Gaps M7

Gaps Series

Light high-performance USBL systems
Gaps M7 in operation


Gaps Series has been designed to provide accurate location, positioning and tracking of subsea assets, from ultra-shallow to deep water depths. The Gaps Series embeds a FOG-based motion sensor for vessel positioning redundancy and subsea telemetry. It is compatible with third-party equipment.

Both Gaps USBL systems are easy to install and operate. In one single step, that is to turn on the system after installing it, the user is ready to operate.

Featuring an embedded gyro and a 3D configuration acoustic antenna, both Gaps are omnidirectional and does not need to be oriented. The 200° acoustic aperture allows Gaps Series to track horizontally and operate with nominal accuracy in ultra shallow waters.


Main features

Gaps Series


  • Gaps Series USBL systems were the first USBL systems embedding an inertial system. This innovation allows for quick deployement on vessel of opportunity, unrivalled positioning of subsea assets and INS/USBL redundancy in case of dynamic positioning.
  • The 3D configuration of the omnidirectional acoustic antenna allows the system to keep a vertical AND horizontal nominal accuracy. Gaps systems are the only USBL systems allowing for true horizontal tracking without the need to change the angle of the antenna.
  • Gaps Series covers the full spectrum of medium frequency applications from the surface to 4,000m water-depths.

iXlive webinar on Gaps M5

How to deploy and operate an USBL system

Session 1: Unboxing & Pre-deployement

 Session 2a : Transponder configuration

  Session 2a : USBL configuration

Session 3: USBL operation

 Session 4: Tips & Tricks

Main performance

  Gaps M5


Gaps M7


  Gaps M5

Gaps M7

Absolute accuracy (% of the distance)* < 0.5 up to 0.06
Operating range (m) 995 4,000
Coverage (°) 200 200
Frequency band (kHz) 20-30 (MF) 20-30 (MF)
Heading (° sec lat) 0.5 0.01
Roll & Pitch (°) 0.1 0.01
Cable length 20 to 190 20 to 190
Power consumption (W) 22 30
Weight (air/water, kg) 14 / -5 16 / -7
Gaps head 520,8 x 296 638 x 296
Gaps box 233 x 330 X 94 233 x 330 X 94

* For more information, please refer to the datasheet below


High accuracy subsea positioning for scientific mission off the coast of Columbia

iXblue recently took part in a scientific mission to study marine ecosystems around the Malpelo Island, off the coast of Columbia. This mission was part of a three-year circumnavigation of the world’s oceans and seas to study the overexploitation of marine resources, pollution, ocean warming and acidification, as well as the disappearance of marine species and coral reefs.


Gaps Subsea applications

Oil & Gas

  • Structure placement, Dynamic Positioning (DP)
  • ROV navigation, AUV & glider operations, Towfish tracking
  • Cable/pipe laying
  • Diver tracking
  • Exploration, drilling, mining, seabed crawler, touch down monitoring, mattress lay, plough/trench positioning
  • Out Of Straightness, BSR positioning, seismic (streamer, nodes, OBC)


  • Diver & AUV tracking
  • Underhull inspection
  • Imagery
  • Mine counter-measure
Canopus in Operation

Earth Science

  • Geodetic fields monitoring and data collection
  • ROV, AUV, gliders and towfish tracking


  • Why is a Gaps calibration free?

    It is mandatory for USBL systems to be calibrated with an INS/AHRS on board of a vessel to give accurate position and attitude to the USBL system. Calibration is not necessary with Gaps M7 because it contains an INS inside of the USBL head. The calibration is done during the manufacture process, making Gaps M7 a precalibrated system. No calibration is necessary by the end user and Gaps M7 can be installed and be operational in less than an hour.

    Gaps M5 does not need to be calibrated, even after an hydrophone replacement.

  • If Gaps is calibration free, why do I need to execute an alignment procedure?

    Gaps systems mixes USBL, INS and GPS technologies to give the most accurate USBL data in its category, it operates in deep or very shallow water and difficult environments where other systems are deficient. As Gaps has an integrated INS, this allows its use without a calibration. Where it is necessary to carefully calibrate all other USBL systems with the INS on board, It is not necessary to carry out this calibration for Gaps, as it is pre-calibrated at our factory with Phins/Octans inside. This will save a lot of time for users and Gaps can be operational in less than an hour. The important point to know is that it is necessary to align Gaps to obtain the best accuracy. This means that it is necessary that the vessel stay with a constant heading during 5 minutes for the coarse alignment, then it is necessary that it move in different directions for 25 minutes for the fine alignment. In only 30 minutes, Gaps will be ready to deliver you the specified accuracy.

  • How can I retrieve the IP address of my Gaps system?

