Clipper groundtracks. The combination of REASON *sounding* and *altimetry* data will also be used to search for the presence of liquid water either within or at the base of the ice shell by way of isostasy. The detection of zones of liquid water within the ice shell will be relevant to a future Europa lander as it signifies an area of on-going geological activity and may host a potentially habitable, near-surface environment. *Reflectometry* measurements of the surface echo at the two REASON frequencies will provide insights to dominant surface and near-surface properties (e.g., roughness) within the radar footprint, to depths of 15 m to 150 m, that are directly relevant to both landing science and risk assessment. That includes porosity (Grima et al. 2014a), roughness (Grima et al. 2014b), fallout-deposit from recent plume activity (Scanlan et al. 2022), near-surface refrozen brines (Rutishauser et al. 2016; Chan et al. 2023b), or the presence of near-surface liquid water potentially accessible to future in-situ sampling (Grima et al. 2016).

## 9.2.2 JUICE/RIME Synergy

JUICE will operate in the Jovian system concurrent with Europa Clipper. One of JUICE's primary mission objectives is to make detailed observations of Ganymede, but it will also collect measurements at Europa and Callisto during its initial orbital insertion phases. The JUICE payload includes RIME (Bruzzone et al. 2011), a 9-MHz sister instrument to REASON HF. RIME and REASON overlapping targets and similar instrumental parameters, offer a unique opportunity for synergistic science. Both instruments will have common observations at intersections, allowing radiometric calibration of their respective measurements. The outcome is to produce a radiometrically homogeneous 9-MHz *reflectometry* and *sound-ing* dataset covering Europa, Ganymede, and Callisto (a first of its kind in planetary exploration), which could be used to relatively compare signal attenuation and scattering from the various englacial and surface structures encountered across the Jovian icy moons. RIME can also set its transmitted bandwidth to either 1 MHz (same as REASON) or 3 MHz. Because surface roughness is wavelength-dependent, observations with REASON and RIME at the same center frequency (9 MHz) but different bandwidths could help to isolate the effects of surface roughness in order to better characterize bulk near-surface properties (e.g., layering) over various types of terrains (Chan et al. 2023a). Assuming the effects of roughness are either negligible, analogous to internal layers in glacial ice (e.g., MacGregor et al. 2015 and references therein), or can be quantified and corrected (e.g., Grima et al. 2019), multi-frequency/bandwidth analyses could also be used to better characterize plume fall-out deposits at the surface (Scanlan et al. 2022) or salt layers at depth (Wolfenbarger et al. 2023). Finally, the concurrent presence of JUICE and Europa Clipper in the Jovian system can help to better characterize its magnetospheric plasma where both radars could be used as transmitters-receivers of each other signal, the relative delay and attenuation of which are indicative of the plasma density along the propagation path.

## 9.2.3 Passive Radar Sounding

REASON has the capability to perform passive sounding using signals of opportunity originating from the Jovian magnetosphere (Cecconi et al. 2012; Romero-Wolf et al. 2015; Schroeder et al. 2016). In this mode of operation, REASON would not be transmitting a signal, but would record incoming Jovian emissions and their possible reflections from the surface and subsurface of Europa. A processing step analogous to range-compression would then reveal geological features as in standard, active sounding (Gerekos et al. 2020). Jovian decametric emissions are coherent and highly beamed, making them good candidates
for passive sounding (Carrer et al. 2021; Gassot et al. 2022). These characteristics could similarly be favorable for estimating TEC and studying the ionosphere (Peters et al. 2020). The use of passive sounding could complement the standard concept of operations on the sub-Jovian side of Europa in a high-noise environment. While the spatial extent of solar radio-sources likely prevents their use for passive subsurface sounding (Peters et al. 2022), passive observations of solar bursts from the outer solar system can be envisioned. Recent re-analyses of archival SHARAD and MARSIS data have shown that radar sounders have the capability to image type-III bursts as high resolution from Martian orbit (Gerekos et al. 2024), and such multi-point observations can inform about the processes behind burst generation (Jebaraj et al. 2020) and their propagation across interplanetary space (Reid and Kontar 2021).

# 10 Summary

REASON is a combined HF/VHF ice penetrating radar specifically conceived, developed, and operated to provide a top-to-bottom window into many of the most critical and poorly understood processes and conditions governing the behavior, evolution, and habitability of Europa's ice shell. At the highest level, REASON is designed to probe the exosphere, the surface/near-surface, the shallow subsurface to three kilometers and the full depth of Europa's ice shell to an ice-ocean interface at depths as great as thirty kilometers with nadir-directed active radar signals centered at both 9 MHz and 60 MHz. This pair of frequencies provides both robustness (with the 9 MHz band less sensitive to surface roughness and volume scattering and the 60 MHz band less sensitive to Jovian radio noise) and richness (capturing cross-frequency signatures of ionosphere and ice-shell properties and processes) to the full suite of Europa Clipper radar assessment and sounding investigations. Similarly, the unique dual-channel, cross-track interferometric capability of REASON's VHF band allows for potentially obfuscating cross-track surface clutter with similar delays as subsurface signals to be discriminated internally to the instrument, increasing the fidelity and reliability of radargram interpretation. REASON will be calibrated through carefully coordinated combinations of transmitting and receiving HF and VHF before and after Jupiter orbit insertion, d. With these calibrations, REASON will be operated and its data analyzed to explore physical constraints on the potential habitability of Europa through a full suite of *altimetry*, *reflectometry*, *sounding*, *interferometry*, *plasma characterization*, and *ranging* measurements—each described briefly below with their implications for REASON guiding and extended science (Table 2) as well as complementary science.

