The need to obtain information from depths on the seabed is one of the properties of the USBL system. Due to environmental conditions, obtaining reference points in these environments, we are clearly unable to use systems such as GNSS.

USBL systems, whose acronym stands for Ultra Short Baseline, is, in simple terms, an underwater acoustic positioning system.

To calculate the underwater position, the USBL system must be connected on the surface to a GNSS system and a gyrocompass or a GNSS system that has the property of obtaining position and orientation, which uses a double antenna; one obtains the position and the second obtains the vessel's orientation. The distance between antennas must not be less than 2 meters.

 

Figure 1. Dual antenna installation

 

The USBL system incorporates a transducer, which is installed on the vessel via a pole located underwater, and a transponder or beacon, which we are trying to track and can be carried by a diver, ROV, or installed fixed on the seabed.

Figure 2. Applied Acoustics EasyTrak Transducer

 

 Figure 3. Transponder, Beacon 1100 series

 

Figure 4. Installation of beacons

The principle is simple: the transducer emits an acoustic pulse, which is detected by the transponder or beacon, which responds with an acoustic pulse, and is then detected by the transducer. The USBL system measures the time between the transmission and reception of the pulse, obtaining a range:

 

RANGE =    VOS x  pulse travel time (m)
2

*VOS: Precise Speed of Sound


    Figure 5. Basic system installation diagram

     

    Now we have the following information: position in the plane (N, E) thanks to the GNSS system; with this, if the transducer is installed on the pole where our GNSS antenna is located, we obtain the location of our transducer; we add the distance between the transducer and transponder, in addition to the angular information; coordinates can be assigned to the transponder or Beacon, using Applied Acoustics' EASYTRAK software and its algorithms, which can be summarized as a coordinate transfer.

    The visualization is carried out by the EasyTrak software, in which we can upload an image or the design of the study area and visualize the position of the beacons, their depths, and mark any event or waypoint we need.


       

      Figure 6. Applied Acoustics EASYTRAK Software.

       

      Depths are provided directly by the USBL system and, depending on the need, can originate, for example, from the chart datum.

       

      Discover more solutions from our Marine & Port area in our new Geocom webinar.