G. B. Hospodarsky
george-hospodarsky@uiowa.edu
W. S. Kurth
william-kurth@uiowa.edu
C. W. Piker
chris-piker@uiowa.edu
Dept. of Physics & Astronomy
The University of Iowa
Iowa City, IA 52242
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This document provides an overview of the calibration test data for the Juno - Waves instrument. All Calibration tables my be found in the CALIB directory of the PDS 3 volume JNOWAV_1000.
This document describes the calibration of the Juno Waves Receivers and Sensors. The bulk of the Waves calibrations were carried out on the ground prior to integration on the spacecraft with spot checks of these carried out during ATLO. The calibrations involve precisely measuring the gains of the preamps, gain amplifiers, and attenuators in the receivers, the filter responses of each of the receivers, the transfer function of the search coil, and the base capacitance of the electric preamp. The ultimate goal of the calibrations is to accurately relate the telemetered values to physical electric and magnetic field strengths and spectral densities. However one significant calibration aspect was not accounted for during pre-launch testing. Namely the effect of a large spacecraft ground plane on comparatively short conducting antenna elements.
Release 14 (Sept. 2020) of the JNOWAV_1000 volume contains a new set of calibration tables with version numbers listed in the applicable sections below. These incorporate a number of aspects of re-calibration, with the primary one being the use of a more realistic effective length for the electric dipole antenna. However, other modifications include, improved calibrations near the noise floor of the receivers and better matching of channels across frequency ranges. All data product files previously archived as version V01 have been recalibrated and archived under version V02. All new product files starting with release 14 were created using these new calibrations, though as this is the first release of these products they carry the version designation V01. If in doubt as to the calibrations applied, note that the applicable calibration tables for a particular release are always present in the CALIB directory of the volume.
External references:
Waves Project Documents:
The Juno Waves engineering team performed an extensive series of amplitude calibrations, frequency responses, and instrument performance checks prior to launch, both before and after integration on the spacecraft. The primary calibration information is to derive physical units (spectral density, etc.) from the instrument telemetered outputs. Calibrations were primarily performed by applying to the instrument inputs signals at known amplitudes and frequencies, and recording the output of instrument. Calibrations tests were performed on each of the individual receivers, on the search coil sensor (MSC), and finally as an end-to-end calibrations (sensor + receiver). The primary calibration was performed at +22° C, with additional testing performed at -35° C and +75° C to characterize the instrument at the predicted temperature extremes.
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 the ground calibration data.
The amplitude response of the search coil sensor and amplifier were determined in a two step process. Initially, a detailed calibration of the MSC was performed in a Mu-metal shield by applying a known signal to a calibration coil to produce a magnetic field with a known magnitude. The resulting data relate input field strength to the voltage at the magnetic preamplifier output over the frequency range of the sensor. The second step related the voltage input to the receiver and the output telemetry value. Combining these two steps provides an overall end-to-end calibration of the magnetic field wave measurements. The MSC was also attached to the Waves instrument and end-to-end calibrations were performed by driving the calibration coil to produce a known magnetic field strength and verifying the expected telemetry values.
In very simple terms, the Waves instrument measures the differential potential between the two elements of the electric antenna. The electric field E is simply -V/Leff where V is the measured potential and Leff is the effective antenna length.
The pre-launch calibration utilized the geometric antenna length which is basically the distance between the mid-points of the two conducting antenna elements, 2.41 m. The second revision calibration modifies this length by two important electrical considerations. These are discussed in detail in Kurth et al. (2017), but the first involves taking the complex and large surrounding spacecraft structure, including the solar panels, into account. This structure is the ground plane for the antenna system. Given the very short antenna elements (2.8 m) in the presence of the spacecraft with ~ 8-m solar panels and associated structure, the spacecraft effectively decreases the effective length of the antenna system. This effect was studied by Sampl et al. (2012; 2016) using both an analog rheometry analysis as well as a surface patch model of the spacecraft. The result is that the antenna has an effective length, after taking into account the complex ground plane of the spacecraft of 1.46 m.
The second effect is a capacitive divider effect due to the base capacitance of the antenna and the capacitance of the antenna to space. While the base capacitance is somewhat uncertain, this is effectively a decrease in sensitivity (equivalently, another decrease in effective length) of 8 db. Combining these, we've used an effective antenna length of 0.5 m for the Juno electric antenna in the second revision calibration tables. Clearly, this means the newly-calibrated electric field associated with a 1-V potential difference is 4.8 times greater than the old one. And, spectral densities that are proportional to E**2 will increase by a factor of about 23.
The Waves calibrations all consist of transfer functions from telemetered data numbers (dn) to field strengths. Near the bottom of the receiver dynamic range, these can be challenging to reconstruct from pre-launch measurements because of noise sources in the lab and the fact that noise levels in space can be quieter, in many cases. The initial method used to produce dn-to-field strength transformations near the noise floor for some of the channels were unrealistically non-linear in the V1.0 calibration. The second revision tables use an improved fitting method and the inflight noise levels to provide a smoother transformation in this lower portion of the dynamic range. It should be noted that this will affect low magnetic signal levels as well as those for electric field.
During the recalibration of the electric and magnetic fields near the receiver sensitivity floor, the calibrations performed before launch were reanalyzed as a verification of the overall V2.0 calibration. This reanalysis produced some small changes (at most a few dB) in the receiver response over frequency, resulting in a smoother (flatter) spectrum between adjacent channels when a signal is detected in multiple channels.
