3.0 Overview of Instrumentation

The RPWS instrumentation consists three electric antennas, three magnetic antennas, a Langmuir probe and its associated electronics, and five specialized receivers designed to address the scientific objectives and performance requirements discussed in the two previous sections. In this section we describe each of these elements of the instrument.

3.1 Block Diagram

A simplified block diagram of the RPWS instrument is shown in Figure 3.1.1. Three monopole electric field antennas, labeled Eu, Ev, and Ew, are used to provide electric field signals

Figure 3.1.1

to the various receivers. The orientations of these three antennas relative to the x, y, and z axes of the spacecraft are shown in Figure 3.1.2. By electronically taking the difference between the voltages on the Eu and Ev monopoles, these two antennas can be used as a dipole, Ex, aligned along the x axis of the spacecraft. The Eu and Ev antennas also can be used to sound the local plasma by transmitting short pulses. In an alternate mode of operation, they can be biased and used as Langmuir probes to measure the phase velocity of density structures in the plasma. The tri-axial search coil magnetic antennas, labeled Bx, By, and Bz in Figure 3.1.1, are used to detect

Figure 3.1.2

three orthogonal magnetic components of electromagnetic waves. The search coil axes are aligned along the x, y, and z axes of the spacecraft. The spherical Langmuir probe, shown at the bottom of the block diagram, is used for electron density and temperature measurements. Both the electric antennas and the Langmuir probe can be used to detect dust impacts.

Next, we consider the function of the five receivers shown in the middle column of the block diagram in Figure 3.1.1. These receivers are connected to the antennas described above by a network of switches in the block labeled "antenna selection switches," the details of which are

not shown. The high frequency receiver (HFR) provides simultaneous auto- and cross-
correlation measurements from two selected antennas over a frequency range from 3.5 kHz and 16 MHz. By switching the two inputs of this receiver between the three monopole electric antennas, this receiver can provide direction-of-arrival measurements, plus a full determination of the four Stokes parameters. The high frequency receiver includes a processor that performs all of its digital signal processing, including data compression. The high frequency receiver also includes a sounder transmitter that can be used to transmit short square wave pulses from 3.6 to 115.2 kHz. When used in conjunction with the high frequency receiver, the sounder can stimulate resonances in the plasma, most notably at the electron plasma frequency, thereby providing a direct measurement of the electron number density. The medium frequency receiver (MFR) provides intensity measurements from a single selected antenna over a frequency range from 24 Hz to 12 kHz. This receiver is usually operated in a mode that toggles every 32 seconds between the Ex electric dipole antenna and the Bx magnetic search coil, thereby providing spectral information for both the electric and magnetic components of plasma waves. The low frequency receiver (LFR) provides intensity measurements from 1 Hz to 26 Hz, typically from the Ex electric dipole antenna and the Bx magnetic antenna. The five-channel waveform receiver (WFR) collects simultaneous waveforms from up to five sensors for short intervals in one of two frequency bands, either 1 to 26 Hz, or 3 Hz to 2.5 kHz. When connected to two electric and three magnetic antennas, this receiver provides wave normal measurements of electromagnetic plasma waves. The wideband receiver is designed to provide nearly continuous wideband waveform measurements over a bandwidth of either 60 Hz to 10.5 kHz, or 800 Hz to 75 kHz. These waveforms can be analyzed on the ground in either the temporal domain, or in the frequency domain (Fourier transformed) to provide high-resolution frequency-time spectrograms. In a special frequency-conversion mode of operation, the high frequency receiver can provide waveforms to the wideband receiver in a 25-kHz bandwidth that is tuneable to any frequency between 125 kHz and 16 MHz. Table 3.1.1 summarizes the characteristics of the five basic receivers described above.

The Langmuir probe controller shown in the block diagram is used to sweep the bias voltage of the probe over a range from -32 to +32 V in order to obtain the current-voltage characteristics of the probe, and thereby the electron density and temperature. The controller can also set the bias voltage on the Eu and Ev monopoles over a range from -10 to +10 V in order to operate them in a current collection mode for ne/ne measurements.

