5.0 RPWS Calibration Methods

An extensive series of amplitude calibrations, frequency responses, phase calibrations, and instrument performance checks were carried out on the RPWS prior to launch, both before and after integration on the spacecraft. These tests and calibrations were performed at room temperature (25°C), -20°C, and 40°C. While there are calibration signals available in the instrument for in-flight calibration purposes, these are mainly used to check for drifts due to aging or radiation exposure. The primary calibration information to derive physical units (spectral density, etc.) is derived from the prelaunch tests.

5.1. Calibration Procedure

5.1.1. Electric Antennas

Each of the RPWS electric antennas is connected to an amplifier located in the high frequency receiver. In the long wavelength regime where the wavelength is much greater than the tip-to-tip length of the antenna, L, the potential between the two elements of an electric dipole is given by V = E Leff, where Leff is a quantity called the effective length [Gurnett, 1998]. This relation ignores the presence of any electrical load. For a dipole consisting of collinear elements, to a very good approximation Leff is one-half the tip-to-tip length of the antenna. For a V configuration, Leff is the distance between the geometric centers of the two elements. Taking into account the 120° included angle between the two elements of the Ex dipole and the finite distance between the roots of the two elements, the effective length of the Cassini Ex dipole is Leff = 9.26 m. Next we must consider the effect of an electrical load on the antenna. At frequencies above a few tens of Hz, where the antenna impedance is primarily capacitive, the ratio of the output voltage to the input voltage is given by a simple capacitive divider,

(1)

In the above equation CA and CL are the antenna and load capacities, respectively. For a cylindrical antenna, to a good approximation, the antenna capacity is given by

(2)

where L/2 is the length of the element, a is the radius of the element and o is the permittivity of free space. For L/2 = 10 m and a = 0.014 m, CA = 100.2 pF.

The load capacity can be considered to be the sum of two parts: (1) the base capacity, which consists of the internal capacities of all circuits and mechanical structures connected to the base of the antenna; and (2) the capacity between the antenna and the spacecraft structure. Since the electric antenna cannot be fully extended in-flight configuration while the spacecraft is on the ground, there is no way to directly measure the capacitance between the antenna and the spacecraft structure. Thus, the total base capacitances listed in Table 5-1 only give lower limits to the load capacities. Another, and perhaps better way to estimate the load capacities is to determine the half-wavelength resonance frequency of the antenna. This resonance frequency is shifted downward by a small amount due to the load capacity. During the instrument checkout in December 1998 and January 1999, a sharp peak was observed in the Ex dipole noise level at 8.5 MHz (see the discussion of noise levels in Section 14.1). This peak is due to the half-wave resonance. An antenna modeling program was then used with an accurate representation of the spacecraft geometry to determine the load capacitance required to place the resonant frequency at 8.5 MHz. The resulting value was CL = 150 pF. This is our best current estimate of the load capacity for the Ex antenna. Using the capacitive divider relation above with CA = 100.2 pF and CL = 150 pF gives a loading loss of 7.9 dB for the Ex antenna. Chapter 6 discusses the calibration of the electric antenna in more detail.

Due to the complex shape of the Cassini spacecraft that acts as the ground plane for these antennas, the electrical axes of the antennas are expected to be rotated significantly from their physical orientations [Rucker et al., 1996]. Radio signals from Jupiter were used as a calibration source during the Cassini flyby of Jupiter in late 2000 and early 2001 to determine the electrical axes of the antennas. The orientations of the antenna axes can be determined by inverting the direction-finding software, using the direction to Jupiter as the source direction. Preliminary results from this calibration are given by Vogl et al. [2001]. Additional antenna calibrations are scheduled on approach to Saturn and while in orbit after the Huygens probe is released.

Electric field calibrations of the receivers were performed by applying signals with known amplitudes at the preamplifier inputs and relating the input signal strength with the resulting telemetry value. As described below, the telemetry values from the various receivers are related to the input signal strength either via a set of look-up tables or through an analytical function that fits ground calibration data.

