The Hera Saturn Probe mission will conduct in situ measurements of the structure, composition and fundamental processes operating within Saturn’s atmosphere. Measurements will be made by a suite of instruments on the probe as it descends for up to 75-90 minutes under a parachute from the tropopause near 100 mbar, through the upper cloud decks, down to at least 10 bars. The Tier 1 instruments, designed to address the highest priority science goals, include a Mass Spectrometer and an Atmospheric Structure Instrument. The instruments comprising the Tier 2 payload address lower priority science goals, and include a Net Flux Radiometer, a Nephelometer, and a Radio Science experiment. While all instruments are located on the Hera probe itself, one ultrastable oscillator for the Radio Science experiment will be mounted on the Carrier.

All instruments can operate on both the day and night side of Saturn, although the visible channel of the Net Flux Radiometer can only measure the altitude profile of solar energy absorption if the descent is on the dayside. In this section we describe the measurement principle, the instrument design, the resource requirements including mass, power, volume, and data rate, interface and calibration requirements, and a summary of technology readiness, heritage, and critical issues (if any). The total data returned from the probe will range from 15 to 20 megabits per channel (30-40 megabits total). The following table summarizes the characteristics of each instrument included in the Hera Saturn probe.

Summary of Parameters for Hera Science Instruments

Hera Mass Spectrometer

Measurement Principle

The core of the Hera MS is a time-of-flight mass spectrometer (TOF-MS). The TOF-MS has several advantages for space research: i) all masses are measured at the same time, thus there is no need for scanning the mass thereby increasing the sensitivity, ii) the TOF-MS is simple and robust and is very suitable for remote operation on a spacecraft, iii) the TOF-MS is very light-weight. The cadence of mass spectrometric measurements is variable, from mass spectra accumulated every 1-second to integration up to 300 seconds. At suitable times measurements of atmospheric gas are replaced by measurements of calibration gas, and measurements of gas enriched and separated from the bulk atmosphere.

The atmospheric gas will enter the experiment via a gas inlet system with several independent entrances of various conductances, which will cover the pressure range of 0.1 – 10 bar level. The cadence of mass spectra is adjusted such that the vertical resolution is about 1.8 km along descent trajectory, which amounts to about 400 mass spectra along the descent trajectory.

Not all species can be measured directly in the gas entering from the atmosphere, at least not with the desired accuracy. Noble gases, for example, will be separated from the atmospheric inflow and collected by a cryotrap enrichment system. After sufficient enrichment of the noble gases is accomplished they are released to the TOF-MS for a dedicated mass spectrometric measurement while the direct sampling of the atmosphere is interrupted. Similarly, the use of an additional cryotrap for the enrichment of hydrocarbons and other trace species will also be analyzed at regular intervals.

The accuracy of some composition measure­ments will be enhanced by carrying several reservoirs of reference gases with an accurately known gas mixture. For example, for the measurement of the He/H ratio, a gas container with a calibrated He/H mixture is part of the Hera MS experiment that will allow for the measurement of this ratio with an accuracy of 2% or better. Similarly, containers with a calibrated mixture of noble gases, and with reference gases for key isotopes (H, C, N, and O), are included in the Hera Mass Spectrometer.

 

Design Description / Operating Principle

The Hera Mass Spectrometer consists of four units: the TOF-MS, the Tunable Laser Spectrometer (TLS), the gas separation and enrichment system (GSES), and the reference gas system (RGS).

The TOF-MS consists of a pulsed ion source, a time-of-flight drift path, an ion mirror (reflectron), and a fast ion detector. The TOF-MS is a compact instrument and has a mass range of 1 – 1000 u/e, a mass resolution of m/∆m = 1100, and a very high sensitivity [98]. Ions, continuously generated in the ion source, are pulse-extracted, and sent as ion packets along the TOF path with a repetition frequency of 10 kHz, to the detector resulting in a mass spectrum. These spectra are accumulated for a defined integration period (1 – 300 seconds), depending on the desired vertical resolution along the descent trajectory. The integration of many spectra provides for a dynamic range of 6 – 7 decades in each accumulated spectrum; together with various detector gain steps and the gas enrichments at a dynamic range that exceeds 12 decades is achieved.

The Tunable Laser Spectrometer (TLS) [99] will be employed as part of the Hera MS to make high accuracy measurements of isotopic ratios of the molecules H₂O, NH₃, CH₄, CO₂ and others. TLS employs ultra-high spectral resolution tuneable laser absorption spectroscopy (∆υ = 0.0005 cm^–1) in the near infra-red (IR) to mid-IR spectral region. TLS is a direct non-invasive, simple technique that for small mass and volume achieves sensitivities at the sub-ppb level for gas detection. Species abundances can be measured with accuracies of a few percent, and isotope determinations have an accuracy of about 0.1%. The TLS system can measure the isotopic ratios of D/H, ¹³C/¹²C, ¹⁵N/¹⁴N, ¹⁸O/¹⁶O, and ¹⁷O/¹⁶O as demonstrated by the Mars Science Laboratory SAM GC-MS instrument [100, 101].

The GSES consists of a cryotrap, an ion pump, and a non-evaporable getter (NEG), which together are used to achieve the noble gas enhancement. The NEG removes all constituents except methane and the noble gases. The cryotrap traps the products of the NEG process, except for helium and some neon. The ion pump then operates to pump away the helium, which is the second most abundant gas in Saturn’s atmosphere, thus enhancing the signal to noise in the remaining noble gases by a factor of about 200. This enrichment cell will be accessed periodically during descent to allow the noble gases to be analysed. The cryotrap for minor species will have a separate gas inlet. It will be heated periodically and a valve opened to allow the descent measurements to be interrupted for analysis.

