S. Viola VLVνT 2015, Rome – 14/09/2015

Characterization and testing of the KM3NeT acoustic positioning system

F. SimeoneVLVνT 2015 – Rome, 14/09/2015

S. Viola VLVνT 2015, Rome – 14/09/2015

KM3NeT Acoustic positioning systemThe KM3NeT acoustic positioning system is composed of three main sub-system:

1) a Long Base-Line (LBL) of acoustic transmitters (beacons) and receivers, located at known positions

2) an array of digital acoustic receivers (DARs) installed along the detection units (DUs) of the telescope

3) a farm of PCs for the acoustic data analysis

Beacon signals must be detected at distances of 1 km

Suitable frequency range 20 kHz-50 kHz

lowest level of PSD ( ~40 dB re 1 uPa2/Hz) attenuation (0-10 dB/km)

20 kHz – 50 kHz20 kHz – 50 kHz

S. Viola VLVνT 2015, Rome – 14/09/2015

KM3NeT acoustic beacon

Signal Emitted length: from 0 to 50 ms

Maximum emission amplitude: 180 dB @ 34kHz re 1 μPa at 1 m

Type of acoustic signals:- Monochromatic signals configurable from 1 kHz to 80 kHz- Sine Sweep signals configurable from 1 kHz to 80 kHz- Maximum Length Sequence (MLS) signals with lengths from 5.12 ms to 40.96 ms (at 200 ksamples/s)

The commercially available piezo-ceramic Free Flooded Rings (FFR) transducers SX30 from Sensor Tech Ltd have been chosen for LBL beacons. Each beacon can be reconfigured via a dedicated RS232 connection with a Central Logic Board (CLB) installed on the CB electronics, in order to allow reconfiguration of acoustic emission signal parameters (amplitude, waveform, timing) for “in situ” optimization of the signal detection. The signal emission trigger is synchronized with the detector master clock with calibrated time delay of 7±1 µs

S. Viola VLVνT 2015, Rome – 14/09/2015

Calibration Tests5V 20V 40V 60 VCapacitor Charge

20 30 40 50140

160

180

200

Frequency [kHz]

SPL

[dB

re 1

uPa

@ 1

m @

V]

Sound Pressure Level

20 30 40 50140

160

180

200

Frequency [kHz]

SPL

[dB

re 1

uPa

@ 1

m @

V]

Sound Pressure Level

Horizontal position

Vertical position-40 -20 0 20 400

0.2

0.4

0.6

0.8

1

Angle [º]

Norm

alize

d am

plitu

de

Directivity [Horizontal]

Sine 20kSine 30kSine 40k

Acoustic beacon calibration

Calibration tests were performed at the water tank (dimension of 87.5 x 113 x 56.5 cm3) of Laboratory of Ultrasounds in Gandia, UPV (Department of Physics and Acoustics).

Pressure tests were successfully performed at 400 bars in a High Pressure machine at the company Hiperbaric (Burgos)

S. Viola VLVνT 2015, Rome – 14/09/2015

The ‘external’ Digital Acoustic Receiver

Digital hydrophones are hosted on the base of the DUs and on CBs. These ‘external’ hydrophones are used to calibrate the LBL measuring the relative distances among LBL elements. The selected hydrophone is the DG0330 manufactured by Colmar s.r.l.. It consists of a spherical piezo-ceramic element read-out by an analogue board, splitting the signal in two lines with different gains (+46 dB and +26 dB respectively).

S. Viola VLVνT 2015, Rome – 14/09/2015

The ‘internal’ Digital Acoustic ReceiverThe movement of the DUs under sea currents are monitored thanks to “internal” piezo-electric Digital Acoustic Receivers (DAR) glued from the inside to the glass sphere of each KM3NeT Digital Optical Module (DOM)

Power spectral density of the electronic noise for an internal digital acoustic receiver as measured within a DOM in the laboratory. The blue curve shows the distribution for PMTs power off, the red curve for PMTs power on.

