The Airborne Prism EXperiment (APEX) is an airborne pushbroom imaging spectrometer for Earth observation. Its products will become available in 2011. APEX is currently prepared for final acceptance configuration completing final hardware upgrades, refined calibration methodologies and test flights. APEX is composed of an airborne dispersive pushbroom imaging spectrometer, a Calibration Home Base (CHB) for instrument calibration and a data Processing and Archiving Facility (PAF) for operational product generation and delivery. A unique In-Flight Characterization (IFC) unit is integrated within the sensor optical head, providing pre- and post- data-acquisition characterization monitoring the instruments spectral and radiometric stability. This paper outlines the activities performed with a special focus on system calibration and validation procedures, as well as preliminary measurement results.