The Reference Gas System consists of a central manifold and pressure sensor connected to the mass spectrometer via a capillary leak. Reference gas mixtures are stored in stainless steel 1 ml containers at a pressure of approximately 1 bar. Each reference gas will be admitted into the manifold by opening a single valve in a short pulse. These valves have a leak rate of less than 10^–8 mbar l s^–1 and a controllable pulse width of less than 1 ms; they are a development from Rosetta – Ptolemy heritage (TRL 5). Alternative valves are the same as used on Philae (TRL 9) but have a higher power requirement and a longer operating cycle of several minutes.

The baseline instrumnet includes 3 reference gas mixtures; a hydrogen/helium mixture, a noble gas mixture and an isotope mixture. The composition of the isotopic reference gas will be a mixture of relatively inactive molecules, e.g. methane, carbon monoxide and nitrogen, depending upon the scientific targets.

The RGS includes an ion pump and non-evaporable getter to remove gases between analyses and allow calibration of the mass spectrometer during cruise, a few hours before atmospheric entry and during the atmospheric descent. The ion pump adds a significant mass to the RGS, which could be reduced by using the GSES pump instead; however this adds to the complexity in the timing between the two systems and potentially results in cross contamination between the reference gases and the atmospheric samples (see Figure D.1).

 

D.2.4 Mass, Power, Data Rate

Mass and power of the MS elements are summarized in Table D.1. The average data rate during descent is about 2 kbit/s (including 50 bit/s housekeeping data). The total data volume produced during a 90-minute descent is about 10.7 Mbit of compressed data.

 

 

Figure D.1 Schematics of the Hera MS experiment, with the TOF-MS, the GSES, the RGS, and the TLS units. The elements are valves (V), regulating valves (RV), pressure gauges (G), conductance limiters (C), pumps (P), gas reservoirs (RG), and non-evaporative getter (NEG).

 

 

Table D.1 Specifications of the MS elements

	Mass	Power
TOF-MS	8.2 kg (ion optical system, TOF electronics, DPU, gas entrance system, pumping)	25 watts (full operation)
GSES	4.9 kg (Cryotrap, heaters, valves, pump, electronics)	20 watts (maximum, nominally 1 W and the ion pump power)
RGS	1.2 kg (3 reference gas containers, valves, NEG, ion pump and control electronics)	20 watts (maximum before atmospheric entry, nominally 1 W and the ion pump power)
TLS	1.5 kg	3 watts

 

 

Specific/Critical Interface Requirements & Environment Constraints

The Hera Mass Spectrometer must be mounted on the entry probe such that the gas sampling entrance is in the atmospheric gas flow during descent. No chemical contamination from the entry probe should enter the gas sampling system during descent. The Hera MS is accommodated inside the entry probe in a stable thermal environment. There are no special requirements for radiation and magnetic cleanliness.

The Hera MS must be turned on before starting the scientific measurements to execute a warm-up procedure to reach stable operating conditions, including background measurements, instrument optimisation and calibration in time for the scientific operations. The exact turn-on procedure will be elaborated during phase A when more technical details of the entry probe are avaialable. For the direct atmospheric sampling, the GSES, and the TLS and gas chromatograph share a gas flow line with a gas inlet port in the entry probe fore dome at the apex near the stagnation point and an outlet port at the minimum pressure point at the rear of the probe. Metal ceramic devices seal the inlet and outlet ports. The inlet and outlet lines are evacuated after instrument calibration prior to shipment. They will be opened in sequence by redundant pyrotechnic actuators after probe entry and ejection of the probe front shield.

 

Specific Calibration Requirements

The Hera MS will be thoroughly calibrated on ground before delivery for entry probe integration. Before starting scientific operations during descent the Hera MS will execute an automatic optimisation programme to define the instrument operation parameters. This is followed by a calibration programme using the reference gases. A replica reference gas system will be stored in similar environmental conditions to the entry probe to monitor any changes during the long cruise. This is particularly important for the hydrogen/helium mixture as both hydrogen and helium will diffuse through the walls of the stainless steel container. Because of the changing temperature environment during descent, calibration using the RGS will be repeated several times during the trajectory.

 

TRL Assessment & Relevant Heritage

Significant heritage exists for the measurement of the chemical composition during a descent through the atmosphere, for example the Galileo Probe mass spectrometer system [49] or the Huygens Gas Chromatograph Mass Spectrometer (GCMS) [102]. Over the last two decades TOF mass spectrometers have been developed for space research, for example on the Rosetta mission [103], and offer several advantages over the quadrupole mass spectrometers used before. For example, a TOF-MS is over 1000 times more sensitive than the Cassini INMS (ten times from ion source efficiency and 100 times from better duty cycle). Also for the gas inlet system and the gas enrichment system there is plenty of heritage from previous missions, again the Galileo Probe mass spectrometer system, the Huygens GCMS, or more recently the mass spectrometer experiments on Philae, the Rosetta lander. The GSES, as part of the MASPEX instrument, has been selected to fly aboard the NASA mission Europa-Clipper with a launch date scheduled in 2022.

During phase A, a trade study will be conducted where the selected solution of having a high-sensitivity mass spectrometer with medium mass resolution together with a TDS instrument for selected isotope measurements will be compared to a single sensitive high-resolution mass spectrometer. The latter would allow all measurements to be performed with a single instrument, at the price of increased complexity of the instrument.

