The Acoustic Tracking System
An Ultra Short Baseline (USBL) SCOUT acoustic tracking system from Sonardyne was used during all fieldwork sessions to track the Remotely Operated Vehicle and divers.
The SCOUT system consists of three main components: the vessel mounted acoustic transceiver, one or more Remotely Operated Vehicle or diver mounted transponders and the surface command module running the control software.
In a USBL system, the position of subsea targets is calculated by measuring range and bearing from the vessel mounted transceiver to the submerged transponder, which emits acoustic signals.
In a short baseline system, only one transducer (transponder on the seabed) transmits sound, but many transducers on the surface receive signals. In a USBL system such as SCOUT this array of transducers is built into a single transceiver assembly. The baselines between the individual transducers are very (ultra) short, usually in the range of centimetres.
The waterproof and portable surface command module running the SCOUT USBL software was installed in the control room.
The SCOUT USBL Transceiver was mounted on a pole over the side of Flatholm. In order to provide maximum stability for the transceiver the pole had a diameter of 10cm and was attached to a bracket welded onto the rail and supported by another bracket further down the ship’s side. The pole could be swivelled up when Flatholm was in passage.
SCOUT’s very light and rugged Type 7815 HF transponders could easily be attached to either the Remotely Operated Vehicle or diver umbilicals.
The transponder worked on frequencies between 35 and 55kHz. The stated operating range for the system was 500m and the acoustic coverage was +/- 90 degrees below the transceiver.
Even when moored, a vessel yaws, pitches and rolls. As the calculated diver/Remotely Operated Vehicle position is based on the transceiver position, this movement introduces inaccuracies into the positioning and has to be compensated. The SCOUT system allows tracking in two modes: The transceiver includes an internal heading and attitude sensor, which can be used for compensation. Alternatively an external motion reference unit (MRU) and a gyrocompass can be used to compensate for vessel movements.
Using the system on internal sensors is easier, as no external instruments have to be configured, but this decreases the stated accuracy of the system from ± 0.5% of the slant range to ± 2.75% of the slant range.
For both the Remotely Operated Vehicle and diver fieldwork sessions, an external MRU and gyrocompass were installed on the Flatholm.
A TSS HRP-10 MRU was fitted in the survey container. X, Y and Z offset values to the transceiver were entered in the SCOUT software.
An ASG Brown Meridian Surveyor gyrocompass was also installed in the survey container and positioned exactly parallel to the vessel's lubber line.
An onboard GPS provided positioning information. The GPS was a CSI Vector Sensor Differential GPS capable of sub-metre accuracy.
Prior to the fieldwork, all instruments were surveyed on the vessel with the help of a total station by a Wessex Archaeology surveyor to achieve the highest possible positioning accuracy. The offset values to the USBL transceiver were entered into the SCOUT software. All external instruments were connected to the SCOUT surface command module.
Both external and internal sensors had to be calibrated before use. This required the Flatholm to sail predefined patterns around a transponder on the seabed and to steam a full 360 degree circle. A calibration wizard in the SCOUT software automatically applied the necessary corrections to the USBL settings.