Technical Descriptions - System Functions

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SOUNDING REPETITION RATE

The user may set the rate at which soundings are taken, on-line. This has the effect of changing the range of the data collection. Swath width is approximately twice this range when using a twin transducer system. A 300m range corresponds to a ping repetition frequency (PRF) of around 2.5Hz (times per second). A 70m range gives a PRF of 10.7Hz. The time to gather a full swath at a given nominal range depends on the speed of sound. The operator can select the PRF required to maximize along-track coverage for the required swath width.

MOTION SENSORS AND POSITION

Swath bathymetry requires that the full motion (also known as attitude) of the platform be recorded. The essential parameters are roll, pitch, heave, heading and position. Bathyswath is fully compatible with, amongst others, SMC, Applanix, CodaOctopus, Ixblue, Kongsberg, SBG Systems and TSS motion reference units (MRU).

The position of the sonar sensor also needs to be measured. Bathyswath interfaces to a wide range of position sensors. Most such sensors are based on GNSS. Some sensor systems provide motion (attitude), heading and position as an integrated package.

Positioning information is accepted in both angular (latitude and longitude) format and grid projection (easting and northing). A range of conversion parameters and ellipsoids is available to the operator. A remote platform will often collect its own attitude and position data and pass it on to the Bathyswath system using an interface port.

SPEED OF SOUND

Bathyswath provides interfaces to both a speed of sound profiler and a continuous reading speed of sound meter. The profiler is used by the surveyor at suitable intervals, to measure speed of sound at intervals of depth. This is used by the system software to correct for refraction of the sound as it passes between layers of water with different sound speeds. The continuous reading meter is mounted near the sonar transducers. It ensures that the sound signals are correctly converted to measurements of angle.

The profiler is essential, but the continuous reading meter is only necessary if the speed of sound at the surface changes strongly within the survey area.

TIDE AND VERTICAL POSITION

In order to relate the depths measured by the sonar system to a chart datum height, for example, LLWS (lowest low water springs) sea level, or position height datum (e.g. WGS-84) the height of the sensor needs to be measured in real time. Bathyswath supports two ways of doing this: 

  • Tide measurements: the height of the sensors relative to the water surface is measured, using measuring tape or equivalent methods, and entered as an offset in software. The height of the water surface relative to datum, against time, (i.e., a tide table) is recorded and entered into the processing software. This tide information can come either from recording or real-time tide sensors, or from published tide tables. For accurate work in inshore waters, more than one tide sensor is needed, including offshore tide buoys. The Bathyswath software can integrate between such multiple tide sensors in both position and time.
  • GNSS (GPS) height measurements: if the positioning system is able to provide height information at the accuracy required by the users’ application, then this information can be used in processing instead of tide. However, standard GPS and differential GPS (DGPS) systems do not usually provide height to sufficient accuracy; a system such as real-time kinematic (RTK) is needed.

In either case, heave measurements from the motion sensor are merged with the height measurements in the Swath software.

OTHER SENSORS

Bathyswath provides interfaces to a range of other sensors and systems. These include:

  • An arbitrary data stream, which is time-tagged and logged with the sonar data. The information in this stream can be extracted and used for the operator’s own purposes during post-processing.
  • Echosounder: serial data outputs from single-beam echosounders can be logged by the Bathyswath software. The depth information from the echosounder can be corrected for motion and position, and placed in the gridded depth model for comparison or inclusion with the swath data.
  • Acoustic Ground Discrimination Systems (AGDS)
  • Tide: on-line tide information can be logged and used for processing.

Acoustic pulse triggering is possible to and from other sonar systems, in order to minimize acoustic “cross-talk”.

EXTERNAL CONTROL

Bathyswath can be controlled by external systems, either through an RS232 serial line or by TCP/IP, for example through an Ethernet link. This is used in remote systems, such as USVs, AUVs and ROVs.

SOFTWARE FUNCTIONS

Bathyswath provides all the hardware and software functions needed to produce bathymetric information. In addition, it is provided with links to many industry-standard software packages.

STANDARDISED RAW DATA OUTPUT

In real-time, Bathyswath data is recorded in both a generic data format, and as ‘end product’ files. The latter file format is selected to suit the post-processing system chosen. The Processing/QA off-line software can export data in x,y,z and gridded digital terrain model (DTM) formats, and as depth-and-contour graphics.

These outputs are available in common forms, including ASCII.

SIDESCAN SEAFLOOR IMAGING

Bathyswath provides sidescan imaging, with bathymetry fully co-registered. Imaging data may be displayed on the computer screen. Bathyswath also provides links to several commercially available sidescan-processing platforms.

PATCH-TEST CALIBRATION

The system software includes support for automatic patch-test calibration, which uses data from overlapping survey lines to determine the precise mounting angles of the transducers, and other similar correction factors.

WRECK AND OBJECT DETECTION

The imaging function of Bathyswath allows the operator to detect and identify objects on the seabed, including wrecks. The use of sidescan and colour-coded swath bathymetry displayed coincidently is an extremely powerful tool for this task.

The links to commercial sidescan processors referred to above also provide the opportunity for extended capabilities in this area.

 

SEAFLOOR SEDIMENT CLASSIFICATION
The combination of swath bathymetry and sidescan imaging allows the user to identify the type of seabed being surveyed. Automatic seabed classification is available through several 3rd party software suites.

INTEGRATION WITH SURFACE LASER SCANNERS

Bathyswath has been successfully integrated with laser scanning systems, with excellent results. Further information can be provided on request.

Livorno Castle Moat, subsea data Bathyswath-Standard-2-468, above water OPTECH ILRIS. Image courtesy Codevintec, Srl >