Deep Water Navigation With an Inertial System

Abstract

Abstract Integrated satellite-navigation systems have been employed extensively for positioning marine geophysical surveys in the past three years. Since most exploration has been in relatively shallow water, dead-reckoning with doppler sonar velocities and gyrocompass heading reference has proven adequate. For programs in deep water, we have mechanized an inertial system to provide velocity and heading data for real-time positioning and postprocess mapping with the precision required by the geophysical industry. This is accomplished through real time software control. Inertial-platform drift is determined initially by comparing satellite fix positions with inertial coordinates. Inertial drift curves are predicted, and ship position is controlled by compensating for this drift. As drift characteristics change, drift prediction is updated. Continuous experience at sea since early December, 1971, had demonstrated that system drift can be effectively controlled. On the basis of all satellite fixes regardless of elevation angle, for operations on or near the equator, drift rates have been less than 300 m/hr. about 70% of the time, and less than 450 m/hr. about 90% of the time. Research with more sophisticated prediction methods led to software changes, introduced in October 1972 which showed some performance improvement but also demonstrated a shortcoming in the error model. Software changes, to be introduced shortly, are expected to show a 20%-30% improvement in drift rates. At higher latitudes where more good angle passes are available, system performance will be even better! INTRODUCTION There are a variety of radio systems available for marine navigation. However, none are able to provide continuous, world-wide navigation with the accuracy required for marine geophysical surveys. Consequently, integrated satellite navigation systems were introduced in 1969 to satisfy this need and used extensively with varying degrees of success. These systems rely upon periodic position fixes from the Navy Navigation Satellite System. Orbital data and frequency measurements are used in the position calculation (Stansell (12)). On the average, an acceptable position fix is obtained about once everyone to two hours, depending upon latitude. Since most geophysical exploration has been in relatively Shallow water (less than 100 fathoms), dead-reckoning between fixes with doppler sonar velocities and gyrocompass heading has proven adequate. In deep water, doppler sonar performance is degraded due to an inability to track the ocean floor. Under this condition the sonartracks a return from the water mass and hence the sonar velocities are in error by the amount of any sea current. Halamandaris and Ozdes (S) proposed a solution to the problem of deep water navigation with an integrated satellite doppler-sonar system by the use of a sea-current biasing technique.