Surface returns from the REASON VHF (with ionospheric-delay corrections provided by concurrent HF surface returns) are a significant but often underappreciated observation at Europa, representing the primary *altimetry* measurement for the Europa Clipper payload. This *altimetry* measurement will be used, in combination with EIS stereo imagery, to provide registration and measurement of the surface geometry of Europa which in turn provides a framework for critical hypothesis tests of floatation from any shallow subsurface water bodies. In addition to providing context for both interior ice shell processes and their expression in surface geology through *altimetry*, REASON near-surface *reflectometry* will provide a quantitative assessment of Europa's surface roughness and near-surface material properties (including brines, salt layers, porosity, and plume deposits). *Reflectometry* measurements will also provide wavelength-scale statistical constraints on surface roughness, which are also critical for landing-zone reconnaissance. As a *sounding* instrument, REASON will be able to probe both shallow (to three kilometers) and deeper subsurface
processes of Europa's ice shell (up to thirty kilometers) by exploiting the sensitivity of HF and VHF (assisted by interferometry) radar signals to both the compositional and geometric character of the shallow subsurface (e.g. water and salt bodies) as well as the thermophysical structure (e.g. temperature, brine pockets) across the full depth of the ice shell potentially including the ice-ocean interface or its thermophysical proxy. The detection and characterization of reflectivity, attenuation, and scattering signatures in REASON *sound-ing* data at intersections, along profiles, and across panels will place unique constraints on subsurface properties and processes such as the depth of pore-closure, the existence of eutectic horizons, shallow and relict water bodies, and the thickness and salinity of the ice shell and ocean. The global distribution of REASON VHF returns from *ranging* between the spacecraft and the surface at flyby crossovers compared with simulated surface returns (from EIS digital elevation models including both returns at nadir and off-nadir "clutter") will provide the opportunity to place additional constraints on tidal deformation as part of larger, mission-wide, geodetic inversions to constrain ice-shell and ocean thickness. The combination of REASON HF and VHF surface returns provide a critical *plasma characterization* measurement that will not only allow for ionospheric correction of *altimetry* and *ranging* measurements but also provide essential constraints on the nadir TEC, which will enable both better characterization of the ionosphere and local plume detection. Taken together, the REASON investigation and instrument are designed to enable a unique suite of radar observations and analyses to test hypotheses and reveal processes, by assessing and sounding Europa from the ocean to the near-surface of the ice shell.

## Appendix: Ensemble of Point Models (EPM) Tables

Tables 12–14 present the regime modes assumed in the construction of the point models described in Sect. 4.3. For point models in parentheses, salt layers represent the target interface, as opposed to the subsurface water interface.

<table><caption>**Table 12** Shallow Subsurface Structure EPM</caption><thead><tr><th>Regime</th><th>SS01(07)</th><th>SS02(08)</th><th>SS03(09)</th><th>SS04(10)</th><th>SS05(11)</th><th>SS06(12)</th></tr></thead><tbody><tr><td>Surface Roughness</td><td>Ridged Plains</td><td>Chaotic Terrain</td><td>Chaotic Terrain</td><td>Ridged Plains</td><td>Ridged Plains</td><td>Chaotic Terrain</td></tr><tr><td>Near-Surface Density</td><td>Non-Porous</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Near-Surface Thickness</td><td>Thin</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Near-Surface Composition</td><td>Clean Ice</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Off-Nadir Surface Feature</td><td>Ribbons & Curbs</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Deep Regolith</td><td>Absent</td><td>Present</td><td>Absent</td><td>Absent</td><td>Absent</td><td>Absent</td></tr><tr><td>Surface Temperature</td><td>Polar & Equatorial</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ice Shell Composition</td><td>Marine</td><td>Pure</td><td>Pure</td><td>Marine</td><td>Pure</td><td>Marine</td></tr><tr><td>Ice Shell Thickness</td><td>Thin</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Subsurface Water Interface</td><td>Sharp</td><td>Sharp</td><td>Sharp</td><td>Mush</td><td>Sharp</td><td>Sharp</td></tr><tr><td>Basal Temperature</td><td>Melting</td><td>Melting</td><td>Melting</td><td>Eutectic</td><td>Melting</td><td>Melting</td></tr><tr><td>(Salt Layer Thickness)</td><td>1 m</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>(Salt Layer Depth)</td><td>1/2 Ice Shell Thickness</td><td></td><td></td><td></td><td></td><td></td></tr></tbody></table>
<table><caption>**Table 13** Full Depth Subsurface Exchange EPM</caption><thead><tr><th>Regime</th><th>FD01(07)</th><th>FD02(08)</th><th>FD03(09)</th><th>FD04(10)</th><th>FD05(11)</th><th>FD06(12)</th></tr></thead><tbody><tr><td>Surface Roughness</td><td>Ridged Plains</td><td>Chaotic Terrain</td><td>Chaotic Terrain</td><td>Ridged Plains</td><td>Ridged Plains</td><td>Chaotic Terrain</td></tr><tr><td>Near-Surface Density</td><td>Non-Porous</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Near-Surface Thickness</td><td>Thin</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Near-Surface Composition</td><td>Clean Ice</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Off-Nadir Surface Feature</td><td>Ribbons & Curbs</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Deep Regolith</td><td>Absent</td><td>Present</td><td>Absent</td><td>Absent</td><td>Absent</td><td>Absent</td></tr><tr><td>Surface Temperature</td><td>Polar & Equatorial</td><td></td><td></td><td></td><td></td><td></td></tr><tr><td>Ice Shell Composition</td><td>Marine</td><td>Pure</td><td>Pure</td><td>Pure</td><td>Pure</td><td>Marine</td></tr><tr><td>Ice Shell Thickness</td><td>Thin</td><td>Thin</td><td>Thin</td><td>Thick</td><td>Thick</td><td>Thick</td></tr><tr><td>Subsurface Water Interface</td><td>Mush</td><td>Mush</td><td>Mush</td><td>Sharp</td><td>Mush</td><td>Sharp</td></tr><tr><td>Basal Temperature</td><td>Eutectic</td><td>Eutectic</td><td>Eutectic</td><td>Melting</td><td>Eutectic</td><td>Melting</td></tr><tr><td>(Salt Layer Thickness)</td><td>1 m</td><td>1 m</td><td>1 m</td><td>1 m</td><td>1 m</td><td>0.2 m</td></tr><tr><td>(Salt Layer Depth)</td><td>1/2 Ice Shell Thickness</td><td></td><td></td><td></td><td></td><td>10 km</td></tr></tbody></table>