 

Critical Issues: None

 

D.3 Hera Atmospheric Structure Instrument

D.3.1 Investigation Overview

The Hera Atmospheric Structure Instrument (Hera-ASI) will make in situ measurements during entry and descent into the Saturn’s atmosphere in order to investigate the atmospheric structure, dynamics, and electricity. The scientific objectives of the Hera-ASI are to determine the atmospheric profiles of pressure and temperature, the evaluation of the density and mean molecular weight profile along the Probe trajectory and the investigation of the atmospheric electricity (e.g. lightning) by in situ measurements. Hera-ASI data will also contribute to the analysis of the atmospheric composition. Moreover Hera-ASI will have a primary engineering function by establishing the entry trajectory and the probe altitude and vertical velocity for correlating all Probe experiment data and to support the analysis of the Radio Science / Doppler Wind Experiment (DWE).

 

D.3.1.1 Atmospheric Structure and Stability

In situ measurements are essential for the investigation of the atmospheric structure and dynamics. Hera ASI will measure the atmospheric state (P, T and density) as well as constraining atmospheric stability, dynamics and its effect on atmospheric chemistry.

The determination of the lapse rate can be used to identify the location of condensation and eventually clouds, and to distinguish between saturated and unsaturated, stable and conditionally stable regions. The variations in the density, pressure and temperature profiles provide information on the atmospheric stability and stratification, on the presence of winds, thermal tides, waves and turbulence in the atmosphere.

 

D.3.1.2 Atmospheric Electricity

Hera ASI will measure unknown properties of Saturn lightning, determine the conductivity profile of the Saturnian troposphere, and detect the atmospheric DC electric field.

It is well known that 2000-km sized atmospheric storm systems on Saturn produce superbolt-like lightning discharges with energies up to 10¹³ J. To date the strong Saturn lightning radio emissions have only been measured from outside Saturn’s ionosphere, i.e. mostly at frequencies >1 MHz and occasionally down to a few hundred kHz. The Hera Atmospheric Structure Instrument will measure the unknown lightning spectrum in the frequency range of ~1-200 kHz, and will obtain burst waveforms with different temporal resolutions and durations. A Saturn lightning flash typically lasts ~100 ms and consists of many sub-discharges of the order of 0.1 ms, so waveforms over 100 ms with 0.1 ms resolution for the full flash and waveforms over 0.5 ms with 2ms resolution for the sub-strokes would be a sensible choice. The latter requires a sampling frequency of 500 kHz, which is also sufficient for obtaining the spectrum up to 200 kHz.

Atmospheric conductivity and the DC electric field are important basic parameters of atmospheric electricity which provide indirect information about galactic cosmic ray ionization, aerosol charging inside and outside of clouds, properties of potential Schumann resonances and so on.

 

D.3.2 Measurement Principle

The key in situ measurements will be atmospheric density, pressure and temperature profile by measuring deceleration of the entry vehicle and performing direct temperature and pressure measurements during the descent phase [74,104]. Densities will be determined using measurements of the deceleration of the probe during entry. The flight profile of the probe, including variations in speed and angle of attack provide information regarding turbulence and vertical motions. Once the probe heat shield is jettisoned, direct measurements of pressure, temperature and electrical properties will be performed. Hera ASI will monitor the acceleration experienced by the probe during the whole entry and descent phase and will provide the unique direct measurements of pressure, temperature, conductivity, and DC electric field through sensors having access to the atmospheric flow.

 

D.3.3 Design Description / Operating Principle

The Hera Atmospheric Structure Instrument (ASI) consists of several sensors both internal and external to the pressure vessel, and operates during high speed entry in the upper atmosphere and in descent when the probe is subsonic. The proposed instrument design leverages strongly from the Huygens ASI experiment of the Cassini/Huygens mission [105] and the Galileo and Pioneer Venus ASI instruments [106, 107]. The Hera ASI consists of four primary sensor packages: (1) a three axis accelerometer (ASI-ACC), (2) a pressure profile instrument (ASI-PPI), (3) temperature sensors (ASI-TEM) and (4) an Atmospheric Electricity Package (ASI-AEP).

The ASI-ACC will start to operate prior to the beginning of the entry phase, sensing the atmospheric drag experienced by the entry vehicle. Direct pressure and temperature measurement will be performed by the sensors having access to the atmospheric flow from the earliest portion of the descent until the end of the probe mission at approximately 10 bars. AEP will measure the atmospheric conductivity and DC electric field in order to investigate the atmospheric electricity and detecting lighting.

 

D.3.3.1 Accelerometers

The ACC package should be placed as close as possible to the center of mass of the entry vehicle. It consists of 3-axis accelerometers. The main sensor is a highly sensitive servo accelerometer aligned along the symmetry (spin) axis of the Probe, with a resolution of  10^-5 to 10^-4 m/s² (depending on the resolution setting) with an accuracy of 1%. Accelerations can be measured in the 0-200 g range (where g is the Earth’s acceleration of gravity). This sensor is the most sensitive accelerometer ever flown in a planetary entry probe [108]. Having a triaxial accelerometer (namely one sensor located along each probe axis) will allow for an accurate reconstruction of the trajectory and attitude of the probe, and to sense the atmospheric drag in order to derive the atmospheric density profile. Assuming the HASI ACC Servo performance at Titan, a noise performance of some 0.3 µg is expected. The exact performance achievable, in terms of the accuracy of the derived atmospheric density, will also depend on the probe ballistic coefficients, entry speed and drag coefficient, all of which will differ somewhat from the Titan case.