    By default the IP address is indicated on the Gaps BOX sticker and is, with XX being the last two numbers of the serial number. If the IP address has been changed and is not known, the following steps can be taken. If you have changed the address and lost it, you can retrieve this IP address as follows: Connect a hyperterminal on the PC to one of the RS232 sockets. Configure the terminal to the following settings: baudrate 115200 bds, data bits 8, parity: none, stop bits: 1. Reboot the unit by switching it off and on, the IP address will be broadcasted by Gaps on the hyperterminal.

  • How should I set the blanking time on my Gaps for the best performance?

    In order to get the best responses from your beacons, you must always set a blanking time based on the recurrence you have set in the Gaps. The blanking time must always be -200ms from the recurrence of the interrogations from the system. For example, if the recurrence is set to 1s, then you must configure the blanking time to 0.8s. Refer to Gaps user manual for detailed tables for different examples.

  • What is the recommended procedure for Hydrophone and transducer maintenance?

    For preventive maintenance:

    In order to limit growth on the hydrophones, iXblue recommends to use the following anti-fouling: International: TRILUX 33

    • SigmanCoatings: blue Sigma-glide
    • They should be cleaned with isopropylic alcohol (Steps mentioned below)
    • Light abrasions with 100/120 sand paper
    • Clean with isopropylic alcohol
    • Wet the surface with aliphatic hydrocarbon solvent
    • Wait for 2 minutes to dry
    • Apply anti fouling according to the manufacturers recommendations (a thin coating )
    • Wait for 24 hours

    For curative maintenance:

    • To clean the hydrophones; use synthetic soap and water
    • To remove gwth, use ro180 sandpaper and apply minimal pressure on the hydrophone
  • How do I check the battery status of my MT9?

    In the configuration software, press F1. In this command menu, click "diagnostic button" to access the battery diagnosis. When the transponder is connected to the charger, the charger LED turns green when the battery is fully charged.

  • What should be the maximum voltage when my MT9 beacon is fully charged?

    The level of the battery pack when fully charged should be around 8,5V (between 8,4V and 8,6V)

    The battery voltage level can be requested via the following steps.

    1. Disconnect any external charger
    2. Connect the transponder to the PC through the connection box
    3. Launch the Transponder HMI interface
    4. Press the F1 key
    5. In the command utility window that appears, click Diagnostic button to get the battery voltage.
  • What is the minimum voltage for an MT9 beacon to be operational?

    The MT9 does not operate when the battery voltage is lower than 7V.

  • How should I configure my MT9 interrogation codes for responder mode?

    You must apply the same interrogation code to both responder and transponder modes in the configuration software. If your responder signal is not sent to the beacon (in case of a cable cut), the beacon will automatically move to transponder mode.

    In this case it is necessary to have the same interrogation code so that you can continue to follow your beacon. Keep in mind that it will be necessary to change the configuration of both Gaps when the responder signal is not send anymore.

  • What is Gaps systems positioning accuracy ?
    • Gaps M7 positioning accuracy depends mainly on Signal to Noise Ratio. This Signal to Noise Ratio depends on the beacon transmit level, the range to the beacon, the noise level and the vertical angle. In best case (SNR_in > 20 dB and in vertical conditions), the positioning accuracy is 0.1% of the slant range (CEP-50). i.e. 100 cm accuracy at 1000 m of distance. If SNR_in is 10 dB, the positioning accuracy is 0.17% of the slant range (CEP-50). For low SNR (around 0 dB), the positioning accuracy is 0.5% of the slant range (CEP-50). When tracking horizontally, the positioning accuracy is 3 times less.
    • Gaps M5 positioning accuracy depends on the same criterias and is 0.5% of the slant range.
  • What are Gaps M7 and Gaps M5 maximum range?

    Gaps M7

    The maximum range depends on the noise level and beacon transmit level. For a TL = 191 dB ref µPa:

    • if noise level is 60 dB ref µPa: max range is 5 000 m (example of buoy installation - SEA STATE 6 + 10 dB)
    • if noise level is 70 dB ref µPa: max range is 3 600 m (example of quiet vessel)
    • if noise level is 80 dB ref µPa: max range is 2 300 m (example of a vessel manoeuvring, Gaps at a reasonable distance from the thrusters)
    • if noise level is 90 dB ref µPa: max range is 1 300 m (example of a vessel manoeuvring, Gaps very closed from the thrusters)


    Gaps M5

    Gaps M5 maximum range is 995 m, making it free-of-export restrictions.

  • How accurate must my GPS lever arm be?

    Try to use lever arms 10x as accurate as your measurement. For example, within 10cm for a 1 m accurate GPS. This will make sure your lever arms will not affect the performance of your system.

