GAPS - Underwater acoustic positioning system | Flanders Marine Institute

Flanders Marine Institute

Platform for marine research

GAPS - Underwater acoustic positioning system

The underwater acoustic positioning system allows to simultaneously track the position of several objects/vehicles under water based on attached acoustic transponders. In this way, the position of the ROV, AUV or scientific divers can be accurately determined. Furthermore, small amounts of data can be transmitted to the transponders, attached to the devices, using the modem function.

How does the positioning system work?

The positioning system of VLIZ is an IXblue GAPS Ultra Short Baseline (USBL) positioning and communication system of the fourth generation. The GAPS is composed of an USBL-antenna, consisting of a transducer emitting a signal and four hydrophones, registering the signal of the different transponders.

The GAPS, installed on board of the research vessel, emits an acoustic signal and the transponder under water receives this signal. The transponder in turn emits a signal, which is detected by the four hydrophones. Based on the phase difference between both signals, the direction in which the transponder is located can be calculated. Based on the difference in time between emission and reception of the signal and the speed of the signal in the water, the distance between the transducer and transponder can be calculated. Using the direction and distance, the exact position (coordinates) of the object or vehicle can be calculated with respect to the GAPS, of which the exact coordinates are known. Given that the calculations are done on a moving platform (vessel), information regarding the absolute movements of this platform are crucial to determine the correct position. This is done by making use of a built-in, pre-calibrated movement sensor (INS system).

Besides localizing objects, the system can also act as a modem. As such, acoustic communication is possible between the GAPS and the transponders. This implies that several commands can be send to the ROV/AUV/… or vice versa.

What is the acoustic positioning system being used for?

The acoustic positioning system is deployed together with other research infrastructure. Some examples are given below.

  • ROV Genesis: The location of the ROV itself and of its TMS (tether management system) is determined using transponders on both devices. As such, the position of the ROV can be monitored in real-time during its dives.
  • The scientific diving team: Members of the scientific diving team can attach a transponder to their back or push it in front of them to determine their location accurately. In this way, a correct geotag can be given to the footage they gather underwater.
  • Videoframe: The videoframe can be equipped with a transponder allowing the determination of the exact location of the footage taken during its dives.
  • AUV: The AUV of VLIZ is equipped with a pre-installed GAPS transponder, enabling sending accurate localizations to the AUV once it is in reach of the transducer. The accuracy of the collected data is increased immensely by incorporating these data.

Technical details


  • GAPS tranducer system: 296x639 mm
  • Transponder: 350x100x100 mm
  • Connection cables: 50m, 15 mm diameter
  • GAPS box: 361x233x93.5 mm


  • Compact, all-in-one system, deployable in both shallow locations as in the deep sea
  • Accuracy up till 0.06% of the distance travelled, depending on the transducer
  • Range up till 4000m, depending on the transponders.
  • 200 degrees coverage
  • Operational frequency between 21.7 and 30.3 kHz
  • Compatible transponders: MT8x2, MT9x2, ZTAx2c, ET862, MTB-OEM, RTAx2, RAMSES, CANOPES, Applies 1000 series, Sonardyne Wideband 1
  • Bi-directional communication up till 500 bps
  • 4 ethernet and 4 serial gates

How to use the acoustic positioning system?

Scientists associated with universities or scientific institutions can use our equipment. Contact VLIZ for more information.

Use and conditions

Note: if you use the acoustic positioning system as a scientist, please refer to it in your publications as: 'This work was supported by data & infrastructure provided by the Flanders Marine Institute (VLIZ)'.