 

D.3.3.2 Temperature Sensors

The ASI-TEM utilizes platinum resistance thermometers to measure the kinetic temperature during the descent just as in the Huygens Probe ASI and Galileo probe. Two thermometers are exposed to the atmospheric flow and effectively thermally isolated from the support structure. Each thermometer includes two redundant sensing elements: the primary sensor (FINE) directly exposed to the airflow and a secondary sensor embedded into the supporting frame with the purpose to be used as spare unit in case of damage of the primary. The principle of measurement is based on the variation of the resistance of the metallic wire with temperature. The reading of the thermometer is made by resistance comparison with a reference resistor, powered by a pulsed current.

TEM has been designed in order to have a good thermal coupling between the sensor and the atmosphere and to achieve high accuracy and resolution. Over the temperature range of 60-360 K these sensors maintain an accuracy of 0.1 K with a resolution of 0.02 K.

 

D.3.3.3 Pressure Profile Instrument

The ASI-PPI will measure the pressure during the entire descent with an accuracy of 1% and a resolution of 1 micro bar. The atmospheric flow is conveyed through a Kiel probe inside the Probe where the transducers and related electronic are located.

The transducers are silicon capacitive sensors with pressure dependant dielectricum. The pressure sensor contains as dielectricum a small vacuum chamber between the two electrode plates, where the external pressure defines the distance of these plates. Detectors with diaphragms of different pressure sensitivity will be utilized to cover the pressure range to ~10 bar. The pressure is derived as a frequency measurement (within 3-20 kHz range) and the measurements is internally compensate for thermal and radiation influences.

 

D.3.3.4 Atmospheric Electricity Package

The ASI-AEP consists of sensors and a signal processing unit. Since Saturn lightning is very intense and localized, lightning discharges should be detectable by a short electric monopole, dipole loop antenna from distance of several thousands of kilometers. The conductivity of the atmosphere can be measured with a mutual impedance probe. A current pulse is sent through the surrounding medium and the resulting voltage is measured by two passive electrodes, from which the impedance of the medium can be determined. This can be corroborated by determining the discharge time (relaxation) of two charged electrodes. After the discharge, the natural DC electric field around the probe can also be measured with them. The signal processing unit (to be accomodated into the ASI main central unit) will manage to amplify the signals, extract waveforms of bursts with different durations and temporal resolutions, perform spectral analysis at various frequency ranges (1-200 kHz or in the TLF – Tremendously Low Frequency below 3 Hz to detect Schumann resonances), and to provide active pulses and sensor potential control to handle the conductivity and DC electric field measurements.

 

D.3.3.5 Data Processing Unit (DPU)

The control, sampling and data management of the ASI sensors is handled by a central Data Processing Unit including the main electronics for the power supply and conditioning, input/output and sensor control. The ASI-DPU interfaces directly to the entry probe processor.

 

D.3.4 Design Description / Operating Principle

The required resources of the Hera-ASI are based on estimates made from the heritage of the Huygens Atmospheric Structure Instrument.

• Mass: ~2.5 kg (including sensors, electronics, and supporting structure/boom);
• Volume: ~20 x 20 x 20 cm³ (distributed, including DPU);
• Data Rate: 250 bps (without compression)
• Power: ~10 watts

It is expected that the performance and requirements will be improved based on experience from the Huygens ASI, in terms of sensor design, packaging, and mounting, as well as the benefit of nearly two decades of technological improvements in sensor technology.

 

D.3.5 Specific/Critical Interface Requirements & Environment Constraints

The accelerometer packaging should be mounted as close as possible to the Probe’s center of mass. The TEM temperature sensors and the PPI pressure Kiel inlet should be mounted onto a fixed external boom ensuring that the sensors are outside of the probe boundary layer. Electric sensors should be accommodated on a boom in order to measure the DC electric field in vertical direction. EMC requirements and ESD protection have to be taken into account.

Heat shield Thermal Protection System (TPS) sensors, like those for MSL MEDLI and ESA ExoMars, will provide important data on TPS mass loss and heat shield dynamics during entry. These type of measurements proved to be critical for the Galileo entry probe [109].

 

D.3.6 Specific Calibration Requirements

Pre-flight: static and dynamical calibration will be performed at sensor, subsystem and instrument integrated level. Stratospheric balloon drop test experiments with the ASI package could be useful in order to assess sensors performance and to validate data retrieval methods.

In flight: during cruise phase CheckOuts (CO) will be regularly performed, ASI sensor performance will be monitored in order to check any drift due to aging or any degradation. Specifically on Huygens, in-flight data have been used in order to monitor the offset at zero g of the ACC sensor and estimate the long-term stability in the zero offset [108].

 

D.3.7 TRL Assessement & Relevant Heritage

     Each Hera-ASI component has strong application heritage tracking back through Pioneer Venus, and Galileo ASI instruments [110, 111] and Huygens ASI experiment of the Cassini/Huygens mission [105].

The Huygens ASI ACC main servo sensor is the most sensitive accelerometer ever flown in a planetary entry probe [108]. Having a triaxial accelerometer (namely one sensor located along each probe axis) will allow for an accurate reconstruction of the trajectory and attitude of the probe, and to sense the atmospheric drag in order to derive the atmospheric density profile.

