The present invention provides an innovative apparatus and system for onboard spacecraft location determination and celestial navigation by employing spectral observations of extrasolar planetary star system motion. In one apparatus embodiment a gas absorption cell is placed between a sensor and the light from a reference star system with at least one exoplanet, such that the sensor can detect the spectrum through the gas absorption cell. Radial velocities can be calculated via Doppler Spectroscopy techniques and incorporated into a spacecraft navigation solution. The present invention can enable and enhance significant mission capabilities for future manned and unmanned space vehicles and missions.