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The Nautical Ordnance Mapping and iDentification (NOMAD) is a long baseline acoustic positioning system that integrates high-accuracy time synchronization and wireless radio-modem telecommunications between bottom stations and a cabled pinger attached to a vessel-mounted surface station. The pinger can be mounted on towed bodies, divers or remotely operated vehicles (ROV).
The objective of this demonstration was to validate the NOMAD positioning system for underwater Munitions and Explosives of Concern (MEC) detection operations. This demonstration consisted of using NOMAD system to position underwater geophysical mapping in a controlled, open-water environment. The design of this demonstration closely mimicked real-world scenarios in that:
Using NOMAD to reacquire anomalies was originally part of the demonstration plan, but was not performed. Software limitations, software bugs and hardware breakdowns resulted in all available time and resources being required to accomplish the primary objective of assessing the system’s overall performance.
The NOMAD system has three primary elements:
As described above, NOMAD is comprised of purpose-built acoustic positioning hardware, GPS timing hardware, radio link hardware, and specialized software to control the hardware and calculate positions. The following innovations have been integrated into NOMAD that forms the basis of this project:
Demonstration of the NOMAD system occurred during August 2014 at Pat Mayse Lake, TX. The system’s overall performance was assessed. Unforeseen hardware and software problems precluded additional tests to demonstrate anomaly reacquisition and ROV navigation. The positioning accuracy of the system in four separate pinger pole tests ranged from 30 cm with a standard deviation of 24 cm to 65 cm with a standard deviation of 60 cm. Those accuracy statements include an estimated 15 cm error attributed to the tilt of the pinger pole. Accuracies improved with experience, the better accuracies were achieved at the end of the field activities, and are attributed to lessons learned from earlier deployments. Magnetometer positioning proved more difficult. The reproducibility of twelve anomaly source locations from three independent surveys was 1.6 m with a standard deviation of 0.9 m. The cause of the degradation in accuracy from the pinger pole tests to the magnetometer tests is not confirmed, but is suspected to be attributed to a variable, 4.5 to 6 second latency in the NOMAD system. The ease of setup met its performance objectives; the whole system can be deployed and calibrated in 45 minutes or less, and retrieved in less than 10 minutes.
The demonstration showed hardware is working as expected but the software is not. The basic system controls or interfaces were satisfactory. Telephone support from the vendor (Desert Star Systems, LLC) was required to debug some software issues, and two software updates were needed to complete the demonstration.
The software does not automatically adjust position solutions for the depth measured at each bottom station or at pinger attached to the towed asset, which significantly affects the accuracy of the calculated position solutions. Depth sensor accuracy is affected by changes in its temperature, which requires cooling the sensor prior to initiating system calibration. The software is also prone to crashes. Additional software improvements are needed because a 4.5 to 6 second latency exists between the time of a ping event and that event being sent over the RS232 communication port.
A formal cost and performance calculation was not conducted due to software crashes and system variable latency.
The only implementation issues are improving the software robustness to preclude system crashes and solving the variable latency problem. Implementation can be greatly improved by automating temperature compensations to the depth measurements and integrating the depth measurements in to the baseline survey and the asset-tracking algorithm.
The NOMAD system is available for purchase from Desert Star Systems, LLC.