The TEM utilizes platinum resistance thermometers just as in the Huygens Probe ASI, and Galileo and Pioneer Venus probes. The proposed type of the pressure sensors, other than being successfully flown as part of HASI on board the Huygens probe, have been flown onboard for the NASA’s Mars Phoenix 2007 mission and Mars Science Laboratory (MSL) and are part of the meteorological package of ESA’s ExoMars 2016 Schiaparelli (DREAMS) and NASA’s Mars 2020 rover (MEDA).

The Galileo Probe LRD (Lightning and Radio emission Detector) used a ferrite-core RF antenna and two photodiodes behind lenses to measure magnetic field pulses and optical emissions of lightning. Since Saturn lightning storms are quite localized (e.g. in the storm alleys at ±35° latitude) and might not be present all the time, we prefer not to use an optical detector, but rather employ an instrument design similar to the Huygens Probe HASI- PWA (Permittivity, Waves and Altimetry) Analyzer [110] including measurements of atmospheric electrical conductivity and the DC electric field.

In the Hera ASI design, existing flight-proven or commercial, off-the-shelf (COTS) hardware is applied in proven processes and applications. Other possible sensors types/candidates could be evaluated during the Phase A, but all of the components in the Hera ASI are at TRL higher than 6.

 

D.3.8 Critical Issues: None

 

D.4 Hera Net Flux Radiometer Experiment

D.4.1 Investigation Overview

Two notable instruments have flown in the past namely, the Large Probe Infrared Radiometer (LIR) [111] on the Venus Probe, and the Net Flux Radiometer (NFR) on the Galileo Probe [112] for in-situ measurements within Venus and Jupiter’s atmospheres, respectively. Both instruments were designed to measure the net radiation flux and upward radiation flux within their respective atmospheres as the Probe descended by parachute. The NASA GSFC NFR, Figure D.2, builds on the lessons learned from the Galileo Probe NFR experiment and is designed to establish the net radiation flux within Saturn’s atmosphere. The nominal measurement regime for the NFR extends from the tropopause at about 0.1 bar to at least 10 bars, corresponding to an altitude range of ~79 km above the 1 bar level to ~186 km below it. These measurements will help to define sources and sinks of planetary radiation, regions of solar energy deposition, and provide constraints on atmospheric composition, dynamics and cloud layers.

 

Figure D.2 Top : NFR instrument concept showing a 5° field-of-view that can be rotated by a stepper motor into five distinct look angles. Bottom : vacuum micro-vessel that houses the FPA – this is essential to the NFR survival since the probe will be unpressurized. Rotation is about the axis of the shaft and is accommodated via bearings on front and rear shafts (NASA GSFC).

 

Measurement Objective: The primary objective of the NFR is to measure upward and downward radiative fluxes to determine the radiative heating (cooling) component of the atmospheric energy budget, determine total atmospheric opacity, identify the location of cloud layers and opacities, and identify key atmospheric absorbers such as methane, ammonia, and water vapor. The NFR measures upward and downward flux densities in two spectral channels, the specific objectives of each channel are:

Channel 1 (Solar): 0.4-to-5µm spectral range. Net flux measurements will determine the solar energy deposition profile; upward flux measurements will yield information about cloud particle absorption and scattering;

Channel 2 (Thermal): 4-to-50µm spectral range. Net flux measurements will define sources and sinks of planetary radiation. When used with calculations of gas opacity effects, these observations will define the thermal opacity of particles.

 

D.4.2 Measurement Principle

The NFR measures upward and downward radiation flux in a 5° field-of-view at five distinct look angles, i.e., ±80°, ±45°, and 0°, relative to zenith/nadir. The radiance is sampled at each angle approximately once every ~ 2s.

The NFR Focal Plane Assembly (FPA), Figure D.3, comprises a set of bandpass filters, folding mirrors, non-imaging Winston cone concentrators, and radiation hard uncooled thermopile detectors housed in a windowed vacuum micro-vessel that is rotated to the look angle by a stepper motor. NASA GSFC has been working on this approach to develop a Saturn Probe NFR for several years and has extensive experience with the detectors and electronics.

Figure D.3 NFR Focal Plane Assembly housed in the vacuum micro-vessel showing construction of Winston cones to limit FOV in each channel (NASA GSFC).

Assuming a thermopile voltage responsivity of 295 V/W, an optical efficiency of 50%, a detector noise of 18 nV/√Hz and an ASIC input referred noise of 50 nV/√Hz with 12-bit digitization gives a system signal-to-noise ratio of 300 to 470 in the solar spectral channel and 100 to 12800 in the thermal spectral channel for atmospheric temperature and pressure ranges encountered in the descent, i.e., 80 to 300 K and 0.1 to 10 bar respectively.

 

 

D.4.3 Design Description / Operating Principle

A physical and functional block diagram of the NFR is shown in Figure D.4. The focal plane consists of four single pixel thermopile detectors (solar, thermal and two dark channels), bandpass filters and Winston concentrators. The Front End Electronics (FEE) readout, Figure D.5, uses a custom radiation-hardened-by-design mixed-signal ASIC for operation with immunity to 174 MeV-cm2/mg single event latch-up and 50 Mrad (Si) total ionizing dose [113]. The ASIC has sixteen low-noise chopper stabilized amplifier channels that have configurable gain/filtering and two temperature sensor channels that multiplex into an on-chip 16-bit sigma-delta analog-digital converter (SDADC). The ASIC uses a single input clock (~1.0 MHz) to generate all on-chip control signals such as the chopper/decimation clocks and integrator time constants. The ASIC also contains a radiation tolerant 16-bit 20 MHz Nyquist ADC for general-purpose instrumentation needs.