<table><caption>**Table 14** Altimetry Reflectometry EPM</caption><thead><tr><th>Regime</th><th>AR01</th><th>AR02</th><th>AR03</th><th>AR04</th><th>AR05</th><th>AR06</th></tr></thead><tbody><tr><td>Surface Roughness</td><td>Ridged Plains</td><td>Chaotic Terrain</td><td>Ridged Plains</td><td>Ridged Plains</td><td>Chaotic Terrain</td><td>Ridged Plains</td></tr><tr><td>Near-Surface Density</td><td>Non-Porous</td><td>Non-Porous</td><td>Porous</td><td>Non-Porous</td><td>Non-Porous</td><td>Porous</td></tr><tr><td>Near-Surface Thickness</td><td>Thick</td><td>Thick</td><td>Thick</td><td>Thick</td><td>Thick</td><td>Thin</td></tr><tr><td>Near-Surface Comp.</td><td>Clean Ice</td><td>Clean Ice</td><td>Clean Ice</td><td>Brine-Filled Ice</td><td>Brine-Filled Ice</td><td>Clean Ice</td></tr><tr><td>Off-Nadir Surface Feature</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Deep Regolith</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Surface Temperature</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Ice Shell Composition</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Ice Shell Thickness</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Subsurface Water Interface</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>Basal Temperature</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>(Salt Layer Thickness)</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr><tr><td>(Salt Layer Depth)</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td><td>—</td></tr></tbody></table>
Acronyms and Abbreviations

