14-04-2011, 03:59 PM
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1. INTRODUCTION
Although covering more than 70% of the Earth surface the oceans are not well known, due to its dimensions, difficulties of oceanographic data acquisition and the high costs of maritime operations. Nervertheless, there is an increasing interest on oceanographic data, due to its influence on the weather, fishing, navigation, biology, ecology and support for petroleum resources offshore exploration.
The traditional approach for ocean monitoring and data acquisition is based on several sensors gathered in an apparatus operated with batteries. This apparatus is deployed in the ocean bottom in the place of interest. It records data during some programmed time, which may last several months. At the end of the mission the apparatus is recovered to have its data uploaded, processed and analyzed.
This way of data acquisition is useful, has been widely applied and represents the source of most of oceanographic data nowadays available. But it has severe drawbacks: it is limited to one point of survey, prevents monitoring the health of the sensor itself and of the data quality during the mission. Moreover, this equipment has limited storage capacity, the acquisition parameters must be established at deployment time and are unchangeable until the apparatus recovery.
A way to overcome these difficulties is the use of Underwater Sensor Networks (UWSN).
CHAPTER 2
2. UNDERWATER SENSOR NETWORKS
UWSN are based on nodes equipped with sensors and acoustic modems. Figure 1 shows a possible architecture. The nodes can communicate with each other to send their own data, and forward data from other nodes, to a sink node. This sink node has two acoustic modems, one to communicate with the sensor network and the other to communicate with a gateway on the sea surface. This gateway has a radio page link to a land station, which may monitor and control the data acquisition of all nodes in real time. If the land station is connected to the internet, the UWSN data will be available worldwide.
Although underwater communication may be accomplished using electromagnetic or optical waves, these methods are unfeasible for practical UWSN. Due to signal attenuation, electromagnetic transmission through salty water only propagates at long distances with low frequencies. This implies on large antennas and high power consumption.
For optical transmission the limitation relies on the short range achievable, due to light absorption and scattering in water. Moreover, even at short distance, optical communication requires high precision and stable alignment source receiver, difficult to achieve in undewater environment. Thus, the practical way to implement UWSN is through acoustic communication
CHAPTER3
3. UNDERWATER SENSORS
Sensors are used to measure many different parameters such as quality of water and to study its characteristics such as temperature, density, salinity (interferometric and refractometric sensors), acidity, chemicals, conductivity, pH (magnetoelastic sensors), oxygen (Clark-type electrode), hydrogen, dissolved methane gas (METS), and turbidity in marine environments. Underwater sensor applications are still sparsely utilized and relatively expensive which imply the need for long range communication and increase sensors power usage. Currently,small,robust, Cheap and low energy consumption underwater sensor without reducing the sensors performance are needed to assist scientists in various underwater applications especially in rough water applications. Such applications are working on marine environmental in-situ monitoring issue, port security, surveillance, Tsunami early detection and autonomous underwater vehicle (AUV) navigation.
3.1. Current underwater sensors
3.1.1 ACOUSTIC SENSORS: -
Acoustic signals propagate better than light and Radio waves or any other types of energy in underwater applications. The importance of acoustic for underwater applications is undeniable and the enabler device is an acoustic sensor. Acoustics are utilized to fulfill the needs of sonar and underwater communications in scientific explorations, commercial exploitations, defense surveillances and environmental protection for many decades. Acoustic sensors are also utilized in underwater telephones and telemetry. Acoustic signal generations and detections encounter significant challenges due to dynamic properties and extreme nature of the ocean. Issues on multi-path propagation, time variation of acoustic channel, non-linear effects, thermocline, strong signal attenuation, high error rate, limited bandwidth are those already discovered. Recent acoustic related system researches works are more likely to focus on the design to improve performances, smaller size and efficient energy consumption. Owing to microelectronics technology and advanced computer aided design, acoustic sensors are successfully miniaturized for underwater applications. Hence the boundaries in underwater acoustic sensing are widening. One example is underwater sensor nodes for underwater sensor networks(UWSN). It composes of omni-directional and compact acoustic sensor as a receiver. Various research groups and industry players have successfully developed underwater modem for underwater communication and sensor networks.
3.1.2 OPTICAL SENSORS: -
Underwater sensor applications are governed by an acoustic sensor system for AUV object detection in the marine environment. However due to their high manufacturing cost, a mass sensor deployment is not possible. Recently, optical fiber-based sensors have interestingly become an important part of sensor technology. The fiber –optic sensor systems have many advantages such as smaller in size, low costs, distributed measurement over a long distance and eliminates cross talking problem associated when using multiple ultra-sonic sensors. It has been reported that, fiber optic sensor is a powerful sensors for many parameter measurements including temperature, pressure, strain, salinity, mass flow rate and rotation position. Their usage as a probe or a sensing element is growing in measuring the marine chemical and physical environment components. Optical sensors are proven to be an alternative for acoustic based sensors. However biofouling are the limiting factors for measurement accuracies and deployement longevity.
3.1.3 ELECTRO CHEMICAL SENSOR: -
Electrochemical based sensors are widely used in marine environmental monitoring. The success in miniaturization technology has led to the development of real time in situ chemical sensor with high sensitivity that robust for routine applications. Electrochemical sensor systems include potentiometry, Amperometry and voltammetry.
3.2 Future Potential Solutions
3.2.1 BIO-INSPIRED SENSORS:-
Animals display a great variety of sensory systems that easily outperforms existing sensors generated by humans. Getting inspiration from learning and mimicking nature always serve new design approaches that push current information technology to a new paradigm. New discoveries in neurosciences and advancement in recent technologies such as MEMS, neuro interfacing, biocompatible materials, low power computing and communication devices are among the driving forces behind today’s development of bio-inspired information technology. A more recent trend is applicability of bio-inspired approaches in underwater technologies. The jelly-box fish behavior imitation will help create cheap underwater sensor platform for mass sensor deployement, While plankton photoluminescence ability to produce light at a visible wavelength that scattered less as compared to radio wave inspired new way of communication that provide an alternative to acoustic system.
3.2.2 MEMS BASED SENSORS:-
The advancement in MEMS technology has enabled scientists to miniaturize sensors on a dimensional scale of microns. Recently the achievements in MEMS technology help to realize the adaptation of new sensor design approach inspired by animal behavior into a functional biologically inspired underwater sensor.
CHAPTER 4
4. UNDERWATER SENSOR NETWORKS:
Communication architecture
Here we describe the communication architecture of underwater sensor networks. In particular, we introduce reference architectures for two-dimensional and three dimensional underwater networks, and present several types of autonomous underwater vehicles (AUVs) which can enhance the capabilities of underwater sensor networks. We discuss the following architectures:
• Static two-dimensional UWSNs for ocean bottom monitoring. These are constituted by sensor nodes that are anchored to the bottom of the ocean. Typical applications may be environmental monitoring, or monitoring of underwater plates in tectonics
• Static three-dimensional UWSNs for ocean column monitoring. These include networks of sensors whose depth can be controlled, and may be used for surveillance applications or monitoring of ocean phenomena (ocean bio–geochemical processes, water streams, pollution).
• Three-dimensional networks of autonomous underwater vehicles (AUVs). These networks include fixed portions composed of anchored sensors and mobile portions constituted by autonomous vehicles.
