This document describes the Advanced Stellar Compass (ASC) and defines the interfaces between the instrument and the PROBA satellite. The ASC is a highly advanced and autonomous Stellar Reference Unit designed, developed and produced by the Space Instrumentation Group of the Department of Automation of the Technical University of Denmark. The document is structured as follows. First we present the ASC - heritage, system description, performance - then we address more specifically the environmental properties, like the EMC compatibility and thermal characteristics, and the design and reliability issues. Section 6 deals with the testing and the calibration procedures and in section 7 the mechanical and electrical interfaces are given. In section 8 and 9 we address issues like manufacturing, transportation and storage and to conclude we review the ASC specifications against the PROBA requirements.