The Low Frequency Receiver (LFR) comprises two identical low-frequency channels (LFR-Lo) covering the range from ~50 Hz to ~20 kHz and allows measurements of both the electric and magnetic component of waves when utilizing both sensors. A third channel in the LFR (LFR-Hi) analyzes signals only from the electric dipole and covers the frequency range of ~10 kHz to ~150 kHz. The outputs from the two LFR-Lo channels are digitized waveforms consisting of 50 ksps at 16 bit resolution. The waveforms are sent to the Digital Signal Processor (DSP) for on board spectrum analysis. The output from the LFR-Hi channel is digitized at a rate of 375 ksps with 16 bit accuracy. This waveform is also analyzed by the DSP.
Electric field calibrations of the LFR-Lo and LFR-Hi receivers were performed by applying signals at 5 kHz and 25 kHz respectively at the preamplifier inputs. The amplitude of the input signal was stepped in 2 dB increments to cover the complete amplitude range of the receiver and the resulting telemetry value were recorded. This test was repeated for each possible configuration of the Electric Preamp and LFR receiver attenuator settings. A linear fit was performed on the resulting center part of the curves of input voltages vs output telemetry values, producing a look up table of input voltage to output telemetry value at 5 and 25 kHz.
The frequency response of the LFR receivers was determined by applying input signals of fixed amplitude to the input of the Electric preamplifier, sweeping the signal across the frequency band of the receivers, and measuring the output telemetry values of the receivers.
Combining the results of the amplitude calibrations at 5 and 25 kHz with the frequency sweep calibration, a look up table was created for the LFR-Lo and LFR-Hi receivers containing an electric field voltage value for each output telemetry value for each frequency channel of the receivers.
Amplitude and frequency sweep calibrations were performed with the MSC attached to the LFR-Lo receiver by driving a calibration coil in a Mu-metal shield to produce a known magnetic field strength at the MSC sensor and measuring the output telemetry values of the LFR. These test results were used to create a look up table containing the magnetic field (nT) for each output telemetry value of each frequency channel of the receiver.
The Second Revision LFR calibration tables are:
Amplitude calibrations were performed for each attenuator setting of the electric preamp and the LFR receivers. From these tests, attenuation or gain values were determined for each possible combination of attenuation settings, and are found in the following table. This table is unchanged for the second revision calibration.
The Juno Waves instrument contains two nearly identical high frequency receivers labeled Board 44 and Board 45. In normal operations, one of the receivers operates as the HFR (High Frequency Receiver) and the other as the HFWBR (High Frequency Waveform Receiver). HFR products are reduced frequency resolution products and provide a constant background survey. HFWBR products contain roughly 1000 times the frequency resolution of HFR products and correspondingly have much greater storage and telemetry bandwidth requirements.
The HFR utilizes signals from the electric dipole and is capable of covering the range from 100 kHz to 45 MHz, though typical science data from the HFR cover the frequency range from ~137 kHz to ~41Hz. The base band channel of the HFR (100 kHz -- 3 MHz) is a broadband channel that is sampled at a rate of 7 Msps with 12 bit resolution. This waveform is sent to the DSP for on board spectrum analysis and are converted to survey data products covering the range of 137 kHz to 2.98 MHz. The frequency range above ~3 MHz is covered by using a synthesized frequency mixed with the incoming signals in 1-MHz bands and the amplitude in each of the channels is detected sequentially as in a swept frequency receiver. The power in each band is recorded via a log amplifier whose output is sampled with 8 bit resolution. Though the instrument may be commanded to set the center frequency of the 1 MHz bands as high as 44.75 MHz, standard HFR science operations are conducted with a 41 MHz top edge.
Unlike HFR, all HFWBR products are waveforms. The base band (100 kHz -- 3 MHz) is sampled at 7 Msps with 12-bit resolution. High-frequency (above 3 MHz) science products are not derived from log amplifier measurements. Instead two synthesized signals are mixed with the incoming signal. Both synthesized signals are at the same frequency but second signal is phase shifted by 90 degrees relative to the first. Both frequency-mixed signals are sampled at 1.3125 MHz with 12-bit resolution.
Electric field calibrations of the HFR receiver were performed by applying pure sine-wave tones at 1 MHz (base band), 3.5 MHz, 6.5 MHz, 10.5 MHz, 21.5 MHz, 29.5 MHz, and 38.5 MHz at the preamplifier inputs. The amplitude of the input signal was stepped in 2 dB increments to cover the complete amplitude range of the receiver and the resulting telemetry value were recorded. This test was repeated for different configurations of the Electric Preamp and HFR receiver attenuator settings. A linear fit was performed on the resulting center part of the curves of input voltages vs output telemetry values, producing a look up table of input voltage to output telemetry values.
The frequency response of the HFR receivers was determined by applying pure sine-wave tones of fixed amplitude to the input of the Electric preamplifier, sweeping the signal across the frequency band of the receiver, and measuring the output telemetry values of the receivers.
Combining the results of the amplitude calibrations with the frequency sweep calibration, a look up table was created for the HFR receiver containing an electric field voltage value for each output telemetry value of each frequency channel of the receivers.
The Second Revision HFR calibration tables are:
Amplitude calibrations were performed for each a number of attenuator settings of the electric preamp and the HFR/HFWBR receivers. From these tests, attenuation or gain values were determined for each possible combination of attenuation settings, and are found in the following table. This table is unchanged for the second revision calibration.
