Development of a Space-Deployable Dual-Mode LiDAR for Planetary Seismology

LIDAR or Light Detection and Ranging systems have been used in space research for more than 50 years. Besides using LIDARs for altimetry, these devices have been used to study atmospheres and ice. The LIDAR In-space Technology Experiment (LITE) in the 1990’s demonstrated a number of capabilities and applications for space applications on board the space shuttle Discovery. One new potential application is the use of LIDARs for planetary seismology. Typically, seismologists exploit measurements of Primary (P), Secondary (S), and Rayleigh waves to infer structure of the underlying planetary body. The P-waves are longitudinal waves which travel at the compressional wave speed in the body. The S-waves or shear waves are transverse waves which travel at the shear wave speed. Rayleigh waves are vertically- polarized propagating waves that are trapped at the interface between an elastic medium and a vacuum (or low density atmosphere such as Earth). These waves can propagate large distance because their amplitudes decay primarily as one over square root of range, as opposed to compressional (P) and shear (S) waves, which decay as one of range. Rayleigh waves are the cause of much damage due seismic activity on Earth but also can be excited by impactors on the moons and rocky planets. Such waves can propagate in ice sheets on Europa. Interface waves at the seabed are called Scholte waves and those waves at the interface of two elastic media are called Stonely waves. The dispersion of the speed of Rayleigh waves (often referred to as P-SV waves) as a function of frequency is dependent on the underlying vertical profile of elastic medium properties such as the shear modulus/speed. A number of techniques have been devoted to the estimation of the vertical shear speed given measurements of Rayleigh wave dispersion from explosions, impacts and other energetic sources. Measurements of Rayleigh waves have traditionally been taken using geophones at low frequencies and accelerometers at mid- to high frequencies. Recently, we have developed chirp lasers to measure both range and surface motion through a NASA-EPSCoR research seed grant. Here, we propose to build a low SWaP-C (Size, Weight, Power and Cost) with two operational modes (ranging mode and Doppler mode) for remote surface measurement. Under ranging mode, the LiDAR sends off chirped laser pulse for frequency modulated continuous wave (FMCW) based ranging operation. Under Doppler mode, the laser outputs a high-quality narrow-linewidth single wavelength laser, and receives the beat signal between the incident wave and reflection wave to measure the surface velocity. The dual-mode LiDAR will be built around a digital circuit, the mode can be controlled digitally and remotely, and the switch time is well within 1 ms. In the first year of the study, the Science PI (T. Wei, URI) and team (G. Potty and J. H. Miller, also URI) propose to (a) investigate the engineering and signal processing; and (b) develop a model to produce synthetic ground motion data and resulting modeled laser pulse returns. In the second year of the project, we propose to demonstrate the chirp laser in a laboratory setting using techniques developed by the P.I.s described above. We also foresee the continued modeling efforts resulting in the development of algorithms for the estimation of ground properties. In the third year of the project, we propose to demonstrate a prototype system in a field experiment. The field experiment would involve measuring both altitude and ground motion using a chirp laser on board a moving aerial platform such as a drone, helicopter or small aircraft. The refinements necessary for this type of system to be launched into space will be investigated through collaboration with researchers at NASA Goddard.

This is a mature technology (TRL 7+) — the realistic path in is usually NASA's Technology Transfer Program: licensing an existing NASA patent, or a Space Act Agreement to use NASA facilities/expertise directly. NASA also runs a startup licensing program with no upfront fee for companies formed to commercialize a specific NASA technology.

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