The Autonomous Pinger Unit of the Acoustic Navigation Network in EnEx-RANGE: an autonomous in-ice melting probe with acoustic instrumentation

Abstract

The Autonomous Pinger Unit (APU) is an electro-thermal drill with acoustic instrumentation developed for the project EnEx-RANGE in view of a future space mission for the sub-surface exploration of Saturn's moon Enceladus. A main goal is the development of navigation technology for an acoustic guidance system allowing maneuvering a probe through glacial ice. In total 13 APUs were built and tested in terrestrial analog scenarios on alpine glaciers. The APUs form a spatially distributed network that defines a system of reference for the navigation of the maneuverable probe to a point of interest. The APUs have a novel melting head, slow control systems, and a modern system-on-chip (SoC) module that controls the probe and processes the recorded data. The APUs use acoustic emitters and receivers to measure the transit time of acoustic signals between them, allowing for the position reconstruction of all APUs by trilateration. Several auxiliary sensors monitor the internal state of the probe and assist the position estimation. With this instrumentation, the APUs have the ability of dynamically optimizing themselves within the network by changing their position. This paper gives an overview of the developed APU hardware and presents performance results from the field tests.

Performance Studies of the Acoustic Module for the IceCube Upgrade

“…The transducer design of the APU is very similar to that of the AM. Differences are the number of piezo discs (8 compared to 16) and the head-to-tail mass ratio (3:1 compared to 1:2.6) [12].…”

“…The range of the AM is determined by the distance at which a minimum required SNR can be reached. During a measurement at the Langenferner glacier in Italy, the APU achieved an SNR of 100 at a distance of ≈ 30 m from APU to APU by averaging 64 waveforms [12]. The attenuation length of the glacial ice was measured to be ≈ 8.85 ± 0.95 m [13].…”

The Acoustic Module for the IceCube Upgrade

“…The technology of the acoustic calibration system is based on experience with the EnEx-RANGE project, in which an acoustic localization system for melting probes had been developed [7]. The system for the IceCube Upgrade consists of 10 AMs distributed over the new strings, as can be seen in figure 1a.…”

“…Therefore, we replace the SNR by a scaling factor • 0 . Here 0 = 95 × 10 3 m corresponds to the SNR from the previous known EnEx-RANGE emitter-receiver-system that resulted in = 25 : 1 measured at a distance of 38 m on a temperate alpine glacier of = 8.25 m attenuation [7]. After distance and attenuation correction, this defines 0 and the baseline is = 1.…”

Design and performance of the Hotrod melt-tip ice-drilling system

“…While conventional electrothermal drills operate in a liquid borehole, meaning the drill can be raised and lowered within a liquid-filled borehole, the vast majority of electrothermal drill systems developed in recent decades operate in refrozen boreholes, meaning that the borehole refreezes behind the drill as it penetrates (Aamot, 1967(Aamot, , 1968Philberth, 1974;Morton and Lightfoot, 1975;Hansen and Kersten, 1984;Kelty, 1995;Zimmerman et al, 2001;Ulamec et al, 2007;Bentley et al, 2009;Dachwald et al, 2014;Stone et al, 2014Stone et al, , 2018Weinstock et al, 2021;Winebrenner et al, 2013;Wirtz and Hildebrandt, 2016). This reflects a general trend, especially in the past 2 decades, of hot-point research being driven towards electrothermal drills of lower specific power suitable for extraterrestrial investigations with refrozen boreholes (Fig.…”