<table><tr><td>A/D</td><td>Analog/Digital</td></tr><tr><td>Actr</td><td>Actuator</td></tr><tr><td>ADC</td><td>Analog-to-Digital Converter</td></tr><tr><td>ALSE</td><td>Apollo 17 Lunar Sounder Experiment</td></tr><tr><td>ANT</td><td>Antenna</td></tr><tr><td>APL</td><td>Applied Physics Laboratory</td></tr><tr><td>AS</td><td>Antenna Subsystem</td></tr><tr><td>AS-Mech</td><td>Antenna Subsystem – Mechanical</td></tr><tr><td>ATLO</td><td>Assembly, Test, Launch and Operations</td></tr><tr><td>ATTN</td><td>Attenuation</td></tr><tr><td>AWG</td><td>Arbitrary Waveform Generator</td></tr><tr><td>BDS</td><td>Bulk Data Storage</td></tr><tr><td>BFPQ</td><td>Block Floating-Point Quantization</td></tr><tr><td>B/H</td><td>Ratio of magnetic flux density (B) to magnetic field strength (H)</td></tr><tr><td>CA</td><td>Closest Approach</td></tr><tr><td>Caltech</td><td>California Institute of Technology</td></tr><tr><td>CBE</td><td>Current Best Estimate</td></tr><tr><td>CDH</td><td>Command and Data Handling</td></tr><tr><td>CLK</td><td>Clock</td></tr><tr><td>Coax</td><td>Coaxial cable</td></tr><tr><td>COTS</td><td>Commercial Off-the-Shelf</td></tr><tr><td>CRF</td><td>Cycle Repetition Frequency</td></tr><tr><td>CSV</td><td>Comma-Separated Value</td></tr><tr><td>CTRL/TLM</td><td>Control/Telemetry</td></tr><tr><td>CTRL/TMG</td><td>Control/Timing</td></tr><tr><td>CTU</td><td>Control and Timing Unit</td></tr><tr><td>D/A</td><td>Digital/Analog</td></tr><tr><td>DAC</td><td>Digital-to-Analog Converter</td></tr><tr><td>DAD</td><td>Deployment Assist Device</td></tr><tr><td>DC</td><td>Direct Current</td></tr><tr><td>DEM</td><td>Digital Elevation Model</td></tr><tr><td>DES</td><td>Digital Electronic Subsystem</td></tr><tr><td>DMM</td><td>Digital Multimeter</td></tr><tr><td>DTM</td><td>Digital Terrain Model</td></tr><tr><td>DPU</td><td>Digital and Power Unit</td></tr><tr><td>DQA</td><td>Data Quality Analyzer</td></tr><tr><td>DSE</td><td>Digital and Synthesizer Electronics</td></tr><tr><td>DSEPCU</td><td>DSE Power Converter Unit</td></tr><tr><td>DSES</td><td>Digital and Synthesizer Electronics Stacks</td></tr><tr><td>DSN</td><td>Deep Space Network</td></tr><tr><td>E-THEMIS</td><td>Europa Thermal Emission Imaging System</td></tr><tr><td>ECM</td><td>Europa Clipper Magnetometer</td></tr><tr><td>EGSE</td><td>Electronic Ground Support Equipment</td></tr><tr><td>EIS</td><td>Europa Imaging System</td></tr><tr><td>EM</td><td>Engineering Module</td></tr><tr><td>EMC</td><td>Electromagnetic Compatibility</td></tr><tr><td>EMI</td><td>Electromagnetic Interference</td></tr><tr><td>EMI/EMC</td><td>Electromagnetic Interference and Compatibility</td></tr></table>
<table>
  <tr>
    <td>EPM</td>
    <td>Ensemble of Point Models</td>
  </tr>
  <tr>
    <td>EPS</td>
    <td>Experiment Planning System</td>
  </tr>
  <tr>
    <td>ESA</td>
    <td>European Space Agency</td>
  </tr>
  <tr>
    <td>Europa-UVS</td>
    <td>Europa Ultraviolet Spectrograph</td>
  </tr>
  <tr>
    <td>FDVHF</td>
    <td>Full Depth Very High Frequency</td>
  </tr>
  <tr>
    <td>FET</td>
    <td>Field-Effect Transistor</td>
  </tr>
  <tr>
    <td>FJ</td>
    <td>Field Joint</td>
  </tr>
  <tr>
    <td>Flt Sys</td>
    <td>Flight System</td>
  </tr>
  <tr>
    <td>FM</td>
    <td>Flight Module</td>
  </tr>
  <tr>
    <td>FODL</td>
    <td>Fiber Optic Delay Line</td>
  </tr>
  <tr>
    <td>FPGA</td>
    <td>Field-Programmable Gate Array</td>
  </tr>
  <tr>
    <td>Frang</td>
    <td>Frangibolt</td>
  </tr>
  <tr>
    <td>FrngActuator</td>
    <td>Frangibolt Actuator</td>
  </tr>
  <tr>
    <td>Func Gen</td>
    <td>Function Generator</td>
  </tr>
  <tr>
    <td>GaN</td>
    <td>Gallium Nitride</td>
  </tr>
  <tr>
    <td>GPS</td>
    <td>Global Positioning System</td>
  </tr>
  <tr>
    <td>GSE</td>
    <td>Ground Support Equipment</td>
  </tr>
  <tr>
    <td>GSFC</td>
    <td>Goddard Space Flight Center</td>
  </tr>
  <tr>
    <td>H&S</td>
    <td>Health and Status</td>
  </tr>
  <tr>
    <td>HAAMX</td>
    <td>High Frequency Antenna Assembly -X</td>
  </tr>
  <tr>
    <td>HAAPX</td>
    <td>High Frequency Antenna Assembly +X</td>
  </tr>
  <tr>
    <td>HD</td>
    <td>Hard Drive</td>
  </tr>
  <tr>
    <td>HF</td>
    <td>High Frequency</td>
  </tr>
  <tr>
    <td>HFPAU</td>
    <td>High Frequency Power Amplifier Unit</td>
  </tr>
  <tr>
    <td>HFPSU</td>
    <td>High Frequency Power Supply Unit</td>
  </tr>
  <tr>
    <td>HFRx</td>
    <td>High Frequency Receivers Assembly</td>
  </tr>
  <tr>
    <td>HFS</td>
    <td>High Frequency Electronic Stack</td>
  </tr>
  <tr>
    <td>HFTxDA</td>
    <td>High Frequency Transmit Distribution Assembly</td>
  </tr>
  <tr>
    <td>HW</td>
    <td>Hardware</td>
  </tr>
  <tr>
    <td>I/F</td>
    <td>Interface</td>
  </tr>
  <tr>
    <td>I&O</td>
    <td>Inner and Outer</td>
  </tr>
  <tr>
    <td>I&T</td>
    <td>Integration-and-Test</td>
  </tr>
  <tr>
    <td>IC</td>
    <td>Integrated Circuit</td>
  </tr>
  <tr>
    <td>IEP</td>
    <td>Initial Encounter Plan</td>
  </tr>
  <tr>
    <td>IESD</td>
    <td>Internal Electrostatic Discharge</td>
  </tr>
  <tr>
    <td>II&T</td>
    <td>Instrument Integration-and-Test</td>
  </tr>
  <tr>
    <td>Instr</td>
    <td>Instrument</td>
  </tr>
  <tr>
    <td>IOS</td>
    <td>Instrument Operations System</td>
  </tr>
  <tr>
    <td>iSDS</td>
    <td>Instrument Science Data System</td>
  </tr>
  <tr>
    <td>ISOC</td>
    <td>Instrument Science Operations Center</td>
  </tr>
  <tr>
    <td>iST</td>
    <td>Instrument Science Team</td>
  </tr>
  <tr>
    <td>Itfc</td>
    <td>Interface</td>
  </tr>
  <tr>
    <td>JOI</td>
    <td>Jupiter Orbit Insertion</td>
  </tr>
  <tr>
    <td>JPL</td>
    <td>Jet Propulsion Laboratory</td>
  </tr>
  <tr>
    <td>JUICE</td>
    <td>JUPITER ICy moons Explorer</td>
  </tr>
  <tr>
    <td>KRP</td>
    <td>Key Radar Parameter</td>
  </tr>
  <tr>
    <td>LEP</td>
    <td>Leading Edge Position</td>
  </tr>
  <tr>
    <td>LRS</td>
    <td>Lunar Radar Sounder</td>
  </tr>
  <tr>
    <td>LVDS</td>
    <td>Low-Voltage Differential Signaling</td>
  </tr>
</table>
<table>
  <tr>
    <td>m-SDS</td>
    <td>Mission Science Data System</td>
  </tr>
  <tr>
    <td>magamp</td>
    <td>Magnetic Amplifier</td>
  </tr>
  <tr>
    <td>MARSIS</td>
    <td>Mars Advanced Radar for Subsurface and Ionospheric Sounding</td>
  </tr>
  <tr>
    <td>MASPEX</td>
    <td>MAss Spectrometer for Planetary Exploration</td>
  </tr>
  <tr>
    <td>Meas.</td>
    <td>Measurement</td>
  </tr>
  <tr>
    <td>MISE</td>
    <td>Mapping Imaging Spectrometer for Europa</td>
  </tr>
  <tr>
    <td>MN</td>
    <td>Matching Network</td>
  </tr>
  <tr>
    <td>MOS</td>
    <td>Mission Operations System</td>
  </tr>
  <tr>
    <td>MRAM</td>
