Time referencing in offshore survey systems

Abstract

The accuracy of offshore survey sensors such as multibeam echo sounders, GPS, ultra-short baseline acoustic navigation systems and attitude sensors is steadily increasing. Integration of high-precision survey and navigation sensors depends on accurate time referencing. The offshore survey community is increasingly experiencing that time referencing accuracy and integration issues are limiting the accuracy offered by the individual sensors themselves. Methods for merging of asynchronous measurements and strategies for clock synchronization are discussed. Operational principles and correct time stamping of key acoustic sensors are presented. Errors in time referencing leads to position errors in final survey products. Based on derived error models of surface survey systems and underwater survey systems, timing accuracy requirements can be calculated from specifications on position accuracy. Timing accuracy requirements in the order of 1 ms were computed. Besides hard requirements on timing accuracy and data output rate a broad approach is needed to ensure high timing accuracy. The report suggests requirements covering time referencing, clock synchronization, time stamping, timing accuracy, merging of asynchronous measurements, data output rate, sensor latency, data acquisition, data recording, time integrity, and sensor mounting.