Saturn's moon Titan is the only satellite in this solar system with a dense atmosphere and hydrocarbon seas. The Titan Mare Explorer (TiME) mission would splashdown a capsule to float for 3 months on Ligeia Mare, a several-hundred-kilometer wide sea near Titan's north pole. Among TiME's scientific goals is the determination of the depth of Ligeia, to be achieved with an acoustic depth sounder. Since Titan's surface temperature is known to vary around 92 K, all instruments must be ruggedized to operate at cryogenic temperatures. This paper's contributions include an approach to infer key acoustic properties of this remote environment and the extraterrestrial environment's influence on the development of a cryogenic depth sounder. Additionally, an approach is formulated to infer the transducer's response, sensitivity, and performance when in situ calibration is impossible or when replicating key environmental conditions is too costly.

One of the most dominant sources of error in the estimation of sonar performance in shallow water is the geoacoustic description of the sea floor. As reviewed in this paper, various investigators have studied the possible use of ambient noise to infer some key parameters such as the critical angle, geoacoustic properties, or bottom loss. A simple measurement approach to infer the bottom loss from ambient noise measurement on a vertical line array (VLA) is very attractive from environmental and operational perspectives. This paper presents a sensitivity study conducted with simulations and measurements that demonstrates mitigating factors to maximize the accuracy of estimated bottom loss. This paper quantifies the robustness and operational constraints of this measurement approach using an ambient noise model that accounts for wind, shipping, and thermal noise. Also demonstrated are the effects of unaccounted water absorption, array tilt, nearby ship interference, flow noise, calibration error, and array deformation on sonar performance estimation. VLA measurements collected during the Asian Seas International Acoustics Experiment in May-June 2001 were also processed to validate the approach via comparisons with measured bottom loss and transmission loss.

A small expendable wideband low-frequency sound source that will be deployed on the seafloor is being developed to be used for geoacoustic inversion surveys in conjunction with a terrain-hugging AUV. This low-cost deployable source contains a transducer that produces a relatively flat transmit response over the broad frequency band of 100 to 4000 Hz. In operation, a seafloor interface wave will be excited and exploited for geoacoustic inversion by the deployed sound source and a receiving array on the bottom-hugging AUV. A feasibility study is also being performed that includes physics-based sonar simulations to infer the performance of geoacoustic inversion in a number of AUV scenarios and environmental conditions. Based on this study, design trade-offs will be determined to finalize key factors of the transducer, such as its physical size, weight, and production cost. Battery technology is also being developed to optimize the source level, the duty cycle, and the operating life of the signals that will be transmitted during data collection. This effort is being supported by ONR.

Saturn’s moon Titan is the only satellite in our solar system with a dense atmosphere and hydrocarbon seas. The proposed Titan Mare Explorer (TiME) mission would splashdown a capsule to float for 3 months on Ligeia Mare, a several-hundred-kilometer wide sea near Titan’s north pole. Among TiME’s scientific goals is the determination of the depth of Ligeia, to be achieved with an acoustic depth-sounder. Since Titan’s surface temperature is known to vary around 92 K, all instruments must be ruggedized to operate at cryogenic temperatures. This paper’s contributions include an approach to infer key acoustic properties of this remote environment, their influence on the development of a cryogenic depth sounder, and on an approach to infer the transducer’s response, sensitivity and performance when unable to perform in-situ calibration measurements or to replicate key environmental conditions. This effort was conducted under the auspices of the Civilian Space Independent Research and Development program from the Johns Hopkins University Applied Physics Laboratory.

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