The RPWS data processing unit shown on the right-hand side of the block diagram consists of three processors. The first processor, called the low-rate processor, controls all instrument functions, collects data from the high frequency receiver, the medium frequency receiver, the low frequency receiver, and the Langmuir probe, and carries out all communications with the spacecraft Command and Data System (CDS). The second processor, called the high-
rate processor, handles data from the wideband and five-channel waveform receivers and passes the data along to the low-rate processor for transmission to the CDS. The third processor, called

TABLE 3.1.1

Summary of RPWS Receiver Characteristics

Receiver

Section	Measurements	Spectral Range	Spectral Resolution	Temporal

Resolution
High frequency Receiver

(HFR)	For E1= Eu, Ev or Ex and

E2 = Ew;

E1 2, E2|2, Re(E1 E2*), Im(E1 E2*)	3.5 kHz - 318 kHz

0.125-16.125 MHz	f/f = 5, 10, 20% (log)

f = 3.1 kHz - n×25 kHz (linear)	0.1 - 10 s/spectrum
Medium Frequency Receiver (MFR)	One of:

Ex, Eu,

Ev, Ew,

Bx, Bz	24 Hz - 180 Hz

180 Hz - 1.5 kHz

1.5 kHz - 12 kHz	f/f 13%

f/f 7%

f/f 7%	16 s/spectrum

16 s/spectrum

8 s/spectrum
Five-Channel Waveform Receiver (WFR)	Five of:

Ex, Eu,

Ev, Ew,

Bx, By, Bz, LP	~1 Hz - 26 Hz

3 Hz - 2.5 kHz	0.2 Hz, typical

3.5 Hz typical	1 data set/5 min, typical,

1 data set/16 s, max
Low Frequency Receiver

(LFR)	Two of:

Ex, Eu,

Ev, Ew,

Bx, By, Bz, LP	~1 Hz - 26 Hz	f/f = 12 %	1E + 1B spectrum/16s, typical
Wideband Receiver

(WBR)	One of:

Ex, Eu,

Ev, Ew, Bx, LP	60 Hz - 10.5 kHz

0.8 kHz - 75 kHz	13.6 Hz, typical

109 Hz, typical	125ms/spectrum, typical

the data compression processor, is primarily used for data compression, but can also perform specialized operations such as on-board dust detection by using waveforms from the wideband receiver.

As shown in Figure 3.1.2, the RPWS hardware is mounted in several locations around the Cassini spacecraft. The main electronics package, which includes the medium frequency receiver and the digital processing unit, resides in bay 4 of the main spacecraft bus. The electric antenna deployment mechanisms and the high frequency receiver are mounted on a bracket on the +y side of the spacecraft, just below the base of the magnetometer boom. The magnetic search coil assembly is supported by a short boom that is attached to the high gain antenna support structure on the -x side of the spacecraft. The Langmuir probe is mounted on the end of an 0.8-m hinged boom that extends outward from the magnetic search coil assembly.

3.2. Electric Antennas

The three electric monopole antennas and their deployment mechanisms were provided by Orbital Sciences Corporation. The antenna elements consist of conducting cylinders, each 10 meters long and 2.86 cm in diameter. The elements are made of beryllium-copper, silver plated on the exterior surface, and painted black on the interior for thermal control. Approximately 12% of the surface area is perforated with small holes to allow sunlight to heat the shaded side of the element in order to reduce thermal bending. The elements themselves are formed by two opposing semi-cylindrical strips with interlocking tabs. For launch, the elements are flattened and rolled onto a spool. During deployment after launch, a motor-driven deployment mechanism feeds the element through a forming channel that expands the strips into a cylindrical tube.

The physical orientations of the three elements with respect to the x, y, and z axes of the spacecraft are provided in Table 3.2.1. The Eu and Ev elements are extended symmetrically at angles of 60° with respect to the spacecraft y-z plane. The plane containing these two elements is rotated 37° with respect to the spacecraft x-y plane. These two elements can be used to provide a dipole antenna (Ex) with a tip-to-tip length of 18.52 m parallel to the spacecraft x axis. The Ew antenna is extended perpendicular to the plane formed by the Eu and Ev elements (i.e., at an angle of 37° from the spacecraft z axis). Several considerations led to this antenna configuration. First, the three monopoles provide the nearly orthogonal tri-axial configuration required for	TABLE 3.2.1

Comparison of Physical and Electrical Orientations of the Electric Antennas

Physical Orientation*	Electrical Orientation (Rheometry)
Antenna†	(deg)	(deg)	(deg)	(deg)
Eu	107.5	24.8	107.9	16.5
Ev	107.5	155.2	107.3	162.7
Ew	37	90	31.4	91.2
*	The angle is the polar angle measured with respect to the spacecraft +z axis, (cos = ), (and the angle is azimuth angle measured with respect to the +x axis (tan = x/y).

