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Design, Implementation, and Characterization of Precision Timing for Bistatic Acoustic Data Acquisition

One of the challenges presented in using autonomous underwater vehicles (AUVs) for remote data collection is accurate time synchronization. In the case of bistatic acoustics, synchronization is required between AUV and source so that the time in which each ping is sent out is exactly known. There are three key obstacles to achieving this. First, the vehicle is submerged and therefore unable to access common time references directly. Second, the required accuracy in data acquisition far exceeds that possible using a computer clock to trigger data collection. Finally, to achieve accuracy in microseconds, the system must be characterized to eliminate delays introduced by filtering and analog-to-digital conversion. This paper describes the implementation and characterization of an accurate and precise timing and data acquisition system used on an AUV to collect acoustic array data. The timing was achieved using a combination of global positioning system (GPS) pulse-per-second (PPS) for synchronization on the surface and the Microsemi (Aliso Viejo, CA, USA) chip scale atomic clock (CSAC) for timing while submerged, with a PPS triggered data acquisition system. Characterization and calibration procedures were developed to ensure that the system met the experiment requirements, which included less than one percent of a wavelength error in phase, and one tenth of a meter accuracy in range. Analog and digital delays in the system were determined, and a method was demonstrated to further improve accuracy by dynamically estimating digital delays. The steps outlined in this paper for achieving precision data acquisition could be applied to many other remote systems that require similar microsecond accuracy.