Underwater Acoustic Sensor Networks (UW-ASNs) have recently been proposed as a way to explore and observe the ocean, which covers two-thirds of the Earth's surface. In particular, we consider a SEA Swarm (Sensor Equipped Aquatic Swarm) architecture for short-term ad hoc real-time aquatic exploration, such as oil and chemical spill monitoring, submarine detection, and surveillance, by deploying drifting sensor nodes (e.g., UCSD Drogues) to the venue of interest that form a swarm and move as a group with the ocean oceanic current. Each sensor monitors local underwater activities and reports critical data or events in real-time using acoustic multi-hop routing to a distant data collection center, e.g., surface buoys or Autonomous Underwater Vehicles (AUVs).
As SEA Swarm architecture adopts acoustic links as a means of communication, it is accordingly confronted with long propagation delays, low bandwidth, and high transmission power consumption. To put SEA Swarm architecture into practical use and alleviate these limitations, we propose the Delay-aware Opportunistic Transmission Scheduling (DOTS) algorithm to increase channel utilization by harnessing both temporal and spatial reuse. Extensive simulation results show that DOTS outperforms existing solutions, S-FAMA, DACAP, and CS-ALOHA in a line topology, in a highly competitive medium access star topology, and in a random topology with an underwater mobility by harnessing temporal and spatial reuse. Furthermore, in a SEA Swarm architecture, a sensor cloud that drifts with water currents and enables 4D (space and time) monitoring of local underwater events such as contaminants, marine life and intruders, is escorted at the surface by drifting sonobuoys that collect the data from underwater sensors via acoustic modems and report it in real-time via radio to a monitoring center. Thus, to realize SEA Swarm architecture, designing an efficient anycast routing algorithm for reliable underwater sensor event reporting to any one of the surface sonobuoys is imperative. Major challenges are the ocean current and the limited resources (bandwidth and energy). We address these challenges and propose two hydraulic pressure based anycast routing protocols, namely HydroCast and VAPR, which exploit the measured pressure levels to route data to surface buoys. The proposed routing protocols are validated via extensive simulations.
