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Using Speed of Sound Measurements to Constrain the Huygens Probe Descent Profile

H. Svedhem ⁽¹⁾, J. Zarnecki, B. Hathi⁽²⁾

(1) Research and Scientific Support Department, SCI-SB, ESA/ESTEC, PB 299, 2200 AG Noordwijk, The Netherlands
(2) Planetary and Space Science Research Institute, Open University, Walton Hall, MK7 6AA Milton Keynes, United Kingdom

email: Hakan.Svedhem@rssd.esa.int

The Acoustic Properties Investigation (API) is a set of sensors for acoustic measurements in gases or liquids, making a part of the Surface Science Package (SSP) on the Huygens probe. It consists of two units, API-V (Velocity of sound) and API-S (Sounding). The API-V has two ultrasonic transducers sending and receiving acoustic pulses over an unobstructed path of 15 cm. An accurate timing circuit is measuring the time it takes to propagate over the distance. Measurements are made in both directions to eliminate the effect of a constant drift of the medium. The transducers have been optimised to operate at low pressure (high altitude) and will operate from about 50km down to the surface. They will also perform well in case of landing in a liquid. The API-S unit is an acoustic sounder, sending short pulses at 15kHz every second and listening for echoes in between. It will detect droplets in the atmosphere and for the last 100m it will characterise the acoustic scattering properties of the surface below. It will also give an accurate value for the descend velocity during the last 100m. In case of landing in a (liquid) lake/ocean it will measure the depth of the lake/ocean down to a maximum of about 1000m.

The Acoustic Properties Investigation (API) is a set of sensors for acoustic measurements in gases or liquids, making a part of the Surface Science Package (SSP) on the Huygens probe. It consists of two units, API-V (Velocity of sound) and API-S (Sounding). The API-V has two ultrasonic transducers sending and receiving acoustic pulses over an unobstructed path of 15 cm. An accurate timing circuit is measuring the time it takes to propagate over the distance. Measurements are made in both directions to eliminate the effect of a constant drift of the medium. The transducers have been optimised to operate at low pressure (high altitude) and will operate from about 50km down to the surface. They will also perform well in case of landing in a liquid.

The API-S unit is an acoustic sounder, sending short pulses at 15kHz every second and listening for echoes in between. It will detect droplets in the atmosphere and for the last 100m it will characterise the acoustic scattering properties of the surface below. It will also give an accurate value for the descend velocity during the last 100m. In case of landing in a (liquid) lake/ocean it will measure the depth of the lake/ocean down to a maximum of about 1000m.

This paper shall discuss the hardware, the modes of operation, the calibration and the expected performances, including error sources, measurement accuracy and range of operation. Possible ways how to integrate the results into a complete descent model shall be elaborated on.