This paper presents the hardware/software architecture of the Sky-Eye UAS prototype. In particular it details the hardware of the prototype, its operational concept and the software avionics architecture. The software architecture is named UAS Service Abstraction Layer (USAL) and consists on the set of standard services required for most UAS missions. The USAL is a distributed architecture which follows the publish / subscribe communication paradigm, allowing fast development of new functionalities. We describe the USAL services required to properly manage the remote sensing mission of hot spot detection. This includes the sensor management, data storage, communications, image processing, flight plan management and mission management.