  • What is the longest available cable for Gaps systems ?

    The Gaps systems cable can be 95m maximum long because of the limitation of Cat 5 Ethernet maximum length. If longer distances are required between Gaps head and Gaps box, it is possible to connect 2 x 95 m cables through a repeater box (total length is then 190 m).

  • Where is the reference point of the Gaps systems?

    The reference point of the USBL is the SUBCON connector, on top of Gaps M7 and M5 heads.

  • Why are Gaps systems the most adapted for shallow water?

    Because of its specific antenna composed of 4 hydrophones, that form a tetrahedron. This is a 3D antenna than enables to position a beacon with good accuracy on 3 axis (X, Y and Z). Gaps M7 and M5 have an aperture of 200° which means we can track a beacon less deep than the antenna.

  • Which Navigation Softwares are compatible with Gaps M7 and M5?

    Almost all navigation softwares have Gaps driver (QINSY, NAVIPAC, HYPACK, STARFIX, WINFROG, GECDIS). If your navigation software is not in this list, Gaps M7 and M5 can emulate third party protocols (GPGGA, SONARDYNE, KONGSBERG) Most of the time, Gaps M7 must not be declared as a standard USBL system because this is a pre-calibrated system. In QINSY for example, Gaps M7 and M5 must be declared as a positioning system.

  • Do I need to measure the sound velocity profile?

    To get Gaps M7 and M5 nominal accuracy, the measurement of the sound velocity in the water column is mendatory. If the sound velocity profile is not measured and not configured into Gaps M7 and M5, this can lead to unaccuracy and even to non positioning. If no sound velocity probe is available on the vessel, statistical sound velocity profiles can be used. Contact iXblue to get a tatistical sound velocity depending on your location and season.

  • How to optimize Gaps M7 and M5 performances?

    The different possibilities are:

    • increase beacon transmit level
    • reduce noise received by the Gaps (increase the horizontal distance between Gaps and source of noise, increase Gaps depth)
  • What to do if positioning accuracy is not as good as expected?

    If positions are jumpy, this probably comes from a multipath (reflective surface which is too closed from the Gaps M7 or M5 or the beacon) and/or difficult conditions for acoustic propagation (shallow water or horizontal tracking). Increase Gaps M7 or M5 depth or place differently the beacon transducer. If a bias is observed in the positioning:

    • check beacon turn around time (in the beacon and in the Gaps web MMI)
    • check that a correct sound velocity profile has been configured into the Gaps
    • check the GPS lever arm and potential mounting misalignments.
  • What comes in a standard Gaps M7/M5 delivery ?

    Gaps M7 standard package comprises the Gaps M7 head, a 50 m cable and the Gaps M7 box. Beacons, other cable lengths, GPS antenna can be ordered separately.

    Gaps M5 standard package comprises the Gaps M5 head, a 20 m cable and the Gaps M5 box. Beacons, other cable lengths, GPS antenna can be ordered separately.

  • What operating system do Gaps M7/M5 support ?

    Gaps M7/M5 are operated through an Internet browser (Firefox, Chrome, Internet Explorer, Safari…) and operated from any device that runs an Internet browser (laptop, workstation, computer, mobile, tablet...). It means that there is no restrictions regarding the Operating System. iXblue recommends the use of Firefox because this is our qualification environment.

  • What is the best location to mount the Gaps M7/M5 head ?

    As far as possible from any source of noise (thrusters, propellers, engines...) and below the wash created by the thrusters.

  • What is the best location to mount the beacon on ROV ?

    As far as possible from any source of noise (thrusters, propellers, engines...) and on top of the ROV in order there is direct acoustic line of sight between the Gaps M7/M5 head and the beacon transducer.

  • What is the impact of Gaps M7/M5 mounting misalignments (heading, roll, pitch) ?

    If the Gaps M7/M5 to GPS lever arm is small (< 5 m), Gaps mounting misalignment has low impact. If the Gaps M7/M5 to GPS lever is bigger, it may have an impact and it is recommended to enter these misalignments into Gaps M7/M5 web MMI (mechanical installation parameters).

    These misalignments can be estimated at sea (when INS correctly aligned) by comparing Gaps M7/M5 heading roll and pitch to vessel heading roll and pitch. If the Gaps M7/M5 to GPS lever arm are really large (20 m or more), it is recommended to compensate this lever arm into a navigation software (QINSY, NAVIPAC, WINFROG...) and to input into Gaps M7/M5 this compensated position.

  • Do third-party beacons have the same positioning performances compared to iXblue beacons?

    Each type of beacon has its own characteristics (transmit level, directivity, type of signal), the positioning performances are then different from one type of beacon to the other.