Absolute navigation using the downlink communication signals of massive LEO constellations augmented by data from an Inertial Navigation System (INS) is considered. Lack of atomic clocks on-board LEO satellites requires the use of Doppler shift as a navigation observable. Furthermore, footprint limits of downlink signals’ beams necessitates the use of a tightlycoupled INS/radio-navigation filter in order to compensate for the fact that it is not possible to measure carrier Doppler shift simultaneously from 8 or more satellites. The navigation solution error is shown for a situation which demonstrates the feasibility of combining measurements from LEO constellations and INS data to enable GNSS-like navigation capabilities. Finally, an investigation is performed to study how navigation solution error is affected by IMU quality, signal availability, and downlink spot beam steering protocol.