An Unmanned Aerial System (UAS) is an uninhabited airplane, piloted by embed- ded avionics and supervised by an operator on ground. Unmanned Aerial Systems were designed to operate in dangerous situations, like military missions. With the avionics tech- nological evolution, Unmanned Aerial Systems also become a valid option for commercial applications, specially for dull and tedious surveillance applications. Cost considerations will also deviate some mission done today with conventional aircrafts to Unmanned Aerial Systems. In order to build economically viable UAS solutions, the same platform should be able to implement a variety of missions with little reconfiguration time and overhead. This paper describes a software abstraction layer for a Unmanned Aerial System distributed architecture. The proposed abstraction layer allows the easy and fast design of missions and solves in a cost-effective way the reusability of the system. The distributed architecture of the Unmanned Aerial System is service oriented. Func- tional units are implemented as independent services that interact each other using commu- nication primitives in a network centric approach. The paper presents a set of predefined services useful for reconfigurable civil missions and the directives for their communication.