Abstract-Underwater acoustic networks (UANs) are an emerging technology for a number of oceanic applications, ranging from oceanographic data collection to surveillance applications. However, their reliable usage in the field is still an open research problem, due to the challenges posed by the oceanic environment. The UAN project, a European-Union-funded initiative, moved along these lines, and it was one of the first cases of successful deployment of a mobile underwater sensor network integrated within a wide-area network, which included above water and underwater sensors. This contribution, together with a description of the underwater network, aims at evaluating the communication performance, and correlating the variation of the acoustic channel to the behavior of the entire network stack. Results are given based on the data collected during the UAN11 (May 2011, Trondheim Fjord area, Norway) sea trial. During the experimental activities, the network was in operation for five continuous days and was composed of up to four Fixed NOdes (FNOs), two autonomous underwater vehicles (AUVs), and one mobile node mounted on the supporting research vessel. Results from the experimentation at sea are reported in terms of channel impulse response (CIR) and signal-to-interference-plus-noise ratio (SINR) as measured by the acoustic modems during the sea tests. The performance of the upper network levels is measured in terms of round trip time (RTT) and probability of packet loss (PL). The analysis shows how the communication performance was dominated by variations in signal-to-noise ratio, and how this impacted the behavior of the whole network. Qualitative explanation of communication performance variations can be accounted, at least in the UAN11 experiment, by standard computation of the CIR and transmission loss estimate.
Fig. 1. An underwater positioning system (left), a desktop experiment (middle), and a GPS error budget from Wikipedia (right). Abstract-It is well known that the ordinary Global PositioningSystem (GPS) fails to provide location and time information under water. The reason is that the electromagnetic signals from the orbiting satellites are heavily damped in water and hence can not be detected by the receiver in most cases of interest. Acoustic waves are the canonical alternative, and there exist a variety of acoustically based systems.It is important to estimate the accuracy of position estimates and if possible correct for the errors. This is done systematically in the GPS case by the concept of Dilution Of Precision (DOP), and is also the natural approach underwater. The main issue presented below is a discussion and analysis of the concept of DOP in the context of Underwater Positioning Systems (UPS). This includes statistical models that differ from the one canonically used. Some of the sources of errors in the UPS case differs substantially from the GPS case. It is in particular demonstrated that the maximum likelihood estimator is biased. Alternative estimators, including an unbiased estimator and an optimal estimator, are discussed.The results are also briefly discussed in the context of an actual model experiment with ultrasound in air. A side effect of this is the demonstration of certain issues which have been ignored in the previous general discussion. The experiment also indicates that the results are relevant in other contexts. Other important classes of examples are given by Real Time Locating Systems as defined by the standard ISO/IEC 24730, and by Wireless Personal Area Networks as described in the IEEE 802.15 standard. These other contexts also provide most useful sources for research publications with results of relevance for UPS, and more generally for underwater communication systems.
An underwater acoustic network (UAN) represents a communication infrastructure that can offer the necessary flexibility for continuous monitoring and surveillance of critical infrastructures located by the sea. Given the current limitation of acoustic-based communication methods, a robust implementation of UANs is still an open research field. The FP7 UAN project moved along these lines, and it was one of the first cases of successful deployment of a mobile underwater sensor network integrated within a wide-area network, which included above water and underwater sensors.This contribution gives details on the UAN network structure and equipment. It reports statistics on the performance of the system as collected during the project final sea trial, which was held in Trondheim, Norway, in May 2011. The UAN network was in operation for five continuous days and was composed of up to four fixed nodes, two autonomous underwater vehicles and one mobile node mounted on the supporting research vessel. Results from the experimentation at sea are reported in terms of channel impulse response and signal to noise plus interference ratio as measured by the acoustic modems during the sea tests. The performance of the upper network levels are measured in terms of round trip time and probability of packet loss. Finally, the experimental results have been compared with those obtained in simulation using the BELLHOP acoustic code, fed with the environmental data gathered during the sea trial.
No abstract
Acoustic networks are for underwater what wifi is for terrestrial networks. The ocean is a nearly perfect media for acoustic waves in which regards long range propagation but poses a number of challenges in terms of available bandwidth, Doppler spread and channel fading. These limitations originate in the physical properties of the ocean, namely its anisotropy and boundary interaction which are particularly relevant in coastal waters where acoustic propagation becomes predominantly dependent on seafloor and sea surface properties. The acoustic communication channel is therefore multipath dominated and time and Doppler spread variable. The problem is aggravated when involving moving receivers as for instance when attempting to establish communication with or between moving autonomous underwater vehicles. The EU-funded project UAN -Underwater Acoustic Network aims at conceiving, developing and testing at sea an innovative and operational concept for integrating in a unique communication system submerged, surface and aerial sensors with the objective of protecting off-shore and coastline critical infrastructures. UAN went through various phases, including the development of hardware and software specific components, its testing independently and then in an integrated fashion, both in the lab and at sea. This paper reports on the project concept and vision as well as on the progress of its various development phases and the results obtained herein. At the time of writing, a final project sea trial is being planned and will take place two weeks before the conference so, although here we will concentrate on the progress obtained so far, the presentation at the conference may include additional results depending on the outcome of the sea trial.
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