Ad Hoc Networks 3 (2005) 257–279 www.elsevier.com/locate/adhoc Underwater acoustic sensor networks: research challenges Ian F. Akyildiz *, Dario Pompili, Tommaso Melodia Broadband and Wireless Networking Laboratory, School of Electrical and Computer Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA Received 15 July 2004; received in revised form 20 September 2004; accepted 21 January 2005 Available online 2 February 2005 Abstract Underwater sensor nodes will find applications in oceanographic data collection, pollution monitoring, offshore exploration, disaster prevention, assisted navigation and tactical surveillance applications. Moreover, unmanned or autonomous underwater vehicles (UUVs, AUVs), equipped with sensors, will enable the exploration of natural under- sea resources and gathering of scientific data in collaborative monitoring missions. Underwater acoustic networking is the enabling technology for these applications. Underwater networks consist of a variable number of sensors and vehi- cles that are deployed to perform collaborative monitoring tasks over a given area. In this paper, several fundamental key aspects of underwater acoustic communications are investigated. Different archi- tectures for two-dimensional and three-dimensional underwater sensor networks are discussed, and the characteristics of the underwater channel are detailed. The main challenges for the development of efficient networking solutions posed by the underwater environment are detailed and a cross-layer approach to the integration of all communication functionalities is suggested. Furthermore, open research issues are discussed and possible solution approaches are outlined. Ó 2005 Published by Elsevier B.V. Keywords: Underwater acoustic sensor networks; Underwater networking; Acoustic communications 1. Introduction lection, pollution monitoring, offshore explora- tion, disaster prevention, assisted navigation Underwater sensor networks are envisioned to and tactical surveillance applications. Multiple enable applications for oceanographic data col- unmanned or autonomous underwater vehicles (UUVs, AUVs), equipped with underwater sen- sors, will also find application in exploration * Corresponding author. Tel.: +1 404 894 5141; fax: +1 404 of natural undersea resources and gathering of 894 7883. E-mail addresses: [email protected] (I.F. Akyildiz), dar- scientific data in collaborative monitoring mis- [email protected] (D. Pompili), [email protected] (T. sions. To make these applications viable, there Melodia). is a need to enable underwater communications 1570-8705/$ - see front matter Ó 2005 Published by Elsevier B.V. doi:10.1016/j.adhoc.2005.01.004 258 I.F. Akyildiz et al. / Ad Hoc Networks 3 (2005) 257–279 among underwater devices. Underwater sensor ture gradients (thermoclines), which are consid- nodes and vehicles must possess self-configura- ered to be a breeding ground for certain marine tion capabilities, i.e., they must be able to micro-organisms. coordinate their operation by exchanging config- • Undersea explorations. Underwater sensor net- uration, location and movement information, works can help detecting underwater oilfields and to relay monitored data to an onshore or reservoirs, determine routes for laying under- station. sea cables, and assist in exploration for valuable Wireless underwater acoustic networking is the minerals. enabling technology for these applications. Under- • Disaster prevention. Sensor networks that mea- Water Acoustic Sensor Networks (UW-ASNs) sure seismic activity from remote locations can consist of a variable number of sensors and provide tsunami warnings to coastal areas [42], vehicles that are deployed to perform collaborative or study the effects of submarine earthquakes monitoring tasks over a given area. To achieve (seaquakes). this objective, sensors and vehicles self-organize • Assisted navigation. Sensors can be used to iden- in an autonomous network which can tify hazards on the seabed, locate dangerous adapt to the characteristics of the ocean environ- rocks or shoals in shallow waters, mooring posi- ment [1]. tions, submerged wrecks, and to perform The above described features enable a broad bathymetry profiling. range of applications for underwater acoustic sen- • Distributed tactical surveillance. AUVs and sor networks: fixed underwater sensors can collaboratively monitor areas for surveillance, reconnaissance, • Ocean sampling networks. Networks of sensors targeting and intrusion detection systems. For and AUVs, such as the Odyssey-class AUVs example, in [15], a 3D underwater sensor net- [2], can perform synoptic, cooperative adaptive work is designed for a tactical surveillance sampling of the 3D coastal ocean environment system that is able to detect and classify