†	In some previous papers [Ladreiter et al., 1995; Rucker et al., 1996], various engineering documents, and some sections of this calibration document the Eu, Ev, and Ew antenna are labeled Ex+, Ex-, and Ez. The u, v, w, notations are used in this section to avoid confusion with the spacecraft x, y, and z axes.

direction finding. Second, as previously described, the Eu and Ev elements can be operated as a dipole (Ex,), which minimizes common-mode coupling in order to provide the lowest possible level of spacecraft-generated interference. The Eu and Ev elements are also symmetric with respect to the magnetometer boom, which minimizing the effect this boom has on the dipole antenna pattern. Third, the w-axis element is rotated somewhat away from the spacecraft in order to reduce electrical coupling and interference from electrical equipment in the lower bay of the spacecraft. Fourth, the Eu and Ev elements are rotated away from the spacecraft x-y plane so that they do not interfere with the field of view of the stellar reference unit. This orientation also provides clearance for release of the Huygens probe.

The three electric antennas were deployed to their full 10-meter length during a 30-hour activity on 25 October 1997, ten days after launch. Full deployment was confirmed by both a potentiometer reading and a limit switch indication, the latter being the most reliable indication of full deployment. A brief set of measurements performed subsequent to the deployments verified that each antenna was properly connected to the instrument and that there were no shorts to the spacecraft structure. There are no plans to retract or otherwise change the length of the elements for the duration of the mission.

Due to the complex shape of the Cassini spacecraft, the effective electrical axes of the monopole antennas differ significantly from their physical orientations. Electrical measurements using a 1:30 scale model of the spacecraft in a tank of electrolytic fluid (rheometry) were performed by Rucker et al. [1996]. These measurements show angular offsets of the electrical axes by as much as 5 to 6 degrees for the Ew antenna, and 7 to 8 degrees for the Eu and Ev antennas (see Table 4.2). Ladreiter et al. [1995] have developed an analysis technique that can be used to perform in-flight determinations of the electrical axes by using a point source with known polarization characteristics. Jupiter provides the best source for this purpose and in-flight calibrations of the electrical axes of the antennas were carried out during the Jupiter flyby, which took place in December 2000 and January 2001. Preliminary results of this calibration are given by Vogl et al. [2001]. Although this calibration occurred with the Huygens probe attached to the spacecraft, the rheometry results show that the electrical axes shift by only a small amount (less than one degree) when the probe is released.

3.3. Magnetic Antennas

The tri-axial search coil magnetic antennas were provided by CETP in Velizy, France. These antennas are mounted on a short, fixed boom just under the high gain antenna on the -x side of the spacecraft, as shown in Figure 3.1.2. The three axes of the search coils are aligned with the x, y, and z axes of the spacecraft. Each search coil utilizes 10,000 turns of 0.07 mm wire wound around a mu-metal core 25 cm long with a cross-sectional area of 15 mm2. The sensors have a usable frequency range of 1 Hz to 20 kHz. A flux feedback winding is used in each search coil to flatten the frequency response over the range from 30 Hz to 18 kHz. The transfer function in this frequency range is approximately 150 mV/nT. Preamplifiers are mounted at the base of the mounting boom. When the spacecraft -z axis is pointed toward the Sun, as it is whenever the spacecraft is in the inner solar system, the high gain antenna acts as a sun shade for the search coils. Two radioisotope heating units are used to provide an internal heat source for thermal control. A thermal blanketing system surrounds the assembly to minimize the radiated heat loss.

3.4. Langmuir Probe

The Langmuir probe consists of a 5-cm diameter titanium sphere mounted on the end of a 0.8-m hinged boom that folds outward from one leg of the boom that holds the search coil magnetic antenna assembly. A preamplifier is mounted near the base of the search coil boom. To minimize the influence of photoelectrons, the outermost 10.9-cm segment of the boom is made as thin as possible (diameter = 6.35 mm) and has an outer guard shield that is held at the same voltage as the probe sphere. In its deployed configuration, the probe is approximately 1.5 m from the nearest spacecraft surface. When the -z axis of the spacecraft is sun-pointed, as it is in the inner solar system, the probe is in the sunlight. The surface of the probe is coated with titanium nitride by baking at high temperature in a nitrogen atmosphere [Whalström et al., 1992]. This treatment provides a durable surface with a constant work function so that the potential of the probe surface is uniform. (If the potential varies over the surface, the current-voltage characteristic is adversely affected.) This coating effectively eliminates hysteresis effects, which can occur for other surface coatings.

The boom holding the Langmuir probe was deployed on 25 October 1997 as part of the launch sequence. Deployment was verified by noting that the probe emitted the expected photoelectron current when biased to a negative voltage (the spacecraft orientation was such that the probe was in sunlight when deployed).

