Acoustic positioning system in ice for the Enceladus Explorer Ruth Hoffmann for the EnEx Collaboration Erlangen, June 20, 2012
Outline Goals of the Enceladus Explorer Project Design of the IceMole Inertial Navigation Acoustic positioning system Field tests Planned Measurements on the Morteratsch Glacier Simulations on the accuracy of the acoustic positioning system Summary and Outlook June 20, 2012 2
Goals Short term: Development of new technologies for future space missions → Navigation systems in ice (inertial & acoustic) → IceMole probe → Environment recognition with Ultrasound arrays → Decontamination of the probe Tiger stripes Long term: Appliance of acoustic positioning in neutrino astronomy Exploration of the sub-glacial lake which is the origin of the Blood Falls Search for extraterrestrial life on the Saturn moon Enceladus June 20, 2012 3
Enceladus: Tiger Stripes Appearance: South polar region Cryovolcanism: Water erupts from cracks in the ice → Liquid water under the surface! Tiger Stripes: Temperature increased by 25 degrees → Inner energy source! Best Explanation so far: → Tidal heating Search for extraterrestrial life! June 20, 2012 4
Ice Mole Combination of melting and drilling probe Ice-screw to press the head to the ice → digging also horizontally and vertically against gravity 16 heater in IM head, 8 side heater that can be accessed seperately → drive curves with an radius of 10m Dimensions: 15x15 cm² width ~1m length Developed by FH Aachen June 20, 2012 5
Inertial Navigation IMU (Inertial Measurement Unit) Gyrocompass and accelerometers → provides position and orientation consecutively → Errors add up over time Differential magnetometer → provides orientation of the IM Combined with measurement of covered distance by the IM (from cable length) → Calculation of the path through the glacier All Systems still in testphase + IMU will not work on Enceladus Acoustic positioning system! June 20, 2012 6
Acoustic positioning system Positioning by measuring the signal propagation times and Triangulation 3 Subsystems: ASP: acoustic surface pinger (BUW) APS: acoustic positioning sensors (RWTH) ADS: acoustic data system (RWTH) June 20, 2012 7
Acoustic positioning system Challenges: Technical → Coupling of pinger to solid ice → Precise measurement of propagation time (1 μs) → Determination of the exact position of the pingers (1 cm) Signal Propagation → Attenuation → Determination of the sound speed → Cutoff by cracks in the ice June 20, 2012 8
Acoustic Positioning Sensors 4 Sensors integrated in IM head PZT (Lead zirconate titanate) disks coupled to a preamplifier stage Pressure between PZT and IM head shall be tunable Resonance frequency of sensors should match resonance frequency of IM head Coupling to the ice through IM head → good coupling to head needed Optimized for precise arrival time determination June 20, 2012 9
Acoustic Pingers Flexible frequency range (20 – 500 kHz) → match resonance frequency of IM head Peak-to-peak voltage for pinging up to 1kV Optimize signal-to-noise ratio for precise time measurement Coupling to solid ice ● Coupling through water ● Freezing directly on the surface Aluminum Cone Determination of the sound speed Measurements from pinger to pinger of sound waves that were bent PZT Ceramic backwards in the glacier June 20, 2012 10
Field tests July 2012: Morteratsch glacier (Switzerland) April 2013: Matanuska glacier (Alaska) November 2013: Canada glacier (Antarctica) November 2014: Taylor glacier (Antarctica) June 20, 2012 11
Morteratsch Gletscher Setup: 4 Receivers (RWTH) 3 Pinger → ITC 1001 → Hydrophone SQ09 → Hades sensors (PZT rings) Melting probe for hole drilling down to 5m Goals: Measurement of sound speed Measurement of the attenuation length Homogenity of the ice (all in dependence of the depth in the glacier) Development of a model of the depth dependence of the sound speed in a glacier June 20, 2012 12
Simulation Purpose: Study of possible uncertainty sources on the precision of the acoustic positioning system 1. Fixing of the Geometry → 3 Pinger at 3 corners of a square (distance 100 m) → IceMole at Position [50, 50, -50] 2. Calculation of the real propagation times from speed of sound (4000 m/s) and real distances June 20, 2012 13
Simulation 3. Creation of gaussian distributed arrival times from these real times 4. Fitting a Gauss distribution to receive the “measured” times and their errors 5. Calculation of the “measured” distances from the times and sound speed 6. Calculation of the Position of the IceMole and by Triangulation June 20, 2012 14
Triangulation Calculate intercept circle of two spheres Calculate intercept point of the circle with remaining sphere June 20, 2012 15
Results Considered errors sources ● Signal propagation time ● Speed of sound ● Position of the Pingers Variable Error Precision IM Time 1 μs ~1 mm Sound speed 40 m/s ~1 m Pinger Position 1 cm ~1 cm Uncertainty of the sound speed is the most critical error source June 20, 2012 16
Improvements Other methods for the triangulation Usage of more pingers Usage of a smaller distance between the pingers → reduce the influence of the speed of sound Including of a model of the sound speed in the positioning algorithms June 20, 2012 17
Summary and Outlook Summary Introduction of Enceladus Explorer Overview on the current status of the acoustic positioning system Identification of the sound speed as the most crucial error source Outlook Measurements in glacier ice Model of the depth-dependence of the sound speed Upgrade of the simulation on 4 Pingers and calculation of time as a fourth coordinate June 20, 2012 18
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