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Techniques : Acoustic Positioning Systems

Underwater acoustic positioning systems (APS) have been used in marine archaeology almost since the tools were invented. The systems are in common use in the offshore industry and by the military but are not common in marine archaeology surveys.

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Diver positioning the stern of the Resurgam submarine using the Prospector APS

Most acoustic positioning systems work by using pulses of sound to measure distance underwater from a diver unit to a number of beacons. These distances are then used to compute the position of the diver relative to the array of beacons, in this way the process is very similar to that used by Site Surveyor. There are a number of different types of APS but we shall only look at the one that gives the best accuracy, the Long Baseline (LBL) system.

For LBL work an array of four or more electronics pods called beacons are deployed on the seabed around the site. The beacons listen for acoustic sound signals in the water and can reply with a different, individual signal if they receive a signal they recognise.

The positions of the beacons is then determined. The positions can be calculated relative to each other by getting the beacons to acoustically measure the distance between themselves and other beacons. This is identical to setting up control points and then making baseline measurements between the points. The beacons can measure their own depth and relay the information to a controlling system on the surface.

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Prototype ROVTrak system mounted on a Hyball ROV during the positioning survey of the Mary Rose in 1996

 

The positions of the beacons can also be determined in real-world co-ordinates by combining acoustic range measurements from a surface unit on a boat with positions from a DGPS receiver.

Once positioned or ‘calibrated’ the array of beacons can then be used to position a mobile unit carried on a diver, ROV or similar. The diver unit can simultaneously interrogate the beacons and measure the distance from itself to each of the beacons. Knowing the positions of the beacons and the measurements to the beacons we can calculate the position of the diver. This is identical to the process of positioning an artifact using baseline measurements from control points. There are differences between using tape measures and acoustics.

 

Advantages :

  • Acoustic positioning systems can compute the position of a point many times a second.
  • Many acoustic ranges can be measured at the same time for added redundancy.
  • The diver unit can accurately measure its own depth.
  • Many divers can use the array of beacons at the same time with no loss in update rate or accuracy.
  • Systems are available which are accurate to 10mm or so over ranges of a few hundred metres.
  • Systems are also available which are accurate to 50 mm which can cover kilometres.

Drawbacks :

  • APS are expensive and currently quite complicated.
  • Existing systems do not work well in confined spaces.

 
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Sonardyne UHDLS hull inspection system used on the Church Rocks Site, Teignmouth, for the C4 'Time Team' programme

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The Sonardyne UHDLS system being tested on the Breakwater Fort, Plymouth
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Sonardyne ADAN cable-less diver tracking system on trials

APS have not been used on many sites for a number of reasons. The cost of most systems prohibits their use in a number of cases however compared to the expense of making tape measurements they are a viable alternative, especially in deep water.

The complexity of the systems is another drawback although this is now being addressed by the manufacturers. Some bad experiences with APS have been reported in the past, in some cases it was because the system used was not good enough but in other cases the system was not used properly.

 

 
 

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The Sonardyne Homer Pro hand-held locator

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Sonardyne Homer Pro positioning a cannon
Recent work using APS includes:

 

  • Mary Rose site survey in 1996 and Resurgam site survey in 1997 using the Sonardyne ROVTrak system
  • the survey of the Lighthouse at Alexandria using a system from PLSM in 1996 
  • The Skerki Bank survey in 1997 using the EXACT system from the Woods Hole Oceanographic Institute
  • Excavation of the Vliegent Hart for the VOC Anniversary Shipwreck project using the Sonardyne Pharos system
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Early acoustic positioning : a Sonardyne Rangemeter used on the Coronation site in 1981
 

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