As mentioned earlier, two of the electric antennas (Eu, Ev) can also be operated as Langmuir probes in a current-collecting mode. This mode is used primarily for studying plasma density variations associated with waves and turbulence. The current measurement range is 1 nA to 100 A for positive bias voltages, and 100 pA to 100 A for negative bias voltages.

3.5. High Frequency Receiver

The high frequency receiver consists of two sets of four analog receivers followed by a digital signal processing unit. The design is based on a similar receiver flown on the Wind spacecraft [Bougeret et al., 1995]. One set of receivers is connected to the Ew monopole. The other set of receivers can be connected either to the Ex dipole or toggled between the Eu and Ev monopoles. Three of the four receivers have fixed-frequency filters (A, B, and C) that together cover a frequency range from 3.5 to 319 kHz. Each filter covers 2.2 octaves in frequency (i.e., a factor of 4.5). Digital spectral analysis is performed within each of the three filter bands to provide 8, 16, or 32 logarithmically spaced frequency channels, yielding spectral resolutions of 20, 10, or 5%, respectively. The fourth (HF) receiver is tunable over a frequency range from 100 kHz to 16.1 MHz and has a bandwidth of 25 kHz. This receiver can be tuned in two different modes, HF1 and HF2. HF1 provides measurements over a frequency range from 100 kHz to 4.125 MHz in 25-kHz steps. HF2 provides measurements over a frequency range from 125 kHz to 16.125 MHz in 50-kHz steps. Within the 25 kHz passbands of HF1 or HF2, it is possible to have either 1, 2, 4, or 8 linearly spaced channels, thereby giving frequency resolutions of 25, 12.5, 6.25, and 3.125 kHz. The high frequency receiver has an automatic gain control (AGC) that provides a dynamic range of about 90 dB. In addition, a switchable attenuator provides an additional 26 dB of dynamic range. At any specific gain setting of the AGC, the digital spectrum analyzer has a dynamic range of about 30 dB. Table 3.5.1 provides a summary of the basic characteristics of the high frequency receiver.

TABLE 3.5.1

Characteristics of the High Frequency Receiver

Band	Frequency Range	Frequency Steps	Frequency Resolution	Integration Times
A	3.5 kHz - 16 kHz	8, 16, or 32 (logarithmic)	20, 10, or 5 %	.125, .25, .5, 1 s
B	16 kHz - 71 kHz	8, 16, or 32 (logarithmic)	20, 10, or 5 %	.125, .25, .5, 1 s
C	71 kHz - 319 kHz	8, 16, or 32 (logarithmic)	20, 10, or 5 %	.125, .25, .5, 1 s
HF1	125 kHz - 4.125 MHz	n×25 kHz; 1, 2, 4, or 8 linear channels within 25-
kHz band	3.125, 6.25, 12.5, 25, or n×25 kHz	20, 40, 80, 160 msec
HF2	125 kHz - 16.125 MHz	n×50 kHz; 1, 2, 4, or 8 linear channels within 25-
kHz band	3.125, 6.25, 12.5, 25, or n×50 kHz	10, 20, 40, 80 msec

Using the two sets of receivers, complex auto- and cross-correlation measurements can be performed between the Ex dipole and Ew monopole antennas, thereby giving the amplitude and relative phase of signals detected by the two antennas. This mode of operation allows for polarization and direction-finding measurements of purely circularly polarized radio emissions such as cyclotron maser radiation. In an alternate mode of operation, the first of the two sets of receivers can be toggled rapidly between the Eu and Ev monopoles, with the second set on the Ew antenna, in order to provide direction-finding measurements.

For low frequency space-borne radio measurements, the wavelength is usually much larger than the length L of the antenna. Under these conditions ( L) angular resolution is virtually nonexistent. Nevertheless, the direction to the center of the source can be obtained by comparing the amplitudes and relative phases of signals from various combinations of antennas. On a rotating spacecraft, direction-finding measurements can be performed with only two antennas. This technique was used by the Ulysses spacecraft to derive the location of Jovian low frequency radio sources with an accuracy of approximately 1 - 2° [Reiner et al., 1993; Ladreiter et al., 1994]. The main limitation of the rotating antenna technique is that the radio emission characteristics must remain essentially constant during one spacecraft rotation, which usually takes ten seconds or more. This is a serious limitation for auroral radio emissions, which often have rapid intensity variations, sometimes on time scales of a fraction of a second or less. Such rapid variations explain why the technique could be applied to only limited portions of the Jovian radio spectrum [Ladreiter et al., 1994]. Since Cassini is three-axis stabilized the rotating antenna technique obviously cannot be used. Ladreiter et al. [1995] have shown that direction-finding and full polarization information (i.e., four Stokes parameters) can be obtained with three nearly orthogonal antennas. Direction-finding and polarization measurements are strongly interdependent parameters and, ideally, measurements should be made simultaneously on all three antennas. However, since the high frequency receiver can only process signals from two antennas at a time, direction-finding and polarization measurements are made using consecutive correlation measurements with two different pairs of antennas, (Eu, Ew) and (Ev, Ew). The time between these measurements is typically 45 to 325 msec (above 125 kHz) or 250 to 2000 msec (below 125 kHz). Because of the very short time interval between successive measurements, this technique is expected to be relatively insensitive to short-term intensity variations.

