It’s well known that combining acoustic positioning with an Inertial Navigation System (INS) benefits Dynamic Positioning by improving accuracy, update rate and reliability of position data. Sonardyne’s first generation ‘loosely’ integrated solution delivered notable performance gains; four years on, the arrival of a ‘tightly’ integrated DP-INS technology represents a genuine break-through to rival state-of-the-art satellite navigation performance even far offshore in ultra-deep water.
Acoustic positioning and INS have complementary characteristics and their combined use was first suggested in 1975. Since then, technology and systems have matured with acoustic-inertial systems available from different vendors having been in successful operational use for many years. For a long time, LUSBL (Long and Ultra-Short BaseLine) acoustic positioning was the only practical complement to satellite based DP in deep water. Sonardyne’s first introduction of loosely integrated INS in 2010 provided a third type of positioning reference by extending USBL positioning into deeper water via reduction of noise and bridging of small dropouts in acoustic positioning. DP using a single seabed transponder was proven in water depths up to 3,070 metres (10,072 feet) and this configuration remains in use for operation in benign conditions where time is at a premium.
While operationally efficient, single transponder USBL aided inertial navigation does have limitations. Complex acoustic degradation can arise from severe thruster wash, turbulence, aeration or an obstructed line of sight. Under harsh operational conditions, these error sources can be of sufficient magnitude and duration for the INS to drift outside of acceptable positioning tolerances. The latest generation Sonardyne DP-INS tightly integrated acoustic-inertial position reference system has been in operational use since 2013, offering optimal and seamless use of any number of transponders – from single transponder use in benign conditions to full LUSBL array for the most challenging operations
Loose, Tight and Ultra-Tight integration
Using terminology from GNSS (Global Navigation Satellite System), the integration of INS with measurements from external aiding sensors is categorised respectively as Loose, Tight or Ultra-Tight depending on the level at which integration takes place.
Orthogonal accelerometer and gyroscope triads within the IMU (Inertial Measurement Unit) measures change in velocity (Δv) and orientation (ΔΘ). INS algorithms integrate the IMU data to output position, orientation and velocity. Aided INS (AINS) uses an error state Kalman filter to continuously estimate and correct INS error by processing measurements from external sensors – here USBL.
In a loosely integrated configuration, the INS is aided by the positions computed by a conventional acoustic positioning system. Tight integration makes use of the individual acoustic measurements from the USBL transceiver – two way travel time (range) and phase differences (direction). Accuracy and robustness is known to improve with the level of integration but so is the required engineering effort. Ultra-tight integration refers to additionally using inertial measurements to improve the low-level measurement process of the aiding sensor. Military use of ultra-tightly integrated GNSS/INS enable GNSS receivers to maintain tracking of the satellite signals in scenarios with severely reduced signal to noise ratio. Future ultra-tight integration between USBL and INS may similarly enhance acoustic tracking in high-noise conditions. Accuracy break-through acoustic positioning and the difference between loose and tight acoustic-inertial integration in high noise conditions. In deep water, it takes several seconds for the acoustic signals to travel from the USBL transceiver to the seabed transponders and back. Wave or thruster induced vessel motion during this time period is not easily compensated for in a standalone acoustic system. This results in latency and reduced accuracy of the computed position and so also severely limits the performance of a loosely integrated acoustic-inertial solution. It is easily within the capability of a quality INS to measure and compensate for vessel motion during an acoustic cycle. In a tightly integrated configuration, this allows full utility of the robust centimetric level range measurement precision of modern Sonardyne 6G wideband acoustics.
The net result is a genuine break-through in real-world performance. It is now possible to achieve centimetric level positioning accuracy repeatability in water depths beyond 3,000 metres (relative to the seabed transponder array).
Tight integration also offers much increased robustness during periods of acoustic degradation. From the 15 second mark and continuing for 120 seconds until 135 seconds, only a few acoustic measurements are available per acoustic cycle. This prevents standalone acoustic positions from being computed and hence the loosely integrated solution enters free inertial drift (2 metres drift in ~ 2 minutes) until acoustic positioning is restored. The more robust tightly integrated solution makes full use of the reduced set of USBL acoustic measurements and the effect is just a slight decrease in accuracy. Due to tight system integration, the Kalman filter gets direct access to a rich set of low level quality metrics created by the acoustic transceiver’s wideband digital signal processing and so can better assign weights to the acoustic measurements and eliminate outliers.
Deep water operational results
Despite using just three transponders as compared to the standard five for a stand-alone LUSBL acoustic system, the performance and DP weighting is close to state-of-the-art GNSS.
DP-INS results obtained in November 2013 onboard another ultra-deep water drillship operating in 2,700 metres of water. The solution remained operational at about 1 metre accuracy despite being down to just two transponders. The acoustic update rate was just 12 seconds (prolonging battery life) with the transponders deployed within easy ROV reach at less than 10 degrees off the vertical. This previously unheard of level of robustness and operational efficiency is only possible via tight integration. Using three or (preferably) more transponders is recommended for DP drilling applications. However, this example shows how robust accurate positioning will typically carry on even if one transponder is lost.
Bring it all together
Tightly integrated acoustic-inertial navigation provides accuracy, update rate, robustness and hence DP weighting that is on par with state-of-theart GNSS (GPS) when operating within a conventional LUSBL array of transponders. The massively increased performance from tight acousticinertial integration can also be used to reduce the number of transponders and the acoustic update rate. This extends the battery life of seabed equipment and reduces operational cost. The reduced amount of acoustic signals in the water also simplifies simultaneous operations (SIMOPS). Pre-calibrated USBL transceivers with mechanically integrated INS are available. Their use saves vessel time and reduces the risk of down time since a transceiver can now be replaced without the need for a lengthy system calibration