High Precision Acoustic Surveying System
[ General description | Operation | Detailed specification | Further information ]General Description
The High Precision Acoustic Surveying System (HPASS) is a precision underwater surveying system developed by the Centre for Marine Science and Technology and the Western Australian Maritime Museum specifically for maritime archaeology applications. The system allows a diver to rapidly survey points of interest in an area up to 50 m in diameter with an accuracy of +/-1 cm in the horizontal and +/- 2.5 cm in the vertical. For larger surveys the system can readily be transferred to adjacent areas in a manner that maintains accurate relative position control.The system consists of a diver electronics unit and probe and up to eight transponders which are located around the area to be surveyed. In most cases excellent results are achieved with five or six transponders.
The horizontal position of the probe is measured by sending short bursts of ultrasound from the probe to each transponder in turn, and measuring the time taken for a reply to be received. This time is a direct measure of the distance from the probe to each transponder.
The vertical position of the probe is measured using a pressure transducer.
Operation
Operation of the system is extremely simple. The sequence for a complete survey is as follows:- Deploy the transponders in appropriate locations. This can be done either from a boat or by a diver.
- In most cases the only pre-survey set up of the diver unit that is required is to ensure the batteries are charged.
- The diver then visits each transponder in turn, places the probe on top of the transponder and initiates a fix which is later used to determine the actual transponder location. It is only necessary to do this once for each survey, but for surveys spanning many days it is good practice to repeat this procedure at intervals, and it is necessary to do so if any of the transponders are moved.
- The diver then visits each point to be surveyed and takes a fix.
- When taking a fix the diver simply positions the probe on the point to be surveyed, notes the fix number on the writing slate on the top of the diver electronics unit, and pushes the single button to initiate a measurement sequence. LEDs indicate the progress of measurement and the reliability of the results.
- After the diver surfaces the data is downloaded to a computer
via a standard serial link for processing.
The transponders are left in place for the duration of the survey.
Detailed Specifications
| Acoustic centre frequency | 276 kHz |
| Acoustic source level | 180 dB re 1uPa rms @ 1m |
| Horizontal beam pattern | Omni directional |
| Vertical 3dB beam width | +/- 23 degrees from horizontal |
| Range measurement resolution | 1.5 mm |
| Range measurement accuracy | Approx. +/- 5 mm, but limited by knowledge of salinity |
| Maximum acoustic range | Approx. 50 m but varies with conditions |
| Data storage capacity | > 1300 fixes |
| Diver unit battery life between charges | Approx. 48 hours |
| Transponder battery life | Approx. 14 days |
| Depth measurement resolution | 1.6 cm |
| Depth measurement accuracy | Limited by surface waves and tidal effects |
| Temperature measurement accuracy | +/- 0.3C |
Further information
Contact Alec Duncan
Email: A.Duncan
Phone: +61 8 9266 3569
Fax: +61 8 9266 4799
Related Information:
* Marine Science Products