The high frequency receiver utilizes digital filters that have an out-of-band rejection greater than 45 dB. They are preceded by an anti-aliasing filter and an automatic gain control (AGC). The AGC normalizes the signal amplitude to a level that provides optimal performance for the digital signal processor. The first receiver has an input, which for simplicity we call E1, that is selectable from either Ex, Eu, or Ev. The second receiver has an input, which we call E2, from the Ew antenna. The digital processor provides two auto-correlations, E1 2 and E2 2, and two cross-correlation Re(E1 E2*) and Im(E1 E2*), for each frequency channel. In the normal mode of operation, a series of auto- and cross-correlation measurements is averaged in order to reduce fluctuations to the digital quantization level (1 bit = 0.375 dB). The exact number of spectrums integrated is controlled by command and depends on the detailed choices of frequency and bandwidth. The intrinsic noise level of the A, B, and C bands is 4.9 × 10-17 V2 Hz-1 for the E1 receiver and 2.5 × 10-17 V2 Hz-1 for the E2 receiver. For HF1 and HF2, these noise levels are typically double the A, B, and C noise levels due to mixer noise.

The high frequency receiver also has the capability of providing a frequency-converted signal to the wideband receiver. In the frequency conversion mode of operation, the high frequency receiver translates high frequency signals downward into a 50-kHz to 75-kHz passband, which is then sent to the wideband receiver. This passband can be shifted to any frequency between 4.125 and 16 MHz, except those centered at multiples of 50 kHz, and to any frequency between 125 kHz and 4.125 MHz.

The high frequency receiver also includes a capability to measure amplitudes in a selected frequency channel with millisecond temporal resolution. In this mode of operation any frequency between 125 kHz and 16 MHz can be selected. This capability allows very short duration signals such as lightning to be detected and can be used for envelope sampling of bursty emissions.

Since the high frequency receiver is under the control of its own processor, virtually all of the receiver parameters can be selected by command, making for an extremely flexible instrument. This flexibility allows for regular surveys of the radio frequency spectrum of Saturn at low data rates (typically about 450 bps), as well as specialized studies of short duration phenomena at very high spectral resolution (the maximum number of frequency channels across the frequency range of the receiver is approximately 3300) as well as direction-finding and full polarization measurements. To minimize the data rate for a given mode, the processor also includes both lossless (Meander code) and lossy (Rice code) compression algorithms.

3.6. Sounder Transmitter

The high frequency receiver also includes a sounder transmitter. The transmitter design is based on the Ulysses/URAP instrument [Stone et al., 1992]. In the sounder mode of operation, a short pulse consisting of a 26-V peak-to-peak square wave is transmitted on the Ex dipole antenna, and the receiver then "listens" for a resonance response from the plasma. After each pulse the transmitter frequency is increased by a small increment. This process is continued across the entire frequency range of interest, thereby producing a spectrum of the plasma resonances. When a resonance is encountered, the received signal is strongly enhanced for a short period of time after the pulse has been transmitted. The sounder can measure electron densities from 0.2 to 164 cm-3 (fpe 3.6 to 115.2 kHz), and is expected to be active for a few seconds out of every 5 to 10 minutes.

The sounder can transmit in five bands over a frequency range from 3.6 kHz to 115.2 kHz. The five bands are described in Table 4-4. Each band is divided into 18 channels for a total of 90 frequencies. The default bandwidths are given in Table 3.6.1. The upper and lower frequency limits of the frequency sweep can be selected by command.

