# A Heterogeneous Robotic Swarm for Long-Term Monitoring of Marine Environments
## 2 Distributed Marine Monitoring System
### aMussel
#### 2.1.1 Modules  
Since the main idea of the subCULTron project is to have a swarm of heterogeneous units interacting and collaborating, each aMussel is equipped with several communication interfaces. These interfaces can be divided into two groups: interfaces for surface communication and interfaces for underwater communication. Surface communication includes WiFi and GSM, with an additional Bluetooth Low Energy module for diagnostic and logistics-related purposes. Available modes of underwater communication include acoustic communication and short-range modulated light communication using green light modules.  
The aMussel has a WiFi dongle plugged into its Raspberry Pi board giving it access to the swarm wireless network. A GSM module exists in its top cap, allowing it to send and receive SMS messages for communication over greater distances.  
The acoustic modems installed in all members of the swarm are a low-cost miniature acoustic communication and ranging device for underwater vehicles, divers, and subsea instruments developed in Intelligent Sensing and Communications research group at the University of Newcastle, shown in Figure 3. Communication with the modems is achieved using chirp signal modulation in the 24-28 kHz frequency range, allowing for short data messages of up to 7 bytes to be exchanged between units at a transfer rate of 40 bits per second. Furthermore, an efficient \"ping\" protocol is implemented for two-way travel time range measurement between units. The farthest distance the communication was successfully established was around 500 m.  
Additionally, the acoustic modems have a submodule for acquiring information about the moment of arrival and successful reception of an acoustic packet. The former is useful for acquiring time difference of arrival (TDoA) measurements, and the latter for waking the aMussel up from a low-powered sleep state.  
Since the underwater sensor nodes are power limited, there is a significant motivation for implementing energy sharing technologies. The aMussel possesses inductive charging coils which enable battery recharging during mission execution. Together with the low-powered sleep modes mentioned earlier, this can significantly prolong the autonomy of the sensor network.  
Figure 3: Acoustic modem Nanomodem.  
Wireless energy transfer is realised using a system based on inductive charging [10; 11], consisting of a power supply on the transmitter side, a set for wireless energy transfer, a battery charger, and a battery. The used wireless charging set is shown in Figure 4. It consists of a transmitter coil, connected to the appropriate printed circuit board (PCB), which is connected to the power supply and receiver coil connected to the PCB, which is connected to the aMussel battery charger.  
The aMussel itself houses two batteries and two independent battery chargers. The primary battery is charged from two wireless receiver coils, while the secondary battery is charged from one wireless charging coil. Charging stops when the wireless receiver stops transferring energy to the chargers or when batteries are full. Monitoring of the charging process is accomplished by tracking the voltages and the currents of the batteries, as well as the digital status pins of each battery.