ACME first sea trials
Location
The first ACME sea trials took place on the Westerschelde in the South-west of The Netherlands, in June 2001. The Westerschelde is a densely shipped sea arm that serves as the gateway for cargo ships sailing to the port of Antwerp, Belgium. As a first step towards a wireless communication network that conveys ADCP (current) data from the shipping lane ashore, point to point communication experiments were carried out. Several tracks were defined to examine the quality of the communication link under varying circumstances. The location of the first sea trials was near the village of Hansweert, the intended site for the prototype network that will be deployed for the final ACME demonstration trials in 2003.
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| Figure 1. Map of the Westerschelde trials area. |
Measurement ship
The trials vessel was MS Lodycke, a well-equipped research platform operated by Rijkswaterstaat. Its accurate positioning system and skilled crew enabled precise modem deployments and bathymetry measurements over the communication tracks.
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| Figure 2. The remote modem is put overboard in a frame and with a battery pack. |
Acoustic modems
Two modems were specifically adapted for the experiments. The remote modem was capable of transmitting particular signals upon request by the local modem. Among these signals were spread-spectrum, BPSK and FSK communication waveforms, signals using a reference sequence, as well as LFM and CW signals to probe the acoustic channel. A battery pack allowed for autonomous operation of the remote (seabed) modem for extended periods. Two reference hydrophones placed near the local modem were employed to capture communication signals for laboratory analysis.
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| Figure 3. Recovery of the local modem. |
Environment
Despite the favourable weather the conditions for acoustic underwater communication altered significantly during the first ACME sea trials. The incoming tide from the sea brought salty water, while the outgoing tide came with fresh water from the river Schelde. As a consequence, the sound speed profile varied with the ever changing temperature and salinity profiles. Moreover, the frequent presence of dredgers induced lots of ambient noise. Still higher noise levels were measured with passing cargo ships. Only ten meters separated the propellor of some container ships from the seabed modem, which temporarily broke the communication link between the two modems.
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| Figure 4. A passing container ship introduces noise. |
Outcome
The results of the point-to-point communication experiments are encouraging. An example is given for a set of direct-sequence spread-spectrum signals, with data rates from 65 bit/s to 667 bit/s in the passband from 10-14 kHz. The bit error rates (BER) are tabulated for two tracks and several communication ranges [1].
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| Figure 5. Illustrative bit error rates and constellation plots. |
One of the findings is that the quality of the acoustic channel depends strongly on the position of the receiver. The plot at the right shows the bit constellations for the two receiver channels (Track 2, 1000 m, 286 bit/s.) Although the hydrophones are vertically separated by only 2 meters, channel 2 is a much more favourable channel in this particular case.
Data set
A selection of the data acquired during the first ACME trials is available at the data bank of IFREMER/SISMER. The selection comprises acoustic data gathered on two tracks and environmental data as diverse as sound speed profiles, atmospheric data, and bottom grabs. The data set is well documented and becomes available for interested EU parties in 2004.
[1] P.A. van Walree, M.B. van Gijzen, and M. Pattipeilohy, Acoustic communication experiments in the Westerschelde shipping lane using direct sequence spread spectrum signals, Proceedings of the 6th European Conference on Underwater Acoustics ECUA 2002, Gdansk, Poland, 2002.