## 1 Introduction  
Sensor networks are relatively young and a challenging area of research. Their primary aim is to collect and communicate environmental data for cooperatively monitoring and controlling their physical surroundings. Figure 1 shows how research on mobile sensor networking has proliferated in the last decade especially due to advances in computation, communication, sensing and actuation technologies. Mobile sensor networks, which are composed of navigating units such as heterogeneous robots and unmanned air or ground vehicles, transmit data through wireless communication networking. These units can typically communicate with each other via implicit or explicit communication techniques. Many applications have emerged in a variety of fields such as wide area surveillance, reconnaissance, search and rescue in disaster environments, and planetary exploration, which are overviewed in Section 2. The focus of this survey is mainly on networked mobile sensors, which have various advantages in comparison with static ones. The most prominent advantages reside in the cooperation between different robot networks as well as the cooperation of robots within a single network. Sensing, acting, communication, computation and locomotion capabilities of networked mobile sensor systems result in improved efficiency in the execution of hard tasks especially with the help of their mobility capability [1]. As a counter example, monitoring and surveying an uncertain environment using static sensor networks is done through the acquisition and fusion of data by static nodes to the extent of their spread over the area [2, 3]. The major problem faced for reconnaissance tasks is inefficiency in area coverage [4, 5, 6], since deployment control during the spreading of static nodes is weak or non-existent. Static sensor networks are spread over an area using airplane, land vehicle, human or animal, and so sensor coverage is heterogeneous. This problem has been overcome by using mobile sensor networks, which are composed of simple mobile robot units deployed over the area for controlled coverage that can be further optimized during network deployment and be still modified under task changes. Mobile sensor networking requires a large number of sensor nodes to be deployed within an environment for efficient surveillance, monitoring, data harvesting and search tasks [7, 8, 9, 10, 11]; efficient coordination is critical for accomplishing a given task with a large number of networked robot units. We undertake a survey of the evolution of the abilities of mobile sensor networks that have been achieved over the years as reflected in the literature. This will be the focus in Section 3, namely coordination control, fault tolerance, seamless communication, holonic reconfigurability and scalability. Despite those advantages, there are some shortcomings of mobile sensor networks. They face the problem of deployment and adaptive coverage, which are energy devouring processes, by each mobile  
Figure 1: The number of publications on mobile sensor networking research in the Web of Science from 1995 to 2009.  
unit of the sensor networks [12], and mobile sensors are more expensive than static ones. A considerable amount of energy must be spent for the locomotion task, and so the lifetime of the network is very limited, _i.e.,_ a given task has to be accomplished within a very constrained time frame. Changing a mobile wireless sensor network system into a hybrid union of static and mobile sensor subnetworks, cooperating for efficient deployment and coverage in an energy saving mode, makes the usage of hybrid sensor networks a more tempting architecture in order to improve the efficiency of the system. The main idea in hybrid network systems is that mobile nodes collaborate with the stationary ones. Existing works in the literature about hybrid sensor networks are overviewed in Section 4.