# A Heterogeneous Robotic Swarm for Long-Term Monitoring of Marine Environments
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1 LABUSTecosystem. It is heterogeneous and consists of three agent types: five Autonomous Surface Vehicles (ASVs) called artificial lily pads (aPads), a small swarm of highly mobile artificial fish (aFish), and more than 100 underwater sensor nodes called artificial mussels (aMussels). Within the scope of this paper, we will focus on the use cases, interactions, and interfaces of the aPads and aMussels. The aPad surface vehicles are equipped with a propulsion system capable of omni-directional movement, GPS providing absolute localisation in the Earth's coordinate system, WiFi modems and powerful antennae enabling communication between agents while above water, and a low-cost acoustic modem enabling acoustic underwater communication and localisation. In addition to the exchange of information, the aPads also have the ability to exchange energy with other types of agents via the use of specially designed mechanical docking stations. The aPad grabs aMussel in the docking station in order to recharge their batteries or be transported to another location. The aMussel underwater sensor nodes come equipped with a variety of sensors for collecting relevant data, an acoustic modem for underwater communication and localisation, and an active buoyancy system that enables vertical motion and thus provides them with their sole independent movement method. Since they have no other actuators, for most complex movement they must rely on assistance from the aPads. Initial deployment of the aMussel underwater sensor nodes within a chosen area of interest is also executed with the help of the aPad surface vehicles. Once released in the proper location, the aMussels will sink to the seabed, where they remain stationary while collecting data and occasionally communicate their findings to the surface, as well as amongst themselves. A variety of scenarios and behaviours are being developed and tested for the swarm agents, including trust and consensus-based decision making, deciding when to surface and request relocation, and how to determine and further explore points of particular interest. In [8] the authors demonstrated a similar underwater swarm system consisting of heterogeneous robots used for ocean exploration. Underwater sensor nodes use buoyancy control for depth control and underwater localisation is aided by acoustic-capable surface buoys. While the swarm described in [8] passively explores the environment relying exclusively on ocean currents, the approach described in this paper places greater focus on the ability to dynamically relocate the swarm for planned exploration. Furthermore, the subCULTron system is capable of collective decision making enabled  
Figure 1: subCULTron Underwater Acoustic Sensor Networks (UASN) concept. by underwater communication. Due to energy sharing between surface and underwater robots, the longevity of deployment can be prolonged. In project Argo [9] a system for ocean profiling was developed. It uses underwater robots capable of depth control using a variable buoyancy system. The robots have the capability of acquiring underwater measurements while drifting in the ocean. Data transfer to a remote data hub is done through satellite communication allowing wide deployment coverage. Each robot is a standalone unit with no interaction between the agents or swarm behaviours. The paper is organised as follows. In Section 2, vehicles in the monitoring system are described along with detailed explanations of developed and integrated hardware. Section 3 explains which types of communication are used in the swarm and their implementation. Section 4 describes the capabilities of the agents required for more complex behaviours. Section 5 presents developed algorithms for swarm behaviours. In Section 6, some concluding remarks and notes about planned future work are given.