TABLE 3.6.1

Sounder Transmission Characteristics

Band	Freq. Range (kHz)	1/Pulse Width (Hz)
1	3.6 - 7.2	200
2	7.2 - 14.4	400
3	14.4 - 28.8	800
4	28.8 - 57.6	1600
5	57.6 - 115.2	3200

Two modes of operation for the sounder are currently planned, designated PAA and AAA. Each of these two modes involves taking three measurements at the selected frequency. In the PAA (Passive Active) mode, a "passive" measurement is taken before the pulse is transmitted, and then two "active" measurements are taken; the first at a time T0 after the pulse, and the second at a time T1 after the first measurement. In the AAA mode, three "active" measurements are taken sequentially at time intervals of T0, T1, and T2 after transmission of the pulse. The parameters T0, T1, and T2 can be selected by command.

3.7. Medium Frequency Receiver

The medium frequency receiver (MFR) system is based on similar receivers flown on the ISEE, Galileo, and Polar spacecraft [Gurnett et al., 1978; 1992; 1995]. The purpose of the medium frequency receiver is to provide continuous spectral measurements over a frequency range from 24 Hz to 12 kHz, with moderate frequency and temporal resolution and a relatively low data rate. This receiver system has the capability of processing signals from the Bx, Bz, Eu, Ev, Ew and Ex, antennas. In a typical mode of operation, the receiver toggles between the Ex dipole antenna and the Bx magnetic antenna in order to provide alternating electric and magnetic spectrums.

The medium frequency receiver consists of three frequency bands designated 1, 2, and 3, each covering a 3 octave frequency range. Band 1 is divided into 16 logarithmically spaced frequency channels ( f/f 13 %) and bands 2 and 3 each have 32 frequency channels ( f/f 7%). The effective bandwidths for the three bands are given in Table 3.7.1. The frequencies are selectable using a two-stage frequency conversion scheme. Each band of the medium frequency receiver uses a mixer to convert the selected frequency to a fixed intermediate frequency (IF) filter. A second mixer then down-converts the IF signal to a baseband filter. The baseband signal is logarithmically compressed, rectified, and summed to provide a 0-5 volt DC output, and then converted to an 8-bit binary number by an analog-to-digital converter. The dynamic range of the medium frequency receiver, from the lowest signal that can be detected to the saturation level, is about 110 dB.

TABLE 3.7.1

Medium Frequency Receiver Characteristics

Band	Frequency Range	f/f	Effective Bandwidth (Hz)	Sweep Time

(s)
1	24 - 180 Hz	13%	5.6	16
2	180 - 1500 Hz	7%	19.4	16
3	1.5 - 12 kHz	7%	139	8

The upper frequency limits of the three medium frequency receiver frequency bands are determined by three bandpass filters at the front end of the receiver. The lower frequency limits are imposed by a combination of the bandpass filters and the low frequency limit of the front end (IF) mixer. The filter frequencies are scaled upward by a factor of eight for each successive higher band, thereby providing frequency coverage over nine octaves. The low-pass filters provide 55 dB of rejection for out-of-band signals.

3.8. Five-Channel Waveform Receiver

The five-channel waveform receiver provides simultaneous waveforms from up to five separate sensors in passbands of either 1 to 26 Hz, or 3 Hz to 2.5 kHz. The purpose of this receiver is to provide high-resolution spectral measurements and to determine the polarization and wave normal of low frequency plasma waves. When connected to the Langmuir probe (including the Eu and Ev monopoles operating in the current collecting mode), the waveform receiver can also provide ne/ne waveforms. The waveforms are sampled with 12-bit resolution once every 10 ms for the 1 to 26 Hz bands and once every 140 s for the 26 Hz and 2.5 kHz bands. Simultaneous waveform samples from up to five receiver channels are stored in a buffer memory until they are read out via the science telemetry. The waveforms are processed on the ground to produce the auto- and cross-correlations that are needed to compute the wave normal and polarization.

The waveform receiver consists of five parallel analog input channels. The five inputs can be connected to various sensors. Table 3.8.1 summarizes the sensor selections for each of the five channels. Signals from the five sensors are routed directly to a gain select stage that has commandable gains of 0, 10, 20, or 30 dB for each receiver channel. Channels 1 and 2 have independent gain settings, but channels 3 to 5 (which are usually connected to the Bx, By, and Bz magnetic sensors) share the same gain control lines. The programmable gain amplifiers are similar to those used in the wideband receiver. The amplifier gains are controlled by the high rate processor and can be operated in either a fixed gain mode or an automatic ranging mode. The output of the programmable gain amplifiers goes to a 26-Hz low-pass filter and a 3-Hz to 2.5-kHz bandpass filter. The desired analysis passband is selected by spacecraft command and the waveform outputs are sampled simultaneously by sample-and-hold circuits and converted to digital signals by a 12-bit analog-to-digital converter. The five-channel waveform receiver can also be commanded to measure one, two, three or four channels. These special modes allow greater resolution for special observations. For example, the single-channel waveform receiver mode using the 3-Hz to 2.5-kHz passband effectively allows a third wideband receiver channel in addition to the 60-Hz to 10.5-kHz and 0.8-kHz to 75-kHz channels that are included in the wideband receiver. Also, the waveform receiver provides signals to the digital processing unit in order to generate the passband for the low frequency receiver function (see Section 3.9).