The design sensitivity of the sensors is - 160 ± 6 dB re 1V/Pa at 50 kHz with a ± 3 dB variation (long time average) in the range 10 ÷ 70 kHz. Acoustic data are acquired continuously at 195 kHz, with a resolution of 24 bits.

S. Viola VLVνT 2015, Rome – 14/09/2015

The on-shore data analysis

The acoustic positioning data analysis is performed on-shore by a farm of PCs. These PCs receive, on a intranet, data from the DARs, parsed from the main data stream. The Time of Arrival (ToA ) of the acoustic signals detected by DARs are measured, analyzing the DAR raw data flow using software algorithms based on cross correlation filters. Once a LBL-beacon pulse is identified, the code associates to it the detection absolute GPS time (ToA).

The distance between each beacon and each DAR will be then calculated taking into account the time of emission (ToE) of the acoustic signal and the sound velocity (SVP) along the water column. Knowing the location of the LBL beacons, each DAR position is recovered using algorithms based on spherical multi-lateration.

S. Viola VLVνT 2015, Rome – 14/09/2015

Acoustic DAQ: latency time measurements

Hydro CH1 (high gain) Hydro CH2 (low gain)

PPS trigger

Clock + GPS timeAnalog signal

(5 cycles, 20 kHz, 100 mV)

The accuracy on the measurement of the arrival time of acoustic signals on the hydrophones depends on the accuracy of the latency time of the whole data acquisition system.

The time latency of the hydrophone DAQ (including 4.5 m cable) has been measured using a dedicated setup and its value is 50.65 ± 0.02 μs for the low gain channel and 50.71 ± 0.02 μs for the high gain channel.

GPS receiver

Hydrophone Control logic board

Signal generator

S. Viola VLVνT 2015, Rome – 14/09/2015

Expected performance: signal recognition

+

+

Direct wave

Direct wave

Reflection (1 ms)

Reflection (1ms)70% of the direct wave amplitude

70% of the direct wave amplitude

Sin 20 kHz, 5 ms

Sweep 20-21 kHz, 5 ms

Direct wave + Reflection (1 ms late)

Direct wave + Reflection (1ms)

Direct wave Reflection

Direct wave Reflection

S. Viola VLVνT 2015, Rome – 14/09/2015

Expected performance: signal recognition in ambient noiseSignal amplitude: 160dB re 1uPa @1m

Length signal: 5 ms

Emitter – receiver distance : 1000 m

Reflection (0.7 amplitude) @ 0.1 ms (~15 cm in water)

Environmental noise: white noise at 40 dB re 1 uPa2/Hz (typical noise in Capo Passero site)

Sweeps 20 -21 kHz (1s ), 22-23 kHz (2s), 24 -25 kHz (3s)Sin 20 kHz (1s), 22 kHz (2s), 24 kHz (3s)

Xcorr (sin 20 kHz) Xcorr (sweep 20 -21 kHz)

100 entries100 entries

S. Viola VLVνT 2015, Rome – 14/09/2015

Expected performace : signal recognition in ambient noise

Sweeps 20 -21 kHz (1s ), 22-23 kHz (2s), 24 -25 kHz (3s)Sin 20 kHz (1s), 22 kHz (2s), 24 kHz (3s)

Xcorr (sin 20 kHz) Xcorr (sweep 20 -21 kHz)

100 entries 100 entries

Signal amplitude: 140 dB re 1uPa @1m

Length signal: 5 ms

Emitter – receiver distance : 1000 m

Reflection (0.7 amplitude) @ 0.1 ms (~15 cm in water)

Environmental noise: white noise at 40 dB re 1 uPa2/Hz (typical noise in Capo Passero site)

S. Viola VLVνT 2015, Rome – 14/09/2015

Conclusions

11

•The main componenets of acoustic positioning system has been

chracterized, in particular their time calibration.

•The algorithms, used to recostruct the time of arrival of the becon pulses,

is expected to work with good performance.

•The acoustic positioning system contains innovative elements like the

autocalibrating LBL that will help to reduce the systematic errors.

S. Viola VLVνT 2015, Rome – 14/09/2015

Thank you