# Special Issue on Multi-Robot Systems: Challenges, Trends, and Applications
## 2 Challenges  
The contributions to the Special Issue reveal a good amount of challenges for the operation of MRSs. Some are related to multi-agent mission planning, intervention in complex scenarios with uncertainty, and operation under restricted communications. Robot cooperation often helps deal with these issues through sharing information and coordinating actions. [PERSON] [1] reviews the state-of-the-art on teamwork in the context of multi-robot systems and dynamic environments. His paper identifies and analyzes multiple engineering challenges: dynamic coalitions, platform harmonization and configuration, knowledge base, methodology and tools, edge and cloud integration, and human in the loop and other sociotechnical concerns, among other concepts. [PERSON], [PERSON], and [PERSON] [2] address the challenge of Multi-Agent Path Planning (MAPP), which is a resource allocation problem complicated by the highly dynamic and distributed environments. In contrast to most MAPP approaches, they assume soft collisions instead of hard collisions, which means that the agents can share resources or concur at the same location at the expense of reducing the quality of solutions. They propose the Soft-Collision M* (SC-M*) algorithm to solve these constrain satisfaction problems and compare its results with the most common algorithms in terms of path cost, success rate, and run time. Meanwhile, [PERSON], [PERSON], and [PERSON] [3] address the problem of multi-robot collaborative self-localization in the context of target tracking missions. They propose an unscented transformation-based collaborative self-localization algorithm to deal with inter-robot and robot-target correlations during the missions. [PERSON] and [PERSON] [4] address other relevant challenges for MRSs, which are communications and coordination. Their paper presents a time-synchronization solution for operations performed by a heterogeneous set of robotic manipulators grouped into a production cell. They develop a master-slave architecture without an external control element, whose communications are implemented via TCP/IP sockets. Similarly, [PERSON], [PERSON], [PERSON], and [PERSON] [5] consider the multi-robot exploration problem under restricted communications. They propose a novel auto-adaptive multi-objective strategy to support the selection of tasks regarding both exploration performance and connectivity level. Compared with other algorithms, it shows effectiveness and flexibility to tackle the multi-robot exploration problem, decreasing the effects of disconnection periods without noticeable degradation of the exploration time. Cooperation can be carried out in homogeneous fleets but also between robots with different morphology. [PERSON], [PERSON], [PERSON], [PERSON], and [PERSON] [6] analyze air-ground robot cooperation in their paper. The combination of aerial and ground robots is useful in search and rescue tasks, given that aerial robots can provide valuable insight to support the navigation of ground robots in complex scenarios affected by disasters. The article proposes a state machine with algorithms that allow an aerial robot to take off, track, and land on a mobile ground platform. [PERSON], [PERSON], [PERSON], and [PERSON] [7] focus on another relevant challenge in multi-robot missions: uncertainty. They find information sharing as a powerful tool to deal with uncertainty in mission planning. In this way, when a robot finds a new obstacle or blocked path, it can share this information with the rest of the fleet, allowing other robots to plan better paths. The paper proposes a novel method for information sharing that works when robots have different sensors, there is positional uncertainty, and obstacles are dynamic. [PERSON], [PERSON], and [PERSON] [8] also focus on uncertainty but in the context of mobile manipulation. In these applications, humans can collaborate with robots to execute complex actions, sharing their knowledge about the task and scenario.