape and no spurious responses. The effective noise bandwidths are measured by stimulating the instrument with a white noise signal of known spectral density. For the electric field antenna, the electric field strength is computed by assuming that the antenna has an effective length of Leff = 3.5 meters. This length is the distance between the geometric centers of the two dipole elements. For the search coil magnetic antennas, the magnetic field sensitivity and frequency response was calibrated in the IPG magnetic field observatory at Chambon La Foret, France. These calibrations were performed using a Helmholtz coil driven by a known AC current source. The absolute accuracy of the sensitivity calibration is estimated to be about 3 percent. The magnetic noise levels were measured by placing the search coils in a mu-metal chamber, which shields the sensors from external noise sources. The calibration and other details of the GLL PWS are given in Kurth [KURTH1993]. Operational Considerations:Nominally, the instrument is operated anytime low rate science (LRS) or greater data rate capability is available. When LRS is the maximum data rate, the instrument is operated in its power-up mode, with the SA and SFR toggling back and forth between the electric and magnetic antennas. When wideband data can be recorded or transmitted to the Earth, then the wide range of instrument modes and antenna configurations are utilized based on the science objectives for a given time interval. The UVS instrument has a stepper motor that drives its grating which is a major source of magnetic interference in the frequency range from about 100 Hz to 2 kHz. Every attempt is made to work with the UVS team to minimize the times during which the grating is moved while PWS is observing on the magnetic antenna. In many cases, this requires a time- sharing arrangement which allows for some percentage of magnetic viewing time in an interference-free environment, but which also allows UVS to observe with a moving grating in order to achieve its science objectives. Detectors:The plasma wave sensors on Galileo consist of one 6.6 meter tip-to-tip electric dipole antenna and two search coil magnetic antennas. The electric dipole antenna is mounted at the end of the magnetometer boom approximately 10.6 meters from the spacecraft, and the search coil magnetic antennas are mounted on the high gain antenna feed. [illustration] The electric antenna consists of two graphite epoxy elements with a root diameter of 2.0 cm, tapering to 0.3 cm at the tip. The dipole elements are mounted perpendicular to the magnetometer boom to minimize electric field distortion effects due to the spacecraft structure. The antenna axis is also oriented perpendicular to the spacecraft spin axis in order to permit direction finding. Each element is hinged 1.8 meters from the tip so that the antenna can be folded for launch. A housing at the base of the dipole elements contains two preamplifiers. These preamplifiers provide low impedance signals to the main electronics package. Each element is grounded to the spacecraft structure through a 250 MegOhm resistance to limit differential charging effects. The search coil magnetic antenna consists of two high permeability rods, 25.5 and 27.5 cm long, one optimized for low frequencies, 5 Hz to 3.5 kHz, and the other optimized for high frequencies, 1 kHz to 50 kHz. The winding on the low frequency search coil consists of 50,000 turns of 0.07 mm diameter copper wire and the winding on the high frequency search coil consists of 2000 turns of 0.14 mm diameter copper wire. The two search coils are mounted orthogonally to minimize the electrical coupling between the sensors. Both search coils are mounted perpendicular to the spacecraft spin axis. The high frequency sensor is perpendicular to the electric dipole antenna and the low frequency sensor is parallel to the electric dipole antenna. Two preamplifiers mounted in a housing near the search coil are used to provide low impedance signals to the main electronics package. Frequencies below 2.4 kHz are obtained from the low frequency search coil, and frequencies above 2.4 kHz are obtained from the high frequency search coil. Electronics:All of the signal processing for the plasma wave experiment is performed in a single main electronics package. The main electronics package is mounted in the spacecraft body near the base of the magnetometer boom. Signals from the electric dipole antenna and the two search coils are processed by a wideband receiver and three spectrum analyzers: a high frequency spectrum analyzer also called the High Frequency Receiver (HFR), a medium frequency spectrum analyzer also called the Sweep Frequency Receiver (SFR), and a low frequency spectrum analyzer also called simply the Spectrum Analyzer (SA). The HFR provides 42 frequencies from 100.8 kHz to 5.645 MHz with a fractional frequency spacing of delta-f/f ~ 10.0% and a bandwidth of 2 kHz. One spectral sweep is provided every 18.67 seconds with a dynamic range of 100 db. The SFR provides 112 frequencies from 40 Hz to 160 kHz with a fractional frequency spacing