  Precise autonomous navigation remains a substantial challenge to all
underwater platforms. Inertial Measurement Units (IMU) and Doppler Velocity
Logs (DVL) have complementary characteristics and are promising sensors that
could enable fully autonomous underwater navigation in unexplored areas without
relying on additional external Global Positioning System (GPS) or acoustic
beacons. This paper addresses the combined IMU/DVL navigation system from the
viewpoint of observability. We show by analysis that under moderate conditions
the combined system is observable. Specifically, the DVL parameters, including
the scale factor and misalignment angles, can be calibrated in-situ without
using external GPS or acoustic beacon sensors. Simulation results using a
practical estimator validate the analytic conclusions.