    <td>Magnetoresistive Random-Access Memory</td>
  </tr>
  <tr>
    <td>MRAM_SEL</td>
    <td>Magnetoresistive Random-Access Memory Select</td>
  </tr>
  <tr>
    <td>MRO</td>
    <td>Mars Reconnaissance Orbiter</td>
  </tr>
  <tr>
    <td>MX</td>
    <td>-X</td>
  </tr>
  <tr>
    <td>MXI</td>
    <td>-X Inner</td>
  </tr>
  <tr>
    <td>MXO</td>
    <td>-X Outer</td>
  </tr>
  <tr>
    <td>NAIF</td>
    <td>Navigation and Ancillary Information Facility</td>
  </tr>
  <tr>
    <td>NASA</td>
    <td>National Aeronautics and Space Administration</td>
  </tr>
  <tr>
    <td>NSLE</td>
    <td>Normalized Side Lobe Envelope</td>
  </tr>
  <tr>
    <td>O-scope</td>
    <td>Oscilloscope</td>
  </tr>
  <tr>
    <td>OBP</td>
    <td>On-Board Processor</td>
  </tr>
  <tr>
    <td>OP</td>
    <td>Observation Plan</td>
  </tr>
  <tr>
    <td>OTM</td>
    <td>Orbit Trim Maneuver</td>
  </tr>
  <tr>
    <td>OVRO</td>
    <td>Owens Valley Radar Observatory</td>
  </tr>
  <tr>
    <td>OVRO-LWA</td>
    <td>Owens Valley Radar Observatory Long Wavelength Array</td>
  </tr>
  <tr>
    <td>PAU</td>
    <td>Power Amplification Unit</td>
  </tr>
  <tr>
    <td>PA</td>
    <td>Power Amplifier</td>
  </tr>
  <tr>
    <td>PC</td>
    <td>Personal Computer</td>
  </tr>
  <tr>
    <td>PCE</td>
    <td>Planning, Coordination, and Execution Operations</td>
  </tr>
  <tr>
    <td>PCDA</td>
    <td>Power Control Distribution Assembly</td>
  </tr>
  <tr>
    <td>PCU</td>
    <td>Power Converter Unit</td>
  </tr>
  <tr>
    <td>PDS</td>
    <td>Planetary Data System</td>
  </tr>
  <tr>
    <td>PEEK</td>
    <td>Polyetheretherketone</td>
  </tr>
  <tr>
    <td>PI</td>
    <td>Principal Investigator</td>
  </tr>
  <tr>
    <td>PIMS</td>
    <td>Plasma Instrument for Magnetic Sounding</td>
  </tr>
  <tr>
    <td>PLL</td>
    <td>Phase Locked Loop</td>
  </tr>
  <tr>
    <td>PME</td>
    <td>Propulsion Module Electronics</td>
  </tr>
  <tr>
    <td>PPDP</td>
    <td>Partially Processed Data Product</td>
  </tr>
  <tr>
    <td>PRF</td>
    <td>Pulse Repetition Frequency</td>
  </tr>
  <tr>
    <td>Prop</td>
    <td>Propulsion</td>
  </tr>
  <tr>
    <td>PRT</td>
    <td>Platinum Resistance Thermometer</td>
  </tr>
  <tr>
    <td>PSS</td>
    <td>Power Switch Slice</td>
  </tr>
  <tr>
    <td>PSU</td>
    <td>Power Supply Unit</td>
  </tr>
  <tr>
    <td>PWM</td>
    <td>Pulse-Width Modulation</td>
  </tr>
  <tr>
    <td>PWR</td>
    <td>Power</td>
  </tr>
  <tr>
    <td>PX</td>
    <td>+X</td>
  </tr>
  <tr>
    <td>PX/MX</td>
    <td>+X/-X</td>
  </tr>
  <tr>
    <td>PXI</td>
    <td>+X Inner</td>
  </tr>
  <tr>
    <td>PXO</td>
    <td>+X Outer</td>
  </tr>
  <tr>
    <td>RAMBo</td>
    <td>Radar Acquisition & Management Board</td>
  </tr>
  <tr>
    <td>RAP</td>
    <td>Reference Activity Plan</td>
  </tr>
</table>
<table><tr><td>RAW</td><td>Raw Data Product</td></tr><tr><td>REASON</td><td>Radar for Europa Assessment and Sounding: Ocean to Near-surface</td></tr><tr><td>REU</td><td>Remote Engineering Unit</td></tr><tr><td>RF</td><td>Radio Frequency</td></tr><tr><td>RFES</td><td>Radio Frequency Electronics System</td></tr><tr><td>RFES-JPL</td><td>Radio Frequency Electronics System Jet Propulsion Laboratory</td></tr><tr><td>RFI</td><td>Radio Frequency Interference</td></tr><tr><td>RFPSU</td><td>Radio Frequency Power Supply Unit</td></tr><tr><td>RIME</td><td>Radar for Icy Moons Exploration</td></tr><tr><td>Rx</td><td>Receiver</td></tr><tr><td>S/C</td><td>Spacecraft</td></tr><tr><td>SA</td><td>Solar Arrays</td></tr><tr><td>SAR</td><td>Synthetic Aperture Radar</td></tr><tr><td>SC/PropMod</td><td>Spacecraft Propulsion Module</td></tr><tr><td>Sci Sys</td><td>Science System</td></tr><tr><td>SCOS</td><td>Spacecraft Operations Subsystem</td></tr><tr><td>SDS</td><td>Science Data System</td></tr><tr><td>SELENE</td><td>SELenological and ENngineering Explorer</td></tr><tr><td>SEG</td><td>Segment</td></tr><tr><td>SHARAD</td><td>SHAllow RADar</td></tr><tr><td>Sig Gens</td><td>Signal Generators</td></tr><tr><td>Sim</td><td>Simulator</td></tr><tr><td>SIOS</td><td>Science and Instrument Operations Subsystem</td></tr><tr><td>SNR</td><td>Signal-to-Noise Ratio</td></tr><tr><td>Spec-A</td><td>Spectrum Analyzer</td></tr><tr><td>SPLD</td><td>South Polar Layered Deposits</td></tr><tr><td>SPOC</td><td>Science Planning Operations Coordination</td></tr><tr><td>SpW</td><td>SpaceWire</td></tr><tr><td>SRAM</td><td>Static Random Access Memory</td></tr><tr><td>SSPG</td><td>Strategic Science Planning Guide</td></tr><tr><td>STACER</td><td>Spiral Tube and Actuator for Controlled Extension and Retraction</td></tr><tr><td>STALO</td><td>STAble Local Oscillator</td></tr><tr><td>STB</td><td>System Testbed</td></tr><tr><td>STEREO</td><td>Solar Terrestrial Relations Observatory</td></tr><tr><td>SUDA</td><td>SUrface Dust Analyzer</td></tr><tr><td>SVV</td><td>Science Verification and Validation</td></tr><tr><td>Synth</td><td>Synthesizer Assembly</td></tr><tr><td>T/R</td><td>Transmit/Receive</td></tr><tr><td>TEC</td><td>Total Electron Content</td></tr><tr><td>Temp</td><td>Temperature</td></tr><tr><td>TID</td><td>Total Ionizing Dose</td></tr><tr><td>TLM</td><td>Telemetry</td></tr><tr><td>TT</td><td>Telltale</td></tr><tr><td>TT/T</td><td>Telltale/Temps</td></tr><tr><td>TTL</td><td>Transistor-transistor logic</td></tr><tr><td>TVAC</td><td>Thermal Vacuum</td></tr><tr><td>Tx</td><td>Transmit</td></tr><tr><td>Tx/Rx</td><td>Transmit/Receive</td></tr><tr><td>Txda</td><td>Transmit Driver Assembly</td></tr></table>
<table><tr><td>UTIG</td><td>University of Texas Institute for Geophysics</td></tr><tr><td>V&V</td><td>Verification and Validation</td></tr><tr><td>VAA</td><td>Very High Frequency Antenna Assembly</td></tr><tr><td>VAAMXI</td><td>Very High Frequency Antenna Assembly -X Inner</td></tr><tr><td>VAAMXI/O</td><td>Very High Frequency Antenna Assembly -X Inner/Outer</td></tr><tr><td>VAAMXO</td><td>Very High Frequency Antenna Assembly -X Outer</td></tr><tr><td>VAAPXI</td><td>Very High Frequency Antenna Assembly +X Inner</td></tr><tr><td>VAAPXI/O</td><td>Very High Frequency Antenna Assembly +X Inner/Outer</td></tr><tr><td>VAAPXO</td><td>Very High Frequency Antenna Assembly +X Outer</td></tr><tr><td>VDD</td><td>Positive supply voltage</td></tr><tr><td>VHF</td><td>Very High Frequency</td></tr><tr><td>VHFPAU</td><td>Very High Frequency Power Amplifier Unit</td></tr><tr><td>VHFPSU</td><td>Very High Frequency Power Supply Unit</td></tr><tr><td>VHFRx</td><td>Very High Frequency Receivers Assembly</td></tr><tr><td>VHFS</td><td>Very High Frequency Electronic Stack</td></tr><tr><td>VHFTxda</td><td>Very High Frequency Transmitters Distribution Assembly</td></tr><tr><td>WBS</td><td>Work Breakdown Structure</td></tr></table>

