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.
We acknowledge useful discussions with the Huygens SSP Team, in particular Håakan Svedhem, James Garry, Martin Towner, and Mark Leese, and with Tim Leighton of the University of Southampton. The authors are grateful for the support of NASA via the Discovery program's TiME Phase A study and the Johns Hopkins University Applied Physics Laboratory via the Civilian Space Independent Research and Development program, and a Janney Publication Fellowship. We acknowledge useful discussions with Michael Paul, Eric Bienert, and Richard Meyer of the Penn State University Applied Research Laboratory, and with Catherine Neish, Kristin Sotzen, and Kim Strohbehn of the Johns Hopkins University Applied Physics Laboratory.
I. INTRODUCTION II. MEASUREMENT GOALS III. ACOUSTICAL PROPERTIES OF THE TITAN SEA A. Physical setting B. Density C. Speed of sound D. Sound absorption E. Bottom reflection F. Suspended materials IV. BACKGROUND NOISE A. Thermal noise B. Wind-generated noise C. Precipitation noise D. Vehicle noise E. Other noise sources V. MEASUREMENT IMPLEMENTATION A. Instrument accommodation and testing VI. SONAR PERFORMANCE A. Analytic prediction of minimum source level B. Transducer design C. Transducer calibration measurements D. Numerical prediction of transducer performance VII. CONCLUSIONS