subma- [3]. Experiments such as the Monterey Bay rines, small delivery vehicles (SDVs) and divers field experiment [4] demonstrated the advanta- based on the sensed data from mechanical, ges of bringing together sophisticated new radiation, magnetic and acoustic microsensors. robotic vehicles with advanced ocean models With respect to traditional radar/sonar sys- to improve the ability to observe and pre- tems, underwater sensor networks can reach a dict the characteristics of the oceanic envi- higher accuracy, and enable detection and ronment. classification of low signature targets by also • Environmental monitoring. UW-ASNs can per- combining measures from different types of form pollution monitoring (chemical, biologi- sensors. cal and nuclear). For example, it may be • Mine reconnaissance. The simultaneous opera- possible to detail the chemical slurry of antibi- tion of multiple AUVs with acoustic and opti- otics, estrogen-type hormones and insecticides cal sensors can be used to perform rapid to monitor streams, rivers, lakes and ocean environmental assessment and detect mine-like bays (water quality in situ analysis) [51]. Moni- objects. toring of ocean currents and winds, improved weather forecast, detecting climate change, Underwater networking is a rather unexplored under-standing and predicting the effect of area although underwater communications have human activities on marine ecosystems, biolog- been experimented since World War II, when, in ical monitoring such as tracking of fishes or 1945, an underwater telephone was developed in micro-organisms, are other possible applica- the United States to communicate with submarines tions. For example, in [52], the design and con- [39]. Acoustic communications are the typical struction of a simple underwater sensor physical layer technology in underwater networks. network is described to detect extreme tempera- In fact, radio waves propagate at long distances I.F. Akyildiz et al. / Ad Hoc Networks 3 (2005) 257–279 259 through conductive sea water only at extra low fre- selected ocean areas, remote configuration and quencies (30–300 Hz), which require large anten- interaction with onshore human operators. This nae and high transmission power. For example, can be obtained by connecting underwater instru- the Berkeley Mica 2 Motes, the most popular ments by means of wireless links based on acoustic experimental platform in the sensor networking communication. community, have been reported to have a trans- Many researchers are currently engaged in mission range of 120 cm in underwater at developing networking solutions for terrestrial 433 MHz by experiments performed at the Ro- wireless ad hoc and sensor networks. Although botic Embedded Systems Laboratory (RESL) at there exist many recently developed network pro- the University of Southern California. Optical tocols for wireless sensor networks, the unique waves do not suffer from such high attenuation characteristics of the underwater acoustic commu- but are affected by scattering. Moreover, transmis- nication channel, such as limited bandwidth sion of optical signals requires high precision in capacity and variable delays [38], require very effi- pointing the narrow laser beams. Thus, links in cient and reliable new data communication underwater networks are based on acoustic wire- protocols. less communications [45]. Major challenges in the design of underwater The traditional approach for ocean-bottom or acoustic networks are: ocean-column monitoring is to deploy underwater sensors that record data during the monitoring • The available bandwidth is severely limited; mission, and then recover the instruments [37]. • The underwater channel is severely im- This approach has the following disadvantages: paired, especially due to multi-path and fad- ing; • No real-time monitoring. The recorded data can- • Propagation delay in underwater is five orders not be accessed until the instruments are recov- of magnitude higher than in radio frequency ered, which may happen several months after (RF) terrestrial channels, and extremely the beginning of the monitoring mission. This variable; is critical especially in surveillance or in envi- • High bit error rates and temporary losses of ronmental monitoring applications such as seis- connectivity (shadow zones) can be experienced, mic monitoring. due to the extreme characteristics of the under- • No on-line system reconfiguration. Interaction water channel; between onshore control systems and the mon- • Battery power is limited and usually batteries itoring instruments is not possible. This cannot be recharged, also because solar energy impedes any adaptive tuning of the instruments, cannot be exploited; nor is it possible to reconfigure the system after • Underwater sensors