**Acknowledgements** This work was supported by NASA through the Europa Clipper Project (80NM0018F0615). The REASON radar was designed, built, and tested at the Jet Propulsion Laboratory, California Institute of Technology, under a contract with the National Aeronautics and Space Administration (80NM0018D0004). REASON science was supported by a contract with Marshall Space Flight Center (NNM16AA26C). This is UTIG contribution 3970. The contributions of Mauricio Sanchez-Barbety, Duane C. Howard, and Julie Newman were made during their time as employees of the Jet Propulsion Laboratory. We acknowledge the support of the NASA Instrument Concepts for Europa Exploration (ICEE) program (13-ICEE13-18) for the development and field validation of techniques ultimately employed by the REASON investigation and thank the G. Unger Vetlesen foundation for their contribution to validation of terrestrial models for Europa's ice shell. We would like to thank L3Harris Cincinnati Electronics Corporation and Wenzel Associates for their dedication in developing and building the DPUs and Synthesizers, respectively. Additionally, we would like to extend our gratitude to James Chinn, Wousik Kim, Allen Andersen, Dennis Thorbourn, and Eduardo Martin of JPL as well as Stephen Brown, Kenneth O'Connor, and Robert Meloy of GSFC for their invaluable help with IESD analysis and testing. We also want to thank the Space Simulator Laboratory at APL for their support during the cryogenics test campaign. Finally, we are grateful to Sean Peters, Anna Broome, and Thomas Teisberg for their feedback on this manuscript.

