The resulting systems offer improved accuracy, higher updates rates, lower latency, and simplified logistics when compared to traditional underwater positioning methods (i.e., transponder based LongBaseline, Short Baseline or Ultra-Short Baseline systems).
Underwater GPS usually refers to positioning for platforms in dynamic motion to allow navigation and geodetic referencing of measurements collected from mobile undersea platforms. Seafloor Geodesy seeks to determine ultra precise positions of static instruments on the seafloor and measure their slow movement due to plate tectonic drift, seafloor subsidence, or co-seismic displacements. Here we discuss both and their relationship with marine robotic platforms.
Underwater GPS, as the name implies, is an underwater positioning system. However, it is also a misnomer in the sense that no underwater positioning system can provide truly global coverage, and instead achieves coverage over a limited regional area.
Despite this limitation in naming, the use of Underwater GPS persists because of other features and similarities between this form of underwater positioning and its space based counterpart.
In any positioning system, two tenets hold true: 1. timing is everything, and 2. nothing beats baseline.
The positioning performed by GPS (or more generally a GNSS) is often described as employing triangulation. This terminology is not correct as aGNSS performs positioning by hyperbolic multilateration, a long baseline technique. Multilateration is an extension of trilateration (position determination by measuring distances) and hyperbolic refers to the fact that distances are not measured directly due to lack of time synchronization between the satellite transmitters and the user’s receiver, and instead so-called pseudo-ranges are computed from measured time differences.
GNSSs provide users the ability to determine their position and time with unprecedented levels of accuracy, coverage and reliability (provided the requisite number of satellites can be observed). This is achieved by precise timing in the satellite’s atomic clocks and precise satellite orbital reference positions, both of which are embedded in the transmission signal, and then combined in a long baseline geometry for position computations using multilateration. Thus, the basis for the two tenets stated at the outset.
Instead of radio signals, Underwater GPS employs transmission and reception of underwater acoustic signals to perform positioning, but in a manner analogous to that of the GPS satellite. DBV Technology has developed two types of Underwater GPS systems which we refer to asPortable Underwater GPS (PUG™) and Inverted Long Baseline (iLBL™). Both types differ from traditional transponder-based approaches by only using one-way acoustic transmissions.
PORTABLE UNDERWATER GPS (PUG™)
The PUG™ application functions by having several (3 or more)surface platforms transmitting downlink signals to an underwater receiver whose position is to be estimated. The surface platform can be a set of buoys or USVs that transmit acoustic signals in a manner analogous to GPS satellites (reference timing and position information embedded into the transmission). This allows an unlimited number of underwater platforms (ROV, AUV, diver) to determine their position. Pictures of PUG™ implemented on buoys and aUSV are shown in Figures 1 and 2 respectively.
Because PUG™ allows an unlimited number of submerged assets to determine their positions, it is ideal for experimentation and testing of swarms of undersea vehicles or other operations in which multiple undersea vehicles are simultaneously and independently operated within the same area.
If a remote operator wants to know the position of submerged assets navigating using PUG™, then an acoustic telemetry link is employed in which the underwater vehicles periodically send a position report to the surface after they’ve determined their position. This position report could be received by the buoy or USV, or by direct uplink to a surface support vessel deploying and managing the assets.
INVERTED LONG BASELINE (iLBL™)
In the iLBL™ application, submerged assets are equipped with an acoustic transmitter (pinger) that emits uplink signals. This pinger can be self-contained (its own pressure vessel and power supply) or integrated into the underwater vehicle. Positioning is performed by having several (3 or more) surface platforms receive the uplink transmissions and obtain timing measurements relative to surface platform GPS position and time. Measurements from all surface platforms are broadcast to a common base station by radio link for calculation and display of the submerged platforms position.
In the iLBL™ approach, the same benefits of PUG™ are realized, but the operator also now has a continuous real time position of the submerged platforms. The tradeoff here is that there will be a limit to the number of submerged platforms that can be simultaneously tracked.
The primary application for iLBL™ is tracking of ROVs, where the ROV is tethered to the surface ship and the ROV navigator wants to be able to determine ROV position with high accuracy, high reliability, low latency and at a high update. Thus, iLBL™ achieves the benefit of traditional LBL with seafloor-based transponders but without the need for deployment and survey (calibration) or the limitations of acoustic propagation between assets operating in the same stratum. Figures 3 and 4 show the iLBL™ implemented on a buoy and our target micro-USV platform.
A SYSTEM FOR SEAFLOOR GEODESY
Both PUG™ and iLBL™ share common hardware, software and system components. They offer end users the flexibility to select the approach that works best for their operational requirements. Both provide the benefits of high update rate, high accuracy, large coverage area, lower cost, and low logistics. For example, there’s no seabed transponders to deploy and survey as in traditional long baseline systems.
PUG™ has been implemented onto the SeaTrac Systems SP-48 USV and BlueRobotics BlueBoat USV. iLBL™ has been implemented onto our iBuoy™ and we are in the process of integrating with the JAIA Robotics micro-sized aquatic drone.
In addition to its use for dynamic positioning of submerged platforms, DBV has also applied its Underwater GPS technology to develop a system for Seafloor Geodesy.
The surface platform consists of a USV equipped with PUG™ and the submerged platform contains an iLBL™ type receiver combined with a Chip Scale Atomic Clock (CSAC). In this configuration, the instrument is referred to as a Temporary Deep Ocean Geodetic (T-DOG™) sensor.
A prototype sensor underwent engineering development testing in the Puerto Rico trench in January 2023 as part of a longterm research effort with Princeton University.
An overview of Underwater GPS for Seafloor Geodesy is shown in Figure 5 and the T-DOG™ instrument is depicted on the front cover of this magazine. Space restrictions preclude a complete description of this application area, and the interested reader should contact us for details and a re-print of an article describing the system and tests in detail.
The common thread amongst all of DBV Technology’s Underwater GPS configurations is to offer end users underwater positioning with improved accuracy, higher position update rates, lower cost and simplified logistics. All employing a common suite of re-configurable hardware and software.
This story was originally featured in ON&T Magazine’s October/November issue. Click here to read more.
