Description and composition of bio-inspired design patterns: a complete overview <s> Digital pheromone pattern <s> Modern UAV’s reduce the threat to human operators, but do not decrease the manpower requirements. Each aircraft requires a flight crew of one to three, so deploying large numbers of UAV’s requires committing and coordinating many human warfighters. Insects perform impressive feats of coordination without direct inter-agent coordination, by sensing and depositing pheromones (chemical scent markers) in the environment [14]. We have developed a novel technology for coordinating the movements of multiple UAV’s based on a computational analog of pheromone dynamics. The control logic is simple enough that it can be executed autonomously by a UAV, enabling a single human to monitor an entire swarm of UAV’s. This paper describes the technology, its application to UAV coordination, and the results we have obtained. <s> BIB001 </s> Description and composition of bio-inspired design patterns: a complete overview <s> Digital pheromone pattern <s> The use of digital pheromones for controlling and coordinating swarms of unmanned vehicles is studied under various conditions to determine their effectiveness in multiple military scenarios. The study demonstrates the effectiveness of these pheromone algorithms for surveillance, target acquisition, and tracking. The algorithms were demonstrated on hardware platforms and the results from the demonstration are reported <s> BIB002 </s> Description and composition of bio-inspired design patterns: a complete overview <s> Digital pheromone pattern <s> Despite the growing interest in pheromone-based interaction to enforce adaptive and context-aware coordination, the number of deployed systems exploiting digital pheromones to coordinate the activities of situated autonomous agents is still very limited. In this article, we present a simple low-cost and general-purpose implementation of a pheromone-based interaction mechanism for pervasive environments. This is realized by making use of RFID tags to store digital pheromones and by having humans or robots spread/sense pheromones by properly writing/reading RFID tags populating the surrounding physical environment. We exemplify and evaluate the effectiveness of our approach via an application for object-tracking. This application allows robots and humans to find forgotten-somewhere objects by following pheromones trails associated with them. In addition, we sketch further potential applications of our approach in pervasive computing scenarios, discuss related work in the area, and identify future research directions. <s> BIB003
The Digital Pheromone Pattern is a swarm coordination mechanism based on indirect communication. In this pattern, agents deposit digital pheromones in hosts. A digital pheromone is a mark that spreads a gradient over the environment and persists in the environment for a while, fading away with time. Other agents beyond the communication range can then receive the information conveyed by digital pheromones. Digital pheromones are stored in the hosts and stay active even when agents that deposited digital pheromones disappear. Digital pheromones can be identical to each others, like in Ant Colony Optimisation Algorithm  or can be specialised to a specific task, like in swarming vehicle control BIB002 . Digital pheromones are a particular case of stigmergy communication. Stigmergy is more general and stands for any indirect communication through the environment, not necessarily a sign that behaves like a Digital Pheromone. Alias: none to our knowledge. Problem: coordination of agents in large scale environments using indirect communication. Solution: digital pheromone provides a way to coordinate agent's behaviour using indirect communication in high dynamic environments. Digital pheromones create gradients that spread over the environment, carrying information about their distance and direction. Thus, agents can perceive pheromones from the distance and increase the knowledge about the system. Moreover, as time goes by digital pheromones evaporate, providing adaptation to environmental changes. Inspiration: the Digital Pheromone Pattern takes inspiration from ant colonies. Ant colonies are able to find the shortest paths from the nest to food sources using local interactions and indirect communication based on pheromones. Pheromones are deposited in the environment by ants to mark the path they are following from the nest to the food source and back. Pheromones quickly evaporate so they must be continuously released to maintain the information of the path. Colonies are able to adapt to environment changes (such as, new obstacles, new food sources, food sources that become empty, etc. . .). Forces: the implementation of the Digital Pheromone Pattern involves the implementation of the Gradient and Evaporation Patterns in order to create an active gradient (Nagpa 2004). The main difference between active gradients and digital pheromones is that pheromone involves indirect communication, while a gradient spreads from agents to agents. Thus, the main forces to consider are the following: (i) as for the Evaporation Pattern, how much and how frequent evaporation is used at each iteration; (ii) as for the Gradient Pattern, the Digital Pheromone Pattern is composed of the Aggregation and Spreading Patterns, thus, the more frequent the spreading of pheromone, the higher the bandwidth used. In addition, spreading pheromones to far away distances, allows more agents to receive the information, but consumes more memory and bandwidth. Entities-Dynamic-Environment: agents are the only entities that can deposit pheromones. Pheromones are deposited in hosts, infrastructural agents then apply spreading, aggregation, and evaporation mechanisms (see Appendix Table 2 ). Thus, pheromones are spread though the network, aggregated in each host when two or more pheromones' information arrive, and evaporated along the time until they disappear. During a pheromone life time, the pheromone can be perceived even beyond the host's communication range, where the pheromone is actually hosted, due to the effect of the Spreading Pattern. The transition rule for the Digital Pheromone Pattern is obtained composing the three basic patterns: Spreading, Aggregation and Evaporation, as shown in Transition Rules (8).  Similar to the Gradient Pattern, the first transition rule models the spreading of information modifying the PhV concentration attribute by decreasing its value by a DPhV interval, representing for instance the distance between two locations. The selection of the target location is the same as for the Gradient Pattern. The second transition rule models the corresponding case of aggregation where only the pheromone with the biggest value is kept. The third transition rule models the evaporation of pheromones, with the Ev factor in the range [0..1]. Implementation: digital pheromones are usually implemented using multiplicative static evaporation (i.e. the same evaporation factor is used periodically over the pheromone's information). Independently of the patterns used to implement the Digital Pheromone Pattern, pheromones can be deposited in hosts, (i.e. following the proposed model), simulated by software BIB002 , or implemented using RFID sensors BIB003 . In the Digital Pheromone Pattern, the agents just deposit pheromones and sense from them. Infrastructural Agents are in charge of spreading, aggregating and evaporating the pheromones. The way the agents exploit the digital pheromones involves new patterns that are explained in the next sections. Known uses: digital pheromones have been used mainly in autonomous coordination of swarming UAVs BIB001 BIB002 . Moreover, applications of digital pheromones can be found in the Ant Foraging Pattern description (Sect. 5.3.1). Consequences: as reported in BIB002 , the implementation of Digital Pheromones for swarm coordination provides the following issues to the system: (1) simplicity, compared with the logic necessary in a centralised approach, (2) scalability, the digital pheromones work in a totally decentralised manner, i.e. they are applicable in large scale MAS, and (3) robustness, due to decentralisation and the continuous self-organising process the digital pheromones provide, some agents may fail but the system is robust enough to overcome these failures. Related Patterns: the Digital Pheromone Pattern is composed of the Evaporation and the Gradient Patterns, the latter itself composed of the Aggregation and the Spreading Patterns, so that we can say that the Digital Pheromone Pattern involves the basic patterns Spreading and Evaporation. All these patterns are described in Appendix Table 2 . The Digital Pheromone Pattern is exploited by the Ant Foraging Pattern (Sect. 5.3.1) from the high level patterns.