## Declarations

**Competing Interests** The authors have no competing interests to declare that are relevant to the content of this article.

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## Authors and Affiliations

Donald D. Blankenship<fnref n="1" /> · Alina Moussessian<fnref n="2" /> · Elaine Chapin<fnref n="2" /> · Duncan A. Young<fnref n="1" /> ·

G. Wesley Patterson<fnref n="3" /> · Jeffrey J. Plaut<fnref n="2" /> · Adam P. Freedman<fnref n="2" /> · Dustin M. Schroeder⁴,⁵ ·

Cyril Grima<fnref n="1" /> · Gregor Steinbrügge<fnref n="2" /> · Krista M. Soderlund<fnref n="1" /> · Trina Ray<fnref n="2" /> ·

Thomas G. Richter<fnref n="1" /> · Laura Jones-Wilson<fnref n="2" /> · Natalie S. Wolfenbarger⁵ ID ·

Kirk M. Scanlan⁶ · Christopher Gerekos<fnref n="1" /> · Kristian Chan<fnref n="1" />,⁷ · Ilgin Seker<fnref n="2" /> ·

Mark S. Haynes<fnref n="2" /> · Amy C. Barr Mlinar⁸ · Lorenzo Bruzzone⁹ · Bruce A. Campbell¹⁰ ·

Lynn M. Carter¹¹ · Charles Elachi¹² · Yonggyu Gim<fnref n="2" /> · Alain Hérique¹³ ·

Hauke Hussmann¹⁴ · Wlodek Kofman¹³,¹⁵ · William S. Kurth¹⁶ ·

Marco Mastrogiuseppe¹⁷ · William B. McKinnon¹⁸ · Jeffrey M. Moore¹⁹ ·

Francis Nimmo²⁰ · Carol Paty²¹ · Dirk Plettemeier²² · Britney E. Schmidt²³,²⁴ ·

Mikhail Y. Zolotov²⁵ · Paul M. Schenk²⁶ · Simon Collins<fnref n="2" /> · Harry Figueroa<fnref n="2" /> ·

Mark Fischman<fnref n="2" /> · Eric Tardiff<fnref n="2" /> · Andy Berkun<fnref n="2" /> · Mimi Paller<fnref n="2" /> · James P. Hoffman²⁷ ·

Andy Kurum²⁸ · Gregory A. Sadowy<fnref n="2" /> · Kevin B. Wheeler<fnref n="2" /> · Emmanuel Decrossas<fnref n="2" /> ·

Yasser Hussein<fnref n="2" /> · Curtis Jin<fnref n="2" /> · Frank Boldissar<fnref n="2" /> · Neil Chamberlain<fnref n="2" /> ·

Brenda Hernandez<fnref n="2" /> · Elham Maghsoudi<fnref n="2" /> · Jonathan Mihaly²⁹ · Shana Worel<fnref n="2" /> ·

Vik Singh<fnref n="2" /> · Kyung Pak<fnref n="2" /> · Jordan Tanabe<fnref n="2" /> · Robert Johnson<fnref n="2" /> · Mohammad Ashtijou<fnref n="2" /> ·

Tafesse Alemu<fnref n="2" /> · Michael Burke<fnref n="2" /> · Brian Custodero<fnref n="2" /> · Michael C. Tope<fnref n="2" /> ·

