The ngRadar team is analyzing the science use cases suggested and prioritized by the ngRadar Science Advisory Council (SAC) as well as common operational desires as input to shape the ngRadar system.
High-Priority Use Cases
- Near-Earth Objects (NEOs): NEOs encompass the asteroids and comets that venture near Earth's orbit, specifically those that come within 1.3 au (astronomical units) of the Sun. A next-generation radar system should be able to provide physical and dynamical characterization of NEOs to complement the work done by the Goldstone Solar System Radar for planetary science and planetary defense. Future radar observations should be used to characterize the general population of NEOs by pushing to observe smaller objects at greater distances. NEOs should be observed multiple times (on multiple apparitions) to search for temporal variability. Specifically for planetary defense, a next-generation radar systems should be responsive to observing imminent impactors (potential or confirmed) and provide accurate astrometric follow-up of newly discovered NEOs, especially from Rubin Observatory and NEO Surveyor. Radar observations should be used to inform and support spacecraft missions when possible. Radar observations of interstellar objects would be operationally similar to NEOs, albeit with different orbits and faster rates of sky motion. Based on historical context, radar observations of NEOs could make use of several hundred hours of telescope time per year.
- Orbital Debris: With the increase in number of Earth-orbiting satellites and the planned return of humans to the Moon, safety and security in cislunar space is becoming a greater concern. A next-generation radar system should be able to conduct population studies of debris from low-Earth orbit (LEO; 1000-2000 km altitude), through medium-Earth orbit (MEO; ~20,200 km altitude) and geosynchronous Earth orbit (GEO; 36,000 km altitude), and into cislunar space. These altitudes are difficult to study via other means, especially in the centimeter to tens-of-centimeters size range. Rapid response of radar observations to fragmentation events on orbit would be especially helpful for risk assessment and improving models of debris propogation. Based on historical context, radar observations of orbital debris could make use of hundreds of hours of telescope time per year.
- Planetary Surfaces: Radar, both from the ground and by spacecraft, has a long history of characterizing the surfaces and sub-surfaces of the Moon and terrestrial planets as radar can penetrate several wavelengths into the regolith, sometimes revealing obscured geology. A next-generation radar system should complement previous radar studies. Radar observations of the Moon and terrestrial planets could make use of tens to hundreds of hours of telescope time per year, where the Moon would dominate these numbers.
- Moon: Investigation of the lunar regolith is of interest, specifically dielectric permittivity (related to radar-scattering properties) and the regolith response to temperature variations. By peering into the sub-surface, radar could help characterize the shallow volatile inventory on the Moon, especially in persistently shadowed regions. Mapping the lunar nearside could take thousands of hours of telescope time, especially if repeatedly imaging regions searching for temporal changes due to, say, new lunar impacts or human/robotic activity on the lunar surface.
- Mercury: Radar observations of Mercury with the Goldstone Solar System Radar and Arecibo Observatory led to the discovery of purported water ice at the poles, hidden below the surface in persistently shadowed regions. Additional radar imaging of Mercury's poles at a higher frequency (shorter wavelength) would provide an additional shallower constraint on burial depth and volume of volatile material in these regions.
- Venus: A long-standing question about Venus is whether the surface is geologically active. The process of answering this question is hindered by the thick atmosphere of Venus making optical study from Earth impossible. Therefore, long-term monitoring of Venus is left to the longer wavelengths (lower frequencies) of radar and highly desirable. However, even radar is not immune to the thick atmosphere of Venus, where radar imaging with frequencies above 3 GHz is essentially impossible as well.
- Mars: Radar observations of Mars should concentrate on potentially detecting and monitoring long-term changes due to seasons in terms of the effects of the Martian hydrological cycle on the signatures of volatiles in the shallow sub-surface. A transmit frequency higher than that of the Goldstone Solar System Radar could be sensitive to the sub-surface region between the diurnal and annual skin depths where seasonal variations may occur.
- Moon: Investigation of the lunar regolith is of interest, specifically dielectric permittivity (related to radar-scattering properties) and the regolith response to temperature variations. By peering into the sub-surface, radar could help characterize the shallow volatile inventory on the Moon, especially in persistently shadowed regions. Mapping the lunar nearside could take thousands of hours of telescope time, especially if repeatedly imaging regions searching for temporal changes due to, say, new lunar impacts or human/robotic activity on the lunar surface.
- Additional high-priority uses cases suggested by the SAC rapidly require a highly sensitive radar system on the scale of Arecibo Observatory or greater. These include characterization of the physical properties of main-belt asteroids, ranging and imaging of the Galilean satellites of Jupiter, and ranging and imaging of Titan (satellite of Saturn and destination of the Dragonfly mission).
Operational Desires
- Transmission at a frequency higher than X band to complement the Goldstone Solar System Radar (X band; 8.56 GHz) and the decommissioned Arecibo Observatory planetary radar (S band; 2.38 GHz).
- Transmission of circular polarization and reception of dual circular polarizations, i.e., the same- and opposite-sense circular polarization as transmitted, for radar-scattering studies (depolarization, chemical composition, Stokes analysis, m-X decomposition, etc.).
- Well-calibrated system for accurate timing, absolute ranges, radar cross sections and albedos, etc.
- Accessibility to users via semester calls for proposals, triggered proposals, or urgent proposals.
- Rapid deployment on the scale of minutes to hours (rather than days or weeks) to be responsive to new discoveries and transient events.
- Near-real-time feedback (quick-look processing of radar data) to evaluate the success of the observation and inform potential changes to the observation and the ability to modify the observation as needed during the scheduled track.
- Archival of raw data for re-processing at a later date or for algorithm development.
Disclaimer: Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation (NSF), the National Aeronautics and Space Administration (NASA), or other funding agencies.