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Carrier-based sensor placement involves mobile robots carrying and dropping (static) sensors for optimal coverage formation. Existing solutions target the traditional area coverage problem and unrealistically assume that robots carry sensors all together (ignoring the physical dimension of sensors and the finite robot capacity). In this paper, we consider a more realistic scenario in which robots have to repeatedly reload sensors and address the FOCUSED coverage (F-coverage) problem in an unknown 2-D environment. In F-coverage, sensors are required to surround a point of interest (POI) as far as possible, thus maximizing the coverage radius. We propose a Carrier-Based Coverage Augmentation protocol (CBCA) that seamlessly tolerates node failures. Robots enter the environment from fixed locations, called base points, and move toward the POI. As soon as they get in touch with already deployed sensors, they search (by communication) along the network border for best sensor placement spots (to improve F-coverage) and move to drop sensors at the discovered locations. Border nodes store the coordinates of failed sensors (if any exists) inside the network as well as of adjacent available deployment positions outside the network, and recommend them to robots during the search process. Robots return to base points for reloading after deploying their current payload and immediately re-enter the environment to augment existing F-coverage. An optimization technique was introduced to reduce augmentation delay and save robot energy. Extensive simulations were conducted to assess CBCA's energy expenditures and deployment latency.