5.1.2. Magnetic Antennas

The sensitivity, frequency response, phase response, and noise levels of the magnetic search coil antennas were calibrated at a low-noise magnetic field observatory near Chambon La Foret, France. The calibrations were performed using a Helmholtz coil driven by a known AC current source to produce a magnetic field with a known magnitude and phase. The thermal blankets were installed in order to account for any conductivity effects that the blanket may have on the sensor response. The resulting data relate input field strength to the voltage at the magnetic preamplifier output. Additional calibrations were then used to relate the voltage input to the various receivers to an output telemetry value. Combining these two steps provides an overall end-to-end calibration of the magnetic field measurements. This calibration was verified through a series of tests performed with the search coil antennas connected to the RPWS instrument after integration on the spacecraft. An end-to-end check of the phase response was also carried out after integration on the spacecraft. Noise levels were measured on the spacecraft by placing the search coils in a -metal chamber, which shields the sensors from external noise sources. See Chapter 7 for a more detailed discussion of the calibration of the magnetic search coils.

5.1.3. High Frequency Receiver

The calibration of the high frequency receiver was performed at Meudon, France. Sine wave and white noise sources with known spectral properties were applied to the input of the electric antenna amplifiers and the corresponding telemetry values were recorded. The calibration data were then fit to analytical models for each of the receiver bands. From these models, the telemetry values can be converted to physical units. A series of tests was then performed after integration of the instrument on the spacecraft, to verify the high frequency receiver calibrations. See Chapter 11 for a more detailed discussion of the calibration of the HFR.

5.1.4. Medium Frequency Receiver

The medium frequency receiver uses compressors with a piecewise-linear approximation to a logarithmic amplitude response. Over the range of amplitudes, the response of each compressor consists of a series of five distinct linear segments that deviate slightly from a true logarithmic response. Because the compressors for each of the three bands have different amplitude sensitivity characteristics, the amplitude response of each compressor must be measured separately. This was accomplished by applying a signal to the input of the electric antenna preamplifier at the center frequency of the filter channel for each Band [Band 1 (channel 8), Band 2 (channel 16) and Band 3 (channel 10)]. The amplitude was stepped in 2 dB increments to cover the complete amplitude range of the receiver.

Since all the frequency channels of a given medium frequency receiver band utilize the same logarithmic compressors, it is only necessary to measure the amplitude response at one frequency channel for each band. The amplitude responses for the remaining channels can be obtained by calibrating the gain through the entire system as a function of frequency at a fixed amplitude. This calibration is called a channel-to-channel gain test. By combining the amplitude response with the channel-to-channel gain calibration, a complete set of calibrated look-up tables can be produced that convert the output telemetry values to signal strengths at the preamplifier inputs.

The frequency response of the three filters used in the medium frequency receiver was also determined by sweeping a sine wave signal over frequency while monitoring the output of a fixed channel. See Chapter 10 for a more detailed discussion of the calibration of the MFR.

5.1.5. Low Frequency Receiver

The low frequency receiver employs onboard fast Fourier transform processing of one or two channels of the five-channel waveform receiver to produce spectrums over a frequency range from 1 Hz to 26 Hz. The calibration of the low frequency receiver involved both amplitude and frequency calibrations. The amplitude calibration is similar to the medium frequency receiver amplitude calibration. A signal was applied to the input of the electric antenna amplifier at the center frequency of every fourth Fourier frequency component. The amplitude was then stepped in 2 dB increments to cover the complete amplitude range of the receiver.

The sensitivity of the low frequency receiver varies from channel to channel across the band. These frequency variations affect the calibration. A channel-channel gain test, similar to that performed on the medium frequency receiver, was conducted for each of the gain states of the low frequency receiver. This test was performed by applying an input signal of fixed amplitude to the input of the electric preamplifiers, and sweeping the signal from the lowest frequency channel of the low frequency receiver to the highest frequency channel. A combination of the amplitude response and the channel-channel gain measurements is then used to complete the calibration of the low frequency receiver for each gain state. See Chapter 9 for a detailed discussion of the calibration of the LFDR.

5.1.6. Wideband and Five-Channel Waveform Receivers

The response of the wideband and five-channel waveform receivers was determined by applying signals of known frequency and amplitude to the electric preamplifiers, and determining the gain factors required to convert the telemetry values into physical units. These gain factors provide calibrations for both the time-series waveform and the spectrum produced by a Fourier transform of the waveform. The amplitude response of the wideband and five-channel waveform receivers was determined for each gain state, and for every filter mode. The frequency and phase response of the wideband receiver and the five-channel waveform receiver was determined by applying input signals of fixed amplitude to the input of the individual receivers, and sweeping the signal across the frequency band of the receiver. End-to-end calibration checks were performed by repeating the frequency response test and by applying an input signal of white noise with known spectral properties to the input of the electric and magnetic preamplifiers. See Chapter 12 for a discussion of the WBR calibration, and Chapter 13 for a discussion of the WFR calibration.