Author: Malik Chibah – Engineering Manager

We hosted a live, in-water demonstration of our new SPRINT-Nav, acoustically aided inertial navigation system (AAINS), at our Plymouth trials facility, UK. We wanted to put our new technology through its paces, to really demonstrate why SPRINT-Nav will be the new industry leader in subsea vehicle hybrid navigation.

Our SPRINT-Nav combines high-performance INS with unique tight integration of raw, low level digital signal processing from the latest generation Sonardyne Wideband LBL and DVL acoustics, all time synchronised to the micro second level.

It wasn’t all plain sailing though.  The weather was against us, and the environmental conditions around Plymouth are testing at the best of times. DVL performance in particular is adversely affected by the poor sea state as there was excessive acceleration for the DVL itself at the bottom of the pole due to the large pitch and roll of the vessel and increased aeration in the water. However, SPRINT-Nav still performed with outstanding results, which is a real testament to the robustness of this system.

Our equipment and set-up

In order to conduct our demonstrations, we took the following equipment with us; SPRINT-Nav mounted on a through-hull pole on our survey vessel to mimic an AUV/ROV. ROVNav6 LBL transceiver to provide LBL aiding to the SPRINT-Nav by ranging to two Compatt 6s, one Compatt 6 was placed at the Plymouth breakwater and one a few kilometres southwest in Cawsands Bay. An SV sensor was used to give us an accurate reading of the water sound velocity, and reference was provided by RTK GPS/GNSS with a nearby base station to give us a base to compare against.   

Extraordinary autonomous performance

Figure 1 – DVL-INS dead-reckoning and free-inertial AUV dive runs

Despite the rough weather, and rougher sea conditions (over 2 metre high waves!), SPRINT-Nav provided extraordinary high accuracy Doppler-Inertial positioning throughout the demonstration. Observed drift was consistently about 1 meter per 2-3km of travel away from the origin. This level of practical real world performance is unrivalled in the marine industry and forms dependable core functionality for any high performance AUV/ROV.

The map (figure 1) shows all of the runs we completed using autonomous dead-reckoning based positioning, making full use of the diversity in the environment in the Plymouth Sound.

The power of three– using sparse LBL-DVL-INS positioning

We wanted to demonstrate SPRINT-Nav’s ability for tightly integrated sparse LBL. This is where the acoustic two-way-travel time measurements from the passing of a transponder en route of a linear trajectory automatically resets DVL-INS dead-reckoning drift.

Figure 2 Single transponder sparse LBL – DVL – INS positioning

We travelled from the Mountbatten pier (NE), passing the first transponder at the Plymouth Sound breakwater after 3km and arrived at the second transponder located in Cawsand Bay (SW) after about 5.5km, see Figure 2 – transponders are indicated by bright blue diamonds.

The SPRINT-Nav LBL-DVL-INS positioning as compared to the RTK GPS reference is shown below (Table 1). Positioning accuracy was robustly within 2m at all times and reduced to << 0.5m when passing a transponder. Sparse LBL performance and range in the Plymouth Sound is severely compromised by the shallow water and highly complex and variable sound velocity profile.

Table 1 – Wide area sparse LBL-DVL-INS vs RTK GPS reference

In a typical offshore environment the realistic transponder maximum ranging distance would be 1 to 3km and SPRINT-Nav accuracy relative to a single transponder would be ~0.1% of slant range (1DRMS), i.e. 1m @ 1km distance when the vehicle passes a transponder at a right angle. The extremely accurate SPRINT-Nav wide area sparse LBL positioning is a consequence of careful tight INS integration of Sonardyne state-of-the-art Wideband DVL and LBL acoustics.

Ultimate accuracy

In order to get the most accurate data possible, it’s necessary to perform post-processing. We have a software product called Janus (named after a Roman god that can look forwards and backwards in time, at the same time – just how the Kalman smoothing technique our software uses works) which reduces the error in the data collected. We took the data that SPRINT-Nav collected during our dive runs and processed it with Janus.

The graph below (Table 2) shows the results from Janus compared to the reference RTK GPS. Radial error is a few decimetres (RMS) and peak error towards the end of the mission is ~0.5m. This is a threefold improvement as compared to the real‑time navigation in the graph above. Often more importantly, the short term relative accuracy and precision is at the sub-mm level to centimetres over 10-100m distances.

Janus post-processed wide area sparse LBL-DVL-INS

The Janus software is really easy to use. All we had to do was import the data from SRINT-Nav and click a button to perform the post-processing function.

The majority of the radial position error (0.5m) observed towards the end of the test likely stems from LBL sound velocity profile error and potentially the shallow Cawsand Bay seabed transponder experiencing a slight drift along the seabed caused by the very rough seas overnight between time of deployment and the demonstration. Peak post-processed radial position error in a real ROV/AUV operation (10km straight line from known origin with transponders at 5 and 10km) would likely be at the 0.3m level or less.

“Free inertial” AUV dive

On our way back to the marina we took the opportunity to illustrate SPRINT-Nav’s “free-inertial” capabilities. This is where the SPRINT part of the unit is working by itself, without any aiding from the DVL or GPS. The radial peak free-inertial position error was 17m over 8 minutes which reduced to 8.1m following DVL bottom lock. This level of demonstrated performance is on-par with or better than the highest performing competitive product. The key here is that the SPRINT-Nav 500 we used is not our highest performing product. So should you need a higher performing free inertial unit, our SPRINT-Nav 700 variant is the one for you.

Tight beam level DVL integration and beyond

Our final demonstration of the day was to highlight how well the tight beam level DVL integration is within SPRINT-Nav. The SPRINT INS user interface provides the ability to selectively switch off processing of individual DVL beams as a simple means of demonstrating the effect and power of tight DVL beam level INS integration. In sequence, processing of 1, 2 and finally 3 beams were switched off. Disabling of one and two beams had a hardly discernible effect on position drift – it remained at <0.25m after 10 minutes. Disabling the third beam caused a significant increase in drift rate as could reasonably be expected. The accrued position error remained below 2 meters after 13 minutes of single beam DVL-INS dead-reckoning.

There are a number of planned future updates to the capabilities of our DVL products, including allowing Syrinx to output <3 beam velocities continuously meaning that if you managed to severely damage your Syrinx DVL, you will still be able to continue with your operations, rather than having to stop until your unit is repaired and reinstalled on the vehicle.

Working throughout the night

After the demonstrations had ended, we decided it would be a good idea to test out the DVL-INS station keeping capabilities of our SPRINT-Nav. We left the unit working away, using DVL and pressure depth aiding, when we left the vessel at 5pm. We returned the next morning to see what had happened over night. In the almost 16 hours SPRINT-Nav had been running, the drift was ~1m. This is pretty impressive considering the strong currents experienced during the period. From this data we would expect SPRINT-Nav to be capable of ~1m drift in a 24 hour period in more benign ROV/AUV station keeping operations.

The time on-board our survey vessel around Plymouth provided us with some great insight into just what SPRINT-Nav is capable of, and we’re excited to see it used offshore, making a positive difference to your subsea vehicle operations.

If you would like to find out any more information about our SPRINT-Nav family, please get in touch to discuss your requirements with us.