TABLE 3.8.1

Waveform Receiver Input Selections

Channel	Input Selection
1	Ex dipole, Eu*
2	Ew, Ev*
3	Bx, Langmuir probe*
4	By
5	Bz
* Operating in Langmuir probe (current collecting) mode
Since the primary purpose of the five-channel waveform receiver is to provide the amplitude and relative phase of the five measured field components, each receiver channel must have known amplitude and phase responses. To achieve the required performance, all five low-
pass passive filters have been carefully matched and are phase-stable to within a one degree between 1 Hz and 2.5 kHz.

3.9. Low Frequency Receiver

In order to have continuous spectral information in the frequency range from 1 Hz to 26 Hz, waveforms from the five-channel waveform receiver are Fourier transformed by the data compression processor in the digital processing unit. The three-channel mode (Ex, Ew, Bx) of the five-channel waveform receiver is used, although typically only two of these channels, Ex and Bx, are analyzed, thereby providing simultaneous electric and magnetic spectrums. Processor speed limits the temporal resolution between successive spectrums to about 16 seconds. The Fourier transform processing utilizes a 512-point waveform to produce a 256-frequency component spectrum with 0.2-Hz resolution. To minimize the data volume and to produce a spectrum with resolution similar to that produced by the medium frequency receiver, the linear components of the spectrum are binned to form a logarithmically spaced spectrum with 32 frequencies covering a frequency range from 0.2 to 26 Hz.

3.10. Wideband Receiver

The wideband receiver is similar to the wideband receivers previously used on the Voyager, Galileo, Polar, and Cluster spacecraft, and provides high-resolution electric and magnetic field waveform measurements in passbands of either 60 Hz to 10.5 kHz, or 0.8 to 75 kHz. The wideband receiver also serves as the front end for the on-board dust detection function. The data compression processor (described below) is used to search the waveforms for the signature of dust impacts.

The wideband receiver processes signals from a single selected sensor (either Eu, Ev, Ex, Ew, Bx, or Langmuir probe). To provide the capability for obtaining waveforms at higher frequencies, the wideband receiver input can be connected to the frequency conversion output from the high frequency receiver (see Section 3.5).

The instantaneous dynamic range of the wideband receiver is 48 dB. Because of the expected large dynamic range of the input signals, a set of discrete gain amplifiers and an automatic gain control are used to amplify the signal to the proper level in steps of 10 dB over a range of 0 dB to 70 dB. This system provides a total dynamic range of over 100 dB for the wideband receiver. The feedback loop in the automatic gain control has a time constant of 0.1 seconds. However, gain updates are only made prior to a waveform capture. Since waveform captures typically occur once per multiple of 125 ms, the effective gain update can be much slower than 0.1 s. The output from the discrete gain amplifiers goes to the two bandpass filters (60 Hz to 10.5 kHz or 0.8 kHz to 75 kHz). The output of the selected bandpass filter is sent to an 8-bit analog-to-digital converter. The sampling rate is 27,777 samples/s for the 10-kHz channel and 222,222 samples/s for the 75-kHz channel.

3.11. Data Processing Unit

The RPWS data processing unit uses three microprocessors, the low-rate processor (LRP), the high-rate processor (HRP), and the data compression processor (DCP), to perform the tasks necessary for the instrument operation. These processors are responsible for collecting the science and housekeeping data, data formatting and data compression, and provide the interface to the spacecraft command and data system.

The tasks and responsibilities of the three processors are as follows. The low-rate processor is considered the primary processor since it has the sole interface with the Cassini spacecraft via the bus interface unit (BIU). It also provides the interface to the high frequency receiver, the interface and control for the medium frequency receiver and the analog housekeeping analog-to-digital converter, and the control for the antenna motor subsystem. The high-rate processor is used to control the waveform and wideband receivers, and the Langmuir probe instrument. The high-rate processor contains a data compression chip for compression of waveform signals. The data compression processor contains a dedicated math processor and is used only for data processing tasks, such as data compression. This processor performs the on-
board dust-detection and also the Fourier transforms for the low frequency receiver.

