Acoustic Positioning Overview: SBL, LBL, etc


#1

Hello,

What's SBL, LBL, etc?? I made these images a few years back in google Sketchup to help. There is a nice wiki page written by Marco Flagg who has been building these systems a while available here: http://en.wikipedia.org/wiki/Underwater_acoustic_positioning_system but here are the main exerpts. These systems use small ceramic elements to send clicks between each station.

<***Begin wikipedia quote from Marco***

Long-baseline (LBL) systems, as in figure 1 above, use a sea-floor baseline transponder network. The transponders are typically mounted in the corners of the operations site. LBL systems yield very high accuracy of generally better than 1 m and sometimes as good as 0.01m along with very robust positions[6][7] This is due to the fact that the transponders are installed in the reference frame of the work site itself (i.e. on the sea floor), the wide transponder spacing results in an ideal geometry for position computations, and the LBL system operates without an acoustic path to the (potentially distant) sea surface.

Ultra-short-baseline (USBL) systems and the related super-short-baseline (SSBL) systems rely on a small (ex. 230 mm across), tightly integrated transducer array that is typically mounted on the bottom end of a strong, rigid transducer pole which is installed either on the side or in some cases on the bottom of a surface vessel.[8][9] Unlike LBL and SBL systems, which determine position by measuring multiple distances, the USBL transducer array is used to measure the target distance from the transducer pole by using signal run time, and the target direction by measuring the phase shift of the reply signal as seen by the individual elements of the transducer array. The combination of distance and direction fixes the position of the tracked target relative to the surface vessel. Additional sensors including GPS, a gyro or electronic compass and a vertical reference unit are then used to compensate for the changing position and orientation (pitch, roll, bearing) of the surface vessel and its transducer pole. USBL systems offer the advantage of not requiring a sea floor transponder array. The disadvantage is that positioning accuracy and robustness is not as good as for LBL systems. The reason is that the fixed angle resolved by a USBL system translates to a larger position error at greater distance. Also, the multiple sensors needed for the USBL transducer pole position and orientation compensation each introduce additional errors. Finally, the non-uniformity of the underwater acoustic environment cause signal refractions and reflections that have a greater impact on USBL positioning than is the case for the LBL geometry.

Short-baseline (SBL) systems use a baseline consisting of three or more individual sonar transducers that are connected by wire to a central control box. Accuracy depends on transducer spacing and mounting method. When a wider spacing is employed as when working from a large working barge or when operating from a dock or other fixed platform, the performance can be similar to LBL systems. When operating from a small boat where transducer spacing is tight, accuracy is reduced. Like USBL systems, SBL systems are frequently mounted on boats and ships, but specialized modes of deployment are common too. For example, the Woods Hole Oceanographic Institution uses a SBL system to position the Jason deep-ocean ROV relative to its associated MEDEA depressor weight with a reported accuracy of 9 cm[10]

GPS intelligent buoys (GIB) systems are inverted LBL devices where the transducers are replaced by floating buoys, self-positioned by GPS. The tracked position is calculated in realtime at the surface from the Time-Of-Arrival (TOAs) of the acoustic signals sent by the underwater device, and acquired by the buoys. Such configuration allow fast, calibration-free deployment with an accuracy similar to LBL systems. At the opposite of LBL, SBL ou USBL systems, GIB systems use one-way acoustic signals from the emitter to the buoys, making it less sensible to surface or wall reflections. GIB systems are used to track AUVs, torpedoes, or divers, may be used to localize airplanes black-boxes, and may be used to determine the impact coordinates of inert or live weapons for weapon testing and training purposes[11][12][13] references: Sharm-El-Sheih, 2004; Sotchi, 2006; Kayers, 2005; Kayser, 2006; Cardoza, 2006 and others...).




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There are trickier ways to do this kind of thing but I like simple ceramic elements sending clicks from one bottle to the next. If you have good clocks to synchronize things then you can be on track. Then all you need is a small ceramic element on the ROV to receive pings from each station and figure out where he is.

Best regards,

Matthew


Acoustic Location System
#2

This is great stuff Mathew!


#3

Hi everyone.
I’m a surveyor working in the oil and gas industry and I use acoustic positioning systems similar to those described above everyday at work on operations using divers and ROVs. I also have a degree in Hydrography.

It would be fantastic to be able to replicate a similar system for the OpenROV vehicle. However from my perspective I can see this being extremely costly and technically difficult for the average user, even with significant financial backing.

The beacons used currently used in industry are quite heavy. The lightest ones are probably 4-5kg. LBL ROVNAV beacons can be as much as 18kg which is too heavy even for some commercial inspection class ROVs like the Sub-Atlantic Commanche. Buoyancy therefore would be one issue.

To get a position from the ROV via acoustics you would need a transducer from the surface recieving a GPS signal from an antenna in order to get a reference point for the ROVs position. Current systems on the market like Sonardyne USBL and Kongsberg HiPAP poles are complex, precision engineered pieces of kit, easily in the region of £30,000+. A transducer like this attached to the vessel you are launching the ROV from would also require gyros, and an MRU (motion reference unit) to compensate for HPR (Heave, Pitch and Roll) motion.All of which must be accurately measured to a reference point on the vessel in order to obtain any sensible degree of position accuracy. Adequate calibration of the system also requires taking readings of the sound velocity at each depth throughout the column of water you are going to be using the whole positioning system in. This however can easily be done with the OpenROVs current sensors, recording depth, salinity and temperature. It goes without saying that you should also have a computer with navigation software that can interpret and process all this data. If you are running this on the same system as the OpenROV Cockpit I can imagine this would use up alot of CPU which could put the system at significant risk of crashing while on dives.

Current systems have all these problems and more. Getting a position fix on one of these beacons is quite difficult in shallow water as the signal can be distorted by thruster noise from the vessel or ROV and water density is often extremely variable, due to sharp changes in temperature and pressure. At 100m you’d have to take several (20-30 or more) position samples to get enough data that ties the beacons position in to within a couple of metres. At the sort of depths and distances most users would be using the ROV at though I would imagine this would render the whole system somewhat useless if you have a position accuracy of <10m. Having a software addin that can tell the beacon on the ROV to ping at set intervals would eliminate one source of error however and the sound would only have to travel one way to the transducer on the vessel.

Despite all these technical hurdles I would imagine it’s not impossible and if anyone is able to come up with a reasonable solution that doesn’t break the bank I would be extremely interested and I certainly think you’d be onto bit of a mony maker! :stuck_out_tongue:


#4

Mattcatt77

It’s worth you haveing a look at what Jim has been doing over at the Acoustic Location System thread.

Maybe there is some input you could give to the development work going on there

Scott