# Optimization of Acoustic Communication Links for a Swarm of AUVs: The COMET and NEMOSENS Examples
## 3 Importance and Benefits of Underwater Communications  
Navigating in the underwater environment is a complex task [17]. The navigation system is also responsible for detecting underwater objects such as hazardous wreck, rocks, seafloor modifications, and other vehicles, in order to avoid collision. Both COMET and NemoSens AUVs utilizes the Inertial Navigation System (INS) aided by Doppler Velocity Log (DVL), and when available, the Global Positioning System (GPS) or Long Baseline (LBL) [15; 16]. In one configuration of NemoSens, the DVL can be suppressed, reducing drastically the AUV cost. In this case, the acoustic communications among AUVs plays a major role in maintaining an accurate navigation.  
When the AUVs operate together in a swarm mode, the navigation algorithm can be improved with the aid of the signal received from the other vehicles. This means that the navigation can be adapted according to information received. In addition to that, there is the need of an algorithm for coordinating the swarm navigation. As there is no central node in this network, each AUV can use the knowledge of the network topology, which is obtained in the process of information exchange.  
Another important improvement that can be achieved thanks to underwater communication is with respect to localization. When the AUV is at the surface, the localization system utilizes the GPS. Once the AUV dives, the vehicle only gets the measurements acquired by its subsea sensors. These measurements have inherent uncertainties, which are represented by the boxes in Figure 3, part 1. When the AUVs operate in a swarm mode, these vehicles might take profit of the information exchange among them for improving localization. While in hearing mode, the AUV can measure the time of arrival of each signal (see Figure 3, part 2). With this information, the AUV can compute the location range that the transmitting vehicle is situated (see Figure 3, part 3). Using the knowledge of the transmitter position box (that was transmitted in the message), the receiver node can improve the localization accuracy of the transmitter AUV, as shown in Figure 3, part 4. These continuous information exchanges among users enables the contraction of position boxes, increasing the localization accuracy [18].  
A benefit of having a more precise position is a better coverage [18]. Figure 4, depicts the area coverage for the case when the AUVs exchange information and for the case the vehicles do not communicate. In this figure, the red color corresponds to the unexplored areas, and the green intensity designates the amount of scans that were performed.  
Figure 2: AUV NemoSens.  
From this figure, one can observe the improvement on the area coverage when the AUVs communicates with each other.  
Figure 4: Area covered by a swarm of AUVs: left picture (number 1) is the coverage with communication, and the right picture (number 2) without communication. Red color represents no coverage, and the green intensity indicates the amount of scans on the area.  
Figure 3: Localization accuracy procedure.  
Another benefit of having more precise positions, is that the collected data has a more accurate coordinates. As a consequence, the sweep of the surveyed area is less prone to samples gap.  
Notice that all these advantages require a constant information exchange among the network nodes. The periodicity of the data exchange among AUVs depends on the mission, AUVs speed, amount of information, and distance among them. As an example, after theoretical studies which considered the projector bandwidth as 4 kHz, an useful modem bit rate of 2 kbps, and a maximum distance among the AUVs of 1 km, the frame duration and the exchange periodicity were set to 2 s and 30 s, respectively.