of delta-f/f ~ 8.0%. This analyzer gives one spectral sweep every 18.67 seconds with a dynamic range of 100 db. The low frequency SA provides 4 logarithmically spaced frequency channels from 5.62 Hz to 31.1 Hz. All four channels are sampled once every 2.67 seconds with a dynamic range of 110 db. The data from the HFR, SFR, and SA (and survey wideband data as described below) are transmitted to the ground via the low rate telemetry at a bit rate of 240 bits/sec. The wideband waveform receiver provides waveform measurements in three frequency bands, 5 Hz to 1 kHz, 50 Hz to 10 kHz, and 50 Hz to 80 kHz. The frequency band to be used is controlled by the spacecraft Command and Data Subsystem (CDS). An automatic gain control (AGC) circuit is used to control the amplitude of the output waveform. The AGC time constant is 0.1 seconds in the two high frequency bands and 1.0 second in the low frequency band. The waveform from the wideband receiver is digitized by a 4-bit analog-to-digital converter (ADC). The sample rate of the ADC is fixed at either 3,150, 25,200, or 201,600 samples per second, depending on the frequency band selected. The waveform data can be transmitted in real time or recorded on the spacecraft digital tape recorder. The plasma wave instrument has several modes of operation and methods of data transmission. These modes are also controlled by the spacecraft CDS. The medium and low frequency spectrum analyzers and the wideband waveform receiver can be connected to either the electric dipole antenna or the search coil magnetic antennas. In the normal mode of operation, the SFR and SA are cycled between the electric and magnetic antennas so that alternate electric and magnetic spectrums are obtained. Since the search coils do not provide signals in the frequency range covered by the HFR, this analyzer is always connected to the electric antenna. In the cycling mode of operation, the time required for a complete set of electric and magnetic field spectrums is 37.33 seconds. The SFR and SA can also be locked on either the electric or magnetic antennas to provide improved time resolution at the expense of complementary electric and magnetic field coverage. In all cases the HFR, SFR, and SA outputs consist of an 8-bit binary numbers that are approximately proportional to the logarithm of the received signal strength. In the ground data processing the data from the HFR, SFR, and SA will be displayed in the form of color frequency-time spectrograms. The frequency scale of the Galileo spectrograms will extend from 5.6 Hz to 5.65 MHz, and variable time scales will be available, ranging from 30 minutes to more than 24 hours, depending on the application. Normally, 24-hour spectrograms will be used to survey the plasma wave data. These survey spectrograms will be used to select specific intervals for more detailed analysis, such as comparison with charged particle or magnetic field data, or direction-finding analyses. The greatest flexibility in the operation of the plasma wave instrument is available in the wideband waveform receiver. This receiver provides very high resolution measurements of electric and magnetic field waveforms during times of special interest, such as the pass through the Io torus and satellite encounters. The waveform data provide the highest possible frequency and time resolution, subject only to the constraints of Fourier analysis, delta-f*delta-t ~ 1. Although the waveform receiver has only three frequency bands, with bit rates of 12.6, 100.8, and 806.4 kbits/sec, several spacecraft modes are available for recording and transmitting the data to the ground. In the highest time resolution mode, an essentially continuous sample of the electric or magnetic field waveform can be obtained over a bandwidth of 50 Hz to 80 kHz for periods of up to 18 minutes (the time required to fill the spacecraft tape recorder). On the ground the waveform data will be Fourier transformed in discrete packets, usually consisting of 1024 samples, and displayed in the form of a frequency-time spectrogram. These frequency-time spectrograms provide the highest time resolution data available from the Galileo plasma wave instrument. In certain modes of operation, such as MPW, XPW, and PW4, the duration of the wideband recording can be extended at the expense of reduced duty cycle, frequency coverage, or analysis bandwidth. To provide some wideband telemetry even when the high rate telemetry link is not available, a waveform survey output is included in the regular low rate telemetry data. This waveform survey output provides one block of 280 waveform samples every 18.67 seconds in two frequency bands, 5 Hz to 1 kHz and 50 Hz to 10 kHz. Filters:The following three tables describe the 158 frequency channels which make up the low rate science portion of the Galileo PWS. Table 1. Spectrum Analyzer (SA) Channels Channel MOD(mf,4) Center Frequency (Hz) Bandwidth (Hz) 1 0 (4) 5.62 0.832 2 3 10.0 1.86 3 2 17.8 2.75 4 1 31.1 4.79 mf is the minor frame counted from 1 through 28. Table 2. Sweep Frequency Receiver (SFR) Channels Chan mf Freq. (Hz) Bandwidth (Hz) Band 1 1 1 42.1 4.26 2 2 45.6 3 3 49.0 4 4 52.5 5 5 56.0 6 6 59.6 7 7 66.7 8 8 70.4 9 9 77.7 10 10 81.5 11 11 89.0 12 12 96.7 13 13 104.5 14 14 112.5 15 15 120.6 16 16 128.9 17 17 137.3 18 18 150.2 19 19 158.9 20 20 172.5 21 21 186.4 22 22 200.7 23 23 215.5 24 24 235.9 25 25 251.7 26 26 268.0 27 27 290.6 28 28 314.1 Band 2 29 1 337. 