David Hawkins<fnref n="2" /> · Kim Aaron<fnref n="2" /> · Gregory T. Delory³⁰ · Paul S. Turin³⁰ ·

Donald L. Kirchner¹⁶ · Karthik Srinivasan<fnref n="2" /> · Julie Xie<fnref n="2" /> · Brad Ortloff<fnref n="2" /> · Ian Tan<fnref n="2" /> ·

Tim Noh<fnref n="2" /> · Duane Clark<fnref n="2" /> · Vu Duong<fnref n="2" /> · Shivani Joshi<fnref n="2" /> · Jeng Lee<fnref n="2" /> · Elvis Merida<fnref n="2" /> ·

Ruzbeh Akbar<fnref n="2" /> · Xueyang Duan<fnref n="2" /> · Ines Fenni<fnref n="2" /> · Mauricio Sanchez-Barbetty³¹ ·

Chaitali Parashare<fnref n="2" /> · Duane C. Howard³² · Julie Newman³³ · Marvin G. Cruz<fnref n="2" /> ·

Neil J. Barabas³⁴ · Ahmadreza Amirahmadi<fnref n="2" /> · Brendon Palmer<fnref n="2" /> · Rohit S. Gawande<fnref n="2" /> ·

Grace Milroy<fnref n="2" /> · Rick Roberti<fnref n="2" /> · Frank E. Leader<fnref n="2" /> · Richard D. West<fnref n="2" /> · Jan Martin<fnref n="2" /> ·

Vijay Venkatesh<fnref n="2" /> · Virgil Adumitroaie<fnref n="2" /> · Christine Rains<fnref n="2" /> · Cuong Quach<fnref n="2" /> ·

Jordi E. Turner<fnref n="3" /> · Colleen M. O'Shea<fnref n="3" /> · Scott D. Kempf<fnref n="1" /> · Gregory Ng<fnref n="1" /> · Dillon P. Buhl<fnref n="1" /> ·

Timothy J. Urban<fnref n="1" />

<figure><img src="image_27.png" /></figure>

N.S. Wolfenbarger
nswolf@stanford.edu

<figure><img src="image_28.png" /></figure>

<footnote n="1">¹ Institute for Geophysics, University of Texas at Austin, Austin, TX 78758, USA</footnote>

<footnote n="2">² Jet Propulsion Laboratory, California Institute of Technology, Pasadena, CA 91109, USA</footnote>

<footnote n="3">³ Applied Physics Laboratory, Johns Hopkins University, Laurel, MD 20723, USA</footnote>
⁴ Department of Electrical Engineering, Stanford University, Stanford, CA 94305, USA

⁵ Department of Geophysics, Stanford University, Stanford, CA 94305, USA

⁶ Geodesy & Earth Observation Division, DTU Space, Technical University of Denmark, 2800 Kongens Lyngby, Denmark

⁷ Department of Earth and Planetary Sciences, Jackson School of Geosciences, University of Texas at Austin, Austin, TX 78712, USA

⁸ Planetary Science Institute, Tucson, AZ 85719, USA

⁹ University of Trento, Trento, Italy

¹⁰ Smithsonian Institution, Center for Earth & Planetary Studies, MRC 315, Washington, DC 20013-7012, USA

¹¹ Lunar and Planetary Laboratory, University of Arizona, Tucson, AZ 85721, USA

¹² California Institute of Technology, Pasadena, CA 91125, USA

¹³ University Grenoble Alpes, CNRS, CNES, IPAG, 38000 Grenoble, France

¹⁴ Institute of Planetary Research, German Aerospace Center, Berlin, Germany

¹⁵ Centrum Badan Kosmicznych Polskiej Akademii Nauk (CBK PAN), Warsaw, Poland

¹⁶ Department of Physics and Astronomy, University of Iowa, Iowa City, IA 52242, USA

¹⁷ University of Rome “La Sapienza”, Rome, Italy

¹⁸ Washington University in St. Louis, St. Louis, MO 63130, USA

¹⁹ NASA Ames Research Center, Moffett Field, CA 94035, USA

²⁰ Dept. Earth and Planetary Sciences, University of California, Santa Cruz, CA 95064, USA

²¹ Department of Earth Sciences, University of Oregon, Eugene, OR 97403, USA

²² Technische Universität Dresden, 01069 Dresden, Germany

²³ Department of Earth and Atmospheric Sciences, Cornell University, Ithaca, NY, USA

²⁴ Department of Astronomy, Cornell University, Ithaca, NY, USA

²⁵ School of Earth and Space Exploration, Arizona State University, Tempe, AZ 85287, USA

²⁶ Lunar and Planetary Institute, Houston, TX 77058, USA

²⁷ Kinemetrics Inc., Pasadena, CA 91107, USA

²⁸ Planet, San Francisco, CA 94107, USA

²⁹ Laboratory for Atmospheric and Space Physics, University of Colorado, Boulder, CO 80303, USA

³⁰ Heliospace Corporation, Berkeley, CA 94710, USA

³¹ Tomorrow.io, Boston, MA 02210, USA

³² Center for Quantum Computing, Amazon Web Services, Pasadena, CA 91125, USA

³³ The Boeing Company, El Segundo, CA 90245, USA

³⁴ B&R Electronics, Reno, NV 89500, USA