Abstract: A highly accurate in-situ determination of the refractivity of an ambient atmosphere is disclosed, which determination is utilizable to enhance the accuracy of a quantity measurement. The system includes use of a refractometer exposed to an ambient atmosphere and having light directed thereto to form an optical interference fringe pattern having a dependence upon the refractivity of the ambient atmosphere. The fringe pattern is measured as a function of angle either by sequentially scanning a collimated input beam in angle while collecting and detecting the transmitted light, or by imaging (onto a multi-element detector) the angular exit space of the interferometer illuminated with a diverging input beam. The electrical output of the detector is processed to provide an output indicative of the index of refraction of the ambient atmosphere.