 

 

Figure D.4 Block diagram of the NFR showing the major subsystems and Probe interfaces. The redundant features are not shown (NASA GSFC).

 

The Main Electronic Box (MEB) is a redundant electrical system for science and housekeeping telemetry and thermal sensing and control. The two main elements of the MEB are the instrument and motor control board (comprising the instrument control and the motor drive electronics) and the Low Voltage Power Supply (LVPS) board.

The instrument control electronics uses a radiation hard µ-processor (e.g., Intersil HS-80C85RH) to perform the following functions: (i) receive and process the serial digitized data from the thermopile channels as well as provide a master clock and tagged encoded commands to the ASIC command decoders via a single line; (ii) mathematical operations on the science data such as averaging or offset corrections; (iii) data reduction, packetization, and routing of the science and housekeeping data to the Probe via a RS422 protocol; (iv) receive and act upon commands received from the Probe, e.g., active channel selection, setting temperature levels, or motor positions; (v) control stepper motor positions as well as decode their respective positions; (vi) provide stable temperature control to the instrument; and (vii) collect all temperatures and supply and reference voltages to form housekeeping/time stamped header packets that are streamlined into the data output to the Probe. All timing functions are synchronized with a 1 pulse per second (PPS) square wave from the Probe. The LVPS board accommodates DC-DC convertors and other various voltage/current control devices. This board not only conditions and regulates the voltages for various electronic usage but also controls power to the heaters. The Probe +28 VDC bus voltage is filtered and dropped via DC-DC switch mode converter to two main voltages: +3.3 VDC for logic use and +5 VDC for the stepper motor.

 

Figure D.5 Top: NFR detector FPA being fit checked inside a liquid N₂ dewar for cold testing. The FPA ASIC read-out electronics is controlled by a “flight like” Pro-ASIC FPGA board. Bottom: set-up for measuring detector FOV response through electronic chain (NASA GSFC).

 

D.4.4 Volume, Mass, Power, Data Rate

• Mass: ~2.4 kg (incl. harness);
• Volume: ~113mm x 144 mm x 279 mm;
• Power: ~5 W;
• Data Rate: ~55 bps (average);
• Data Volume: ~297 Kbits (90-minutes).

D.4.5 Volume, Mass, Power, Data Rate

The NFR will be mechanically mounted using thermally isolating mounts onto the Probe deck so accurate knowledge of the deck temperature, to better than ±1K, is required. An additional +28 VDC supply is needed for survival heaters that are controlled from the Probe.

The power-on for thermal control of the instrument must also be carefully considered prior to deploying the Probe from the carrier relay spacecraft. The heaters may require turning on at least 36-48 hours prior to the start of the probe descent science collection. Once the parachute is deployed at the top of the atmosphere, the descent latitude/longitude should be known to ±0.1°, the drop and spin rate should be known to ±0.1m/s.

 

D.4.6 Specific Calibration Needs

Prior to launch, the NFR will be radiometrically calibrated, in a thermal-vacuum chamber that simulates the environmental conditions at Saturn to establish both offset and gain uncertainties as a function of temperature. The gain uncertainity during descent is calibrated and removed by performing views of on-board hot and cold calibration targets as the instrument cycles sequentially through the look angles.

 

D.4.7 TRL and Relevant Heritage

The TRL level for all components and subsystems in the NFR is greater than 6. NASA GSFC has decades of experience managing, designing and delivering planetary mission instruments like the NFR.

 

D.4.8 Critical Issues: None

 

D.5 Hera Probe Nephelometer

D.5.1 Investigation Overview

Knowing the micro- and macro-physical properties of the haze and cloud particles in Saturn’s atmosphere is crucial for understanding the chemical, thermodynamic and radiative processes that take place. Full characterization of the various types of haze and cloud particles requires in-situ instrumentation, because Saturn’s stratospheric hazes obscure the lower atmosphere, and because remote-sensing measurements of, for example, reflected sunlight depend on myriads of atmospheric parameters thus prohibiting reaching unique solutions. The Hera Nephelometer (NEPH) will sample haze and cloud particles, illuminates them, and measures the flux and degree of linear polarization of the light that is scattered in a number of directions. The particle properties can be derived from the dependence of the scattered flux and polarization on both the scattering angle and the wavelength.

 

Measurement Objectives: NEPH’s primary objective is to characterize the micro- and macro-physical properties of atmospheric particles by measuring the flux and polarization of light that is scattered by aerosols that are passively sampled along the probe’s descent trajectory. The angular and spectral distribution of the flux and polarization of the scattered light provides the particles’ size distribution, composition, and shape, as well as their number density. NEPH’s secondary objective is to measure the flux and polarization of diffuse sunlight in the atmosphere. This will provide the optical depth along the trajectory and its spectral variation, placing the results on the samples into a broad perspective. Combining NEPH’s results with ambient pressure measurements from the ASI (Sect. 3), the absolute vertical profile of the hazes and clouds along the probe’s descent trajectory can be determined.

Figure D.6 Side-view of LOAC’s design, with the particles crossing the LED light beam from below. The photodiode at a scattering angle Θ=12° captures the forward scattered light.

 

D.5.2 Measurement Principles

NEPH consists of two modules: LOAC (Light Optical Aerosol Counter) to measure the size distribution of particles, and PAVO (Polarimetric Aerosol Versatile Observatory) to measure particle shape and composition. The modules will be placed side by side to sample similar particles. The probe’s descent through the atmosphere allows LOAC and PAVO to sample particles passively. The low solar flux levels in Saturn’s atmosphere require both LOAC and PAVO to use artificial light sources for illuminating their samples.