The essential elements of each of the three processors are an 80C85 microprocessor, two or more random access memory chips, and one or two read-only memory chips. The 80C85 runs with a 3-MHz clock and is used in the expanded configuration which uses an address latch to give a full 16-bit address bus and a bi-directional buffer to expand the data bus. Each processor can communicate with the other two processors via the interprocessor communication bus. The 80C85 has a memory address bus width of 16 bits giving a total addressing capability of 216 or 64 kilobytes (kB). The memory in each of the processors consists of both a read-write memory and a read-only memory.

The low-rate and high-rate processors each have more than 64 kB of random access memory available. The low-rate processor contains 64 kB of read/write memory with 16 kB write-protected and 4 kB of read-only memory. The bus interface unit appears to the processor as an additional 16 kB of random access memory. The high-rate processor has 96 kB of random access memory. The excess memory is handled by switching memory chips under program and hardware control.

All communication with the spacecraft is over a redundant 1-MHz bi-directional serial data bus conforming to MIL STD 1553. The interface is implemented via a bus interface unit.

3.12. Mass, Power, and Telemetry

The RPWS has a total mass of 37.68 kg and requires a power of 16.38 W in its fully operational science modes. This power value does not include a momentary ~3 W increase when the bus interface unit is active. By project policy, this bus interface power is allocated to the command and data system and not to the science instrument. Mass and power breakdowns for the various elements of the instrument are provided in Table 3.12.1.

TABLE 3.12.1

Summary of RPWS Physical Characteristics

Element	Mass (kg)	Power (peak W)	Volume (cm3)	Location
Main Electronics	5.59	5.09	41.7×17.8×16.8	Main Bus, Bay 4
Antenna Bracket Assembly	29.77	11.14	61×67×67	Upper shell structure of Bay 4
Magnetic Search Coils	1.05	0	30×30×30	1-m boom attached to high gain antenna support structure
Magnetic Search Coil Preamps	0.28	0.1	12.6×3.2×8.8	Base of search coil boom
Langmuir Probe	0.85	0	74.3×10×11.5	0.8-m boom on search coil boom
Langmuir Probe Preamp	0.135	0.05	8.7×9×4.8	Base of search coil boom
Total	37.68	16.38

Because of its many different receivers, operating modes, and because of the use of several different data compression schemes, the RPWS has a highly variable science telemetry rate. In all of the defined telemetry modes that include RPWS science telemetry, there are four data pickup rates, or rates at which the RPWS can send data to the command and data system. These rates are 30.464, 60.928, 182.784, and 365.568 kbps. However, because of limitations on the Cassini solid state recorder data volume as well as limits on the total data volume that can be telemetered to the ground in any given day, the RPWS can utilize such high data rates only occasionally and for relatively short periods of time. Therefore, a number of different observing modes have been defined that use minimal data rates for basic survey information and utilize the higher data rates for special observations, usually involving the wideband or five-channel waveform receivers. In the usual case where the actual science telemetry rate is less than the current telemetry mode's data pickup rates, RPWS outputs "zero-length" packets that are discarded by the command and data system to reduce the net data production rate.

The basic low-rate survey mode we have defined generates approximately 1 kbps before compression. Based on limited in-flight experience to date, the actual data rate in this mode after compression is about 700 bps. However, since most of the compression techniques used are data content dependent, it is not clear that this compressed data rate is representative of what will be encountered at Saturn. In addition to the continuous low-rate observations, the RPWS survey includes occasional wideband receiver samples (typically about one minute every couple hours) that are essential to the interpretation of the lower resolution data. On average, these high rate samples add approximately 600 bps to the RPWS survey rate. Typical observation modes that require higher data rates include the 75-kHz wideband receiver observations which generate close to 360 kbps before compression, the maximum duty cycle mode of the five-channel waveform receiver in the 2.5-kHz bandpass which generates approximately 130 kbps before compression, the low-duty cycle mode of the 10-kHz wideband receiver measurements which generates approximately 30 kbps before compression, and the high-resolution temporal and/or spectral modes of the high frequency receiver which generate data rates up to about 4 kbps.

Because of the uncertainty in the compression factors and data volume allocations imposed due to the limited downlink and onboard storage, the RPWS has the ability to monitor its actual data volume and compare it to a model. In the case where the actual data production is exceeding the model, various steps can be taken to decrease the data rate. For example, the rate at which waveform samples are acquired by the five-channel waveform receiver can be slowed down. This approach is effective when no wideband data are being acquired. During wideband receiver operations, the effective data rate can be reduced by decreasing the duty cycle of the wideband data. For example, instead of acquiring a 2048-sample data set once every 125 ms, a data set could be acquired every 250 ms.