ACME second sea trials
Location
The second ACME sea trials took place in the Bay of Douarnenez, France, in September 2002. The Bay is a relatively quiet area with little shipping noise, especially when compared with the Westerschelde environment of the first ACME sea trials. For the 2nd sea trials, the main objective was to test the network protocols. The trials had a flying start as all equipment functioned properly from the beginning and the majority of network experiments worked as well. Two measurement ships were available. MS Lodycke (NL) and MS Thalia (FR), property of and operated by the Dutch Ministry of Transport, Public Works and Water Management (Rijkswaterstaat), and the Institut Français de Recherche pour l'Exploitation de la MER (IFREMER), respectively. Valuable and varied data were collected, from a total of 12 network topologies (cf. Fig. 2.) Four acoustic modems were employed in the network, one Master and three Slaves or Remote nodes.
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| Figure 1. Bay of Douarnenez. Experiments were carried out in the marked areas. |
Topologies
Twelve topologies were tried out in the bay. The sketches in Fig. 2 show just three of them, where the numbers are separations in meters.
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| Figure 2. A few of the tested topologies. M = Master (deployed from the Measurement Ship); Rn = Remote n , dropped on the seafloor. |
Broadcast
In a broadcast experiment the master node first configures the network. It determines propagation delays and defines a time schedule accordingly. Fig. 3 shows a broadcast experiment. The instructions from the Master (A) are received at high intensity because the reference hydrophone used for the recordings is near the Master. Subsequent arrivals are: B - Data from R1. C - Data from R2. D: Data from R3. In this experiment the data are modulated with Phase-Shift Keying in combination with spread-spectrum techniques. Each slave has a unique spreading code.
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| Figure 3. Spectrogram of a broadcast experiment. (Sound levels in dB.) |
Relayed data retrieval
In a relay experiment the Master retrieves data from a Target node via a Relay node. Fig. 4 illustrates the idea by means of a spectogram. A - The Master instructs the Relay node (R2) that it wants data from the Target Node (R3). B - R2 forwards instructions to R3. C - Data transmitted by R3. The data are received and retransmitted by R2 (D). In this experiment Frequency-Shift Keying (FSK) is used for the data transfer.
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| Figure 4. Spectrogram of a relay experiment. (Sound levels in dB.) |
Data set
A selection of the data acquired during the second ACME trials will be available at the data bank of IFREMER/SISMER. The selection comprises acoustic data (recorded experiments) and environmental data such as sound speed profiles and bathymetry. The data set is well documented and becomes available for interested EU parties in 2004.
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| Impression of the experiments with transfer of equipment between the two Measurements Ships. |