Figure D.6 shows a schematic of LOAC. Sampled particles cross a 2-mm diameter LED light-beam and the flux F of the light that is scattered by a single particle across angle Θ = 12° is measured. This flux F is very sensitive to the particle size, but relatively insensitive to its shape and/or composition. LOAC can accurately retrieve particle sizes between 0.1 and 250 mm in 20 size classes.

Figure D.7 shows PAVO’s design. PAVO measures flux F as well as degree P and angle χ of linear polarization [114] of light that is scattered by sampled particles at 9 angles Θ: 12° (the same as for LOAC), 30°, 50° 70°, 90°, 110°, 130°, 150°, 170°. At each Θ, a small optical head (without moving elements) translates the scattered light into two modulated flux spectra F_M:

 

F_M (Θ, λ) = 0.5 F(Θ, λ) [ 1 ± P(Θ, λ) cos ψ],

 

where ψ(Θ,λ) = 2χ(Θ,λ) + 2πδ/λ, with λ the wavelength and δ the retardance of the optical retarder in the head [115, also Keller and Snik, patent application WO2014/111487 A1]. The ± sign in the equation represents the beam-splitter in each head that produces two modulated flux spectra at every Θ that are subsequently fed to the spectrograph and detector with an optical fibre. Each modulated spectrum provides P and χ, while the sum of two spectra yields F. PAVO uses LEDs covering 400 to 800 nm to illuminate its sampled particles.

An extra optical head at Θ=0° monitors variations of non-scattered LED-light to obtain information on the number of particles. By chopping the incident beam, we will be able to derive the local diffuse solar radiation field. Another, outward pointing optical head could be added to directly measure the diffuse solar flux and its polarization state.

From the modulated spectra F_M the scattered fluxes F can be derived with a few nm resolution, and P and χ with 10-20 nm resolution. The required accuracy for P is 0.005 (0.5%), well within the modulation technique’s accuracy [116].

 

D.5.3 Volume, Mass, Power, Data Rate

Table D.2 Specifications of the Nephelometer

	Mass	Power	Data Rate
LOAC	0.3 kg	1 watt	50 bps
PAVO	2 kg	2 watt	100 bps (10 optical heads)

 

Figure D.7 Top-view of PAVO. One optical head captures non-scattered LED-light, and 9 heads capture scattered light. Fibres lead the modulated flux spectra from each head to the detector.

 

The data rate of PAVO increases linearly with the number of optical heads. On-board data processing will keep the data rate low (instead of transmitting the high-spectral resolution, modulated flux spectra, low resolution continuum flux and polarization data will be transmitted). The mentioned data rates assume continuous measurements. The total data transmission of NEPH will be optimized with the desired vertical resolution and the probe’s data rate.

 

D.5.4 Specific/Critical Interface Requirements

The nephelometer should be able to sample atmospheric particles and should therefore be located on the outside of the payload, preferably on the lowest part of the probe, to avoid any biasing of the samples due to the flow around the probe.

 

D.5.5 Specific Calibration Requirements

LOAC includes a stray-light correction system to remove diffuse solar flux. The stability of its LED should be characterized pre-launch. PAVO has no requirement on the shape of the LED-spectrum, although it should be known in order to derive the absolute scattered flux F. The flux (and polarization) spectrum of the LED will be calibrated before sampling particles, using the dedicated optical head at Θ=0°. The components in the optical heads can be chosen such that their temperature sensitivity is minimal [115] and will be calibrated before launch (this calibration can be compared against the in-flight calibration of the 0°-head). The scattered and non-scattered LED-light can be distinguished from diffuse sunlight by chopping the incident beam through switching it on and off. The sensitivity of the LED output to this chopping will be determined before launch. Note that P and χ are independent of the incident LED-spectrum.

D.5.6 TRL and Relevant Heritage

LOAC’s design is based on an instrument already in use as a balloon payload for aerosol size determination in the Earth’s atmosphere [117, 118]. PAVO’s optical design is based on the SPEX instrument [116] that is used on the ground to measure aerosol properties. The SPEX-optics has been tested successfully for radiation hardness with view of ESA’s JUICE mission. A design similar to PAVO’s (except for the polarimetric optical heads) is the nephelometer on the Galileo probe [119].

 

D.5.7 Critical Issues: None

 

D.6 Hera Probe Radio Science Experiment

D.6.1 Investigation Overview

The Hera Probe Radio Science Experiment will comprise two main elements. Radio tracking of the Hera probe from the Carrier Relay Spacecraft (CRSC) while Hera is under parachute will utilize the resulting Doppler shift and provide a vertical profile of zonal winds along the descent path for the duration of the probe telecommunications link detectability to at least ten bars [120, 121, 122]. The possibility for a measurement of the second horizontal component of the winds via a probe signal frequency measurement on Earth when the probe descends on the sub-solar side of Saturn [123, 124] will be carefully explored. The Radio Science/Doppler Wind Experiment (DWE) utilizes the Hera radio subsystem, knowledge of the Hera probe descent location, descent speed, altitude profile, and the CRSC trajectory to make a precise determination of the probe speed relative to the CRSC from which the zonal wind drift can be extracted. Additionally, as the Hera probe is expected to drift by up to several degrees in longitude under the influence of the zonal winds, the reconstruction of the probe descent location will provide an improved geographical context for other probe science investigations.

