Unmanned Aerial Systems (UAS) are slowing becoming efficient platforms that can be applied in scientific/commercial remote sensing applications. UAS may offer interesting benefits in terms of cost, flexibility, endurance, etc. On the other side, the complexity of developing a full UAS-system is currently limiting its practical application. Currently, only large organizations like NASA or NOAA have enough budget and infrastructure to develop such applications.Nowadays, UAS technology offers feasible technical solutions for airframes, flight control, communications, and a wide set of multi-spectrum sensors. However, the generalized development of remote sensing applications are still limited by the absence of systems that support the development of the actual UAS sensing mission.This paper introduces a flexible and reusable hardware/software architecture designed to facilitate the development of UAS-based remote sensing applications. This flexibility is organized into an user-parameterizable UAS Service Abstraction Layer (USAL). The USAL defines a collection of standard services are their interrelations as a basic starting point for further development by users. Functionalities like enhanced flight-plans, a mission control engine, data storage, communications management, etc. are offered. Additional services can be included according to requirements but all existing services and inter-service communication infrastructure can be exploited and tailored to specific needs. The overall USAL architecture is demonstrated by means of a wild land fire remote sensing application currently being developed to support fire fighters in the Mediterranean area.