• Design, simulation and experimental set up of angular sensor at nanoradians scale. • Based on instantaneous phase-shifting single reflection Sagnac interferometer. • Absence of rotating components for compactness, optical gluing, and portability. There is a large inter disciplinary need for high precision angular sensors in state-of-the-art technologies. This particularly applies to optical metrology in the field of Extreme Ultraviolet (EUV) or X-ray where the optics are commonly characterized in terms of slope errors at a nanoradian scale level. This paper presents the in-lab development of an angular sensor based on a single reflection compact Sagnac interferometer using instantaneous phase shifting. Our automated prototype and first results show less than 15 nrad RMS repeatability, at least 200 nrad sensitivity and 280 nrad reproducibility over 100 μ rad angular range. [ABSTRACT FROM AUTHOR]