MILOM
Acoustic positionning system resolution

 

 

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Last update : 01/09/2009

The second essential element to realise the necessary angular resolution of the neutrino telescope, is the knowledge of the position in 3D space of the optical modules with a precision of ~10 cm in real time. These position measurements are provided by the acoustic positioning system.

As for the timing precision, the MILOM will provide the final proof that ANTARES can achieve the necessary spatial position resolution and so complete the confidence that the good angular resolution will be obtained in the full ANTARES detector.

The full acoustic positioning system will consist of a three dimensional array of transponders and receivers, on or near the sea bed, exchanging precisely timed acoustic signals between each other. At the present time only a limited number of devices are installed on the site. On the MILOM line one transponder (RxTx module) and one receiver (Rx module) are installed.

On 10 May an autonomous transponder line was deployed ~84m from the MILOM. Later on 17 June two further transponders (“pyramids”) were deployed at other locations on the sea bed but this very recent data is not presented here.. Only when these  final pyramids are used can a 3D spatial reconstruction be made and so only later will the full system be proven.

Figure 19 shows a schematic of the acoustic positioning devices in the sea used for the data presented here from which only one dimensional measurements between fixed points are possible.

Figure 19. Acoustic positioning devices in the sea before June 17 and used for the data in these pages.

Figure 20 shows the measurements of distance between the two fixed points on the MILOM. From this figure, it can be seen that the spread on the measurement of the distance is < 3 cm for a distance ~100 m. For the moment, corrections for variations in sound velocity and orientation of the storey are not yet made in this data and figure 21 is used to indicate what could be the ultimate precision on measuring this distance. For this plot an average is obtained for each period of data in figure 20 and then the difference to this average is calculated and histogrammed. In figure 21 the resolution is seen to be 0.2 cm for the measurement of a distance ~100m.

 The data of figures 20 and 21 are wholly within the MILOM line, but to obtain three dimensional position reconstruction data with transponders outside the line must be used.

Figure 20. Acoustic distance measurement between two fixed points on MILOM line: RxTx module on the BSS and the Rx module in the first storey. The variations from day to day are believed to be due to a lack of certain calibrations and storey rotation corrections in the present analysis.

Figure 21. Adjusted projection of the data in figure 20 for the acoustic distance measurement between the two fixed points in the MILOM. Here an average position is calculated for each day and the difference is taken to this average.

Figure 22 shows such data taken between the base of the MILOM and the autonomous transponder line close to the MILOM. These raw data taken over a period of ~5 hours have a resolution of ~3cm on a distance measurement of ~84m.

These first available data from the MILOM indicate that all features of the acoustic positioning system are functional on the sea bed and that their precision is well within the required specifications. A conclusion on the ultimate positioning precision must await a complete analysis, including all corrections, with the two recently deployed pyramids; however it seems already that the specification of <10 cm spatial precision will be possible.

The top storey of the MILOM contains a monitoring hydrophone, christened Spy Hydrophone, which enables a check on the acoustic positioning system. For the Spy Hydrophone the acoustic signal is readout to shore with a sampling frequency of 400 KHz whereas the general hydrophones for the acoustic positioning system are analysed locally in the sea with only summary timing being readout to shore. The data in figure 23 show a signal of the acoustic positioning system monitored with the spy hydrophone. As well as monitoring the acoustic positioning system the spy hydrophone will be used for studies of acoustic neutrino detection and also studies of marine animals.

Figure 22. Acoustic distance measurement between the base of the MILOM and the autonomous transponder line.

 

Figure 23. A typical “ping” signal of the acoustic positioning system monitored with the “Spy Hydrophone”. Above the raw amplitude signal and below the signal after frequency analysis.

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Author : Thierry Stolarczyk