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:
- Metal targets were dropped/emplaced into the water bottom,
- NOMAD was deployed and integrated into a geophysical mapping survey of the area,
- The system was recovered, the geophysical data processed and interpreted.
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:
- An underwater acoustic positioning system. It is capable of precisely tracking a sensor towfish or providing navigation information for an autonomous underwater vehicle (AUV) during the mapping phase of a MEC project. During the target re-acquisition phase, the system has the capability to guide divers or a ROV back to mapped targets for identification.
- A Global Positioning System (GPS) and acoustic based pinger-pole used to survey the location of the underwater positioning system following deployment. The pinger pole includes a differential GPS receiver and a GPS triggered pinger. By comparing momentary boat GPS positions and associated acoustic range measurements, the position of each baseline station is quickly and precisely fixed in real-world coordinates.
- Simple, task optimized software for the baseline surveys, mapping operations and target re-acquisition. By feeding precision sensor positions into the mapping software for the geophysical detection sensor or towfish, NOMAD provides a streamlined capability for geophysical surveying and target re-acquisition.
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:
- High accuracy timing synchronization: NOMAD achieves timing synchronization on the order of 100 micro seconds for all system components through GPS time signal and radio links at each bottom station. A surface buoy tethered to the bottom station uses GPS time signals to zero the bottom station clock every second, and uses an RF radio link to broadcast bottom station data to the control computer.
- Unlimited number of bottom stations: a key component to the implementability of long baseline (LBL)systems for munitions operations is to have a sufficiently large network of bottom stations so that large area coverage can be achieved. Currently the system software is limited to four bottom stations.
- Fixed mount for bottom stations’ acoustic components: To minimize position error the bottom stations must be as stationary as possible and sufficiently proud of the sea floor to be effective. The tripod mounts are easily deployed from small vessels and designed to land up-right on the seafloor with the acoustic package situated 1m above the seafloor. The tripods hold the acoustic package in a fixed location eliminating error due to a moving acoustic package.
- Rapid baseline calibration of the bottom station network using a pinger-pole survey: To convert the local network of bottom station coordinates to real-world coordinates the geographic locations of each bottom station must be known. To achieve this, the NOMAD system uses a pinger-pole survey to calculate bottom station coordinates. The pinger-pole survey consists of a vertical pole mounted to a surface vessel that has an acoustic target fixed at its bottom and a Real Time Kinematic (RTK DGPS) mounted to its top. By navigating through the survey area with both the NOMAD acoustic system and GPS systems operating simultaneously, it is possible for the NOMAD software to rotate and translate the vessel path (as measured in the local bottom station coordinate system) to match the vessel path measured by the GPS system. This operation in turn provides the geographic coordinates of the bottom stations.
- Precise temperature compensation: Water temperature is one of the components to accurate speed of sound determinations. Each bottom station is equipped with a factory calibrated temperature sensor accurate to +/- 0.1° Celsius.
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.