Gravity on Precise, Short-Term, 3-D Free-Inertial Navigation

From the standpoint of physical principles, an inertial navigation system (INS) navigates without sensing the earth's gravitational field, yet this field affects a vehicle's total trajectory, which can be tracked by GPS. Thus, the integration of GPS and INS represents a fusion of dissimilar positioning data. Since the integration is motivated by the aiding capacity of INS during interruptions in GPS positioning, the INS data may require gravitational corrections. In this paper, statistical analyses of 3-D free-inertial navigation show that when decimeter-level accuracy is demanded from the INS over periods as short as 100 s, the gravity field should not be neglected. This conclusion is supported by simulated trajectories at low altitude (< 1000 m above ground) using actual gravity signatures that accumulate decimeter- to meter-level errors in position within 100 s, and several centimeters-per-second velocity error, depending on the strength of the gravity signature.