6.76 30 2 364. 31 3 392. 32 4 420. 33 5 448. 34 6 476. 35 7 534. 36 8 563. 37 9 622. 38 10 652. 39 11 712. 40 12 774. 41 13 836. 42 14 900. 43 15 965. 44 16 1.031k 45 17 1.098k 46 18 1.201k 47 19 1.272k 48 20 1.380k 49 21 1.491k 50 22 1.606k 51 23 1.724k 52 24 1.887k 53 25 2.013k 54 26 2.144k 55 27 2.325k 56 28 2.513k Band 3 57 1 2.70k 120. 58 2 2.91k 59 3 3.14k 60 4 3.36k 61 5 3.58k 62 6 3.81k 63 7 4.27k 64 8 4.50k 65 9 4.98k 66 10 5.21k 67 11 5.70k 68 12 6.19k 69 13 6.69k 70 14 7.20k 71 15 7.72k 72 16 8.25k 73 17 8.78k 74 18 9.61k 75 19 10.17k 76 20 11.04k 77 21 11.93k 78 22 12.85k 79 23 13.79k 80 24 15.09k 81 25 16.11k 82 26 17.15k 83 27 18.59k 84 28 20.10k Band 4 85 1 21.6k 1520. 86 2 23.3k 87 3 25.1k 88 4 26.9k 89 5 28.7k 90 6 30.5k 91 7 34.2k 92 8 36.0k 93 9 39.8k 94 10 41.7k 95 11 45.6k 96 12 49.5k 97 13 53.5k 98 14 57.6k 99 15 61.7k 100 16 66.0k 101 17 70.3k 102 18 76.9k 103 19 81.4k 104 20 88.3k 105 21 95.4k 106 22 102.8k 107 23 110.3k 108 24 120.7k 109 25 128.9k 110 26 137.2k 111 27 148.8k 112 28 160.8k (Note that the same bandwidth applies to the entire set of channels in a band.) Table 3. High Frequency Receiver (HFR) Channels HFR Center Frequency Channel mf (MHz) 1 1, 2 0.1008 2 5, 6 0.1134 3 9, 10 0.1260 4 13, 14 0.1386 5 17, 18 0.1512 6 21, 22 0.1638 7 25, 26 0.1764 8 3, 4 0.2016 9 7, 8 0.2268 10 1, 12 0.2520 11 5, 16 0.2772 12 9, 20 0.3024 13 3, 24 0.3276 14 7, 28 0.3528 15 1 0.4032 16 5 0.4536 17 9 0.5040 18 13 0.5544 19 17 0.6048 20 21 0.6552 21 25 0.7056 22 2 0.8060 23 6 0.9070 24 10 1.008 25 14 1.109 26 18 1.210 27 22 1.310 28 26 1.411 29 3 1.613 30 7 1.814 31 11 2.016 32 15 2.218 33 19 2.419 34 23 2.621 35 27 2.822 36 4 3.226 37 8 3.629 38 12 4.032 39 16 4.435 40 20 4.838 41 24 5.242 42 28 5.645 (The bandwidth for all channels is 1340 Hz) Mounting Offsets:The electric antenna is mounted at the end of the magnetometer boom such that its effective axis is parallel to the spacecraft X axis (perpendicular to both the magnetometer boom and the spacecraft spin axis. The low frequency magnetic search coil is mounted with its effective axis parallel to the spacecraft X axis and the high frequency search coil is parallel to the spacecraft Y axis (perpendicular to the X axis and to the spacecraft spin axis. Field of View:The field of view of the PWS, whether from the electric dipole antenna or one of the magnetic search coils is a standard dipole antenna pattern which has a maximum sensitivity to the field along the axis of the sensor. For radio waves which propagate above the characteristic frequencies of the plasma and which do not interact with the local plasma, this means maximum sensitivity is to sources in a plane perpendicular to the antenna axis, since the electric and magnetic fields of a radio wave are normally perpendicular to the propagation vector. Data Rates:The basic low rate science (LRS) data rate of the instrument is 240 bps. Wideband waveform receiver data rates range from _._ kbps to 806.4 kbps, depending on the telemetry mode. Rates of 94._ kbps and lower can be either recorded on the spacecraft tape recorder or transmitted directly to the ground; rates of 403.2 and 806.4 kbps can only be recorded onboard for later playback at lower rates. Instrument Modes:The PWS has several modes of operation. The SA and SFR can either monitor only the electric antenna, only the magnetic antenna, or toggle back and forth between the two, obtaining a complete spectral scan from each antenna before switching to the other. This toggling mode is the most commonly utilized mode. There is also a mode which enables the magnetic search coil calibration tone. When enabled, the calibration signal is excited for 56 mf (37.33 sec) at the beginning of each MOD(RIM,8)=0 until disabled. The wideband receiver has three basic modes (analysis bandwidths) and can be attached to either the electric antenna or the magnetic. The three modes provide analysis bandwidths of 10 kHz, 80 kHz, and 1 kHz although there is an additional mode which toggles between the 10 kHz and 1 kHz mode. In this mode, the waveform data is collected for 14 mf (9.33 sec) in one bandwidth and then for 14 mf in the other bandwidth. A wide range of telemetry formats are available for the wideband data. For any one of the three wideband modes, the instantaneous data rate is fixed (806.4 kbps for the 80 kHz mode, 100.8 kbps for the 10 kHz mode, and 12.6 kbps for the 1 kHz mode. However, the different telemetry modes differ primarily in the number of consecutive samples collected during an RTI. This results in a variation in the duty cycle depending on the data rate allocated to this data stream in the selected telemetry mode. References:
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larry-granroth@uiowa.edu
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