Additionally, the Radio Science/Atmospheric Absorption Experiment (AAbs) will utilize the Hera radio subsystem mounted on the CRSC to monitor the signal strength of the probe signal, providing a measurement of the integrated atmospheric absorption along the signal propagation path. The Galileo probe used this technique at Jupiter to strongly constrain the atmospheric NH₃ profile, complementing the atmospheric composition measurements of the probe Mass Spectrometer [125].

The primary objectives of the Hera Probe Radio Science Experiment are therefore to 1) use the probe radio subsystem with elements mounted on both the probe and the CRSC to measure the vertical profile of zonal winds along the probe descent path with an accuracy of better than 1.0 m/s, and 2) to measure the integrated profile of atmospheric absorption, expected to be primarily due to NH₃ between the probe and CRSC. Secondary objectives include the analysis of Doppler modulations and frequency residuals to detect, locate, and characterize regions of atmospheric turbulence, convection, wind shear, and to provide evidence for and characterize atmospheric waves. From measurements of the probe relay signal strength, the effect of refractive-index fluctuations in Saturn’s atmosphere including scintillations and atmospheric turbulence can be characterized. [73, 125].

 

D.6.2   Measurement Principle

The Hera Transmitter UltraStable Oscillator (TUSO) will generate a stable signal for the probe radio link. The receiver USO (RUSO) will provide a time base from which very accurate measurements of the probe link frequency can be made. Knowledge of the probe descent speed and the CRSC trajectory will allow the retrieval of Doppler residuals due to unresolved probe motions including wind. While in terminal descent beneath the parachute, the vertical resolution of the zonal wind measurements will depend upon the probe descent speed [126]. In the upper atmosphere the vertical resolution will be on the order of 7 km, while in the deeper atmosphere variations with a vertical scale size on the order of one km can be detected. The accuracy of the wind measurement will primarily be limited by the reconstruction accuracy of the Hera probe descent location and the CRSC trajectory, the stabilities of the TUSO and RUSO, and the relative geometry of the Hera and CRSC spacecraft. Assuming a UHF link frequency, a wind measurement accuracy better than 0.2 m/s is expected. [73, 127].

 

D.6.3 Design Description / Operation Principle

The Hera probe telecommunications system will consist of the relay radio transmitter subsystem on the probe and the receiver subsystem on the CRSC. The carrier receiver will be capable of measuring the Hera telemetry frequency at a sampling rate of at least 10 samples per second with a measurement accuracy of 0.1 Hz in frequency. The signal strength will be measured with a sample rate of 20 Hz and a signal strength resolution of .01 dBm [125]. This sampling rate will enable probe microdynamics such as probe spin and pendulum motion, atmospheric waves, aerodynamic buffeting and atmospheric turbulence at the probe location to be detected and measured.

The long-period stability of both the TUSO and RUSO, defined in terms of 30-minute fractional frequency drift, should be less than Df/f =10^-11, with an 100-second Allan Deviation of ~10^-13. The expected USO drift during a 90-minute probe descent is on the order of .01 Hz.

 

D.6.4 Volume, Mass, Power, Data Rate

• Ultrastable Oscillator Mass: ~5 kg;
• Ultrastable Oscillator Power: ~3 W steady state (higher during warmup);
• Volume: cylinder, ~4cm diam. x 14cm length;
• Data Rate: Very low. USO temperatures and voltages should be measured several times/minute with a corresponding data rate on the order of 2 bps. [73, 128, 129].

 

D.6.5 Specific/Critical Interface Requirements & Environment Constraints

To avoid spurious contributions to the Doppler profile from probe dynamics, the TUSO on the Hera probe should be mounted as close to the probe center of gravity as reasonably possible. Although the Hera and CRSC USO’s will be contained within thermal ovens, the TUSO and RUSO temperatures should be maintained to better than ±1.0 K. A carefully considered warmup plan is necessary to assure adequate TUSO and RUSO frequency stability. Different USO types (e.g., Galileo probe quartz and Huygens probe Rubidium) require significantly different warmup times and steady state power requirements. [73, 128, 129]. The Hera probe link frequency and signal strength should be sampled by the relay receivers on the CRSC. Upon deployment of the parachute at the top of the atmosphere, the Hera descent latitude/longitude should be reconstructed to ±0.1deg, and the Hera descent speed based on ASI measurements of pressure and temperature should be known to ±0.1m/s. [73].

 

D.6.6 Specific Calibration Needs

The short-term frequency stability and long term aging of the TUSO and RUSO should be characterized prior to launch. In particular, the repeatability of the frequency drift profile over periods of 30 minutes to several hours should be carefully characterized. After launch, both the TUSO and RUSO should be powered on at least several times during cruise for calibration, and aging and drift characterization.

 

D.6.7 TRL and Relevant Heritage

There are two types of USO technologies with outer solar system entry probe heritage. The Galileo probe and orbiter carried SC-cut quartz crystal USOs, and the Huygens (Titan) probe and Cassini orbiter carried Rb USOs. The Galileo quartz crystal USOs provides a very stable frequency reference for short time periods, although the absolute frequency may be unknown to 100’s of Hz, the changes in frequency were the observable of interest. The similarities between the Galileo Jupiter and Hera Saturn probe missions suggests that the crystal USO may be better for the planned Doppler Wind investigation, although further evaluation is needed. [73, 127, 128, 129].

 

D.6.8 Critical Issues: None