Thermospheric mass density (TMD) measurements are invaluable to accurately estimate and predict the position and velocity of orbiting objects in Low Earth Orbit (LEO). Existing observational methods and empirical models fail to describe and predict, with enough accuracy and resolution, the actual air-drag variations required for practical applications. With the increasing number of LEO satellites equipped with high-precision Global Navigation Satellite System (GNSS) receivers, precise orbit technology can be used to obtain non-gravitational accelerations, and therefore estimate accurate TMD variations. In this work, TMD is estimated from CASSIOPE precise orbits, and data from the February 2014 geomagnetic storm can be investigated to high accuracy and resolution. Using this method, a more accurate description than previous methods and empirical models, that are unable to describe short-term TMD variations, during geomagnetic storm conditions is given.