SPRINT-Nav DP provides you with reliable station keeping data, without relying on other external reference sources such as GNSS. To get the very best performance from your SPRINT-Nav DP system in a variety of operations, there are a few key things to consider.
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Heading quality
The SPRINT-Nav DP heading quality provides the 1Drms uncertainty of the INS-calculated heading. This quality improves when the vessel is in motion and when multiple aiding sensors such as GNSS and DVL are available.
If heading quality does not improve through dynamic manoeuvres (e.g. heading changes and variations in pitch and roll – see tip 2 below), the system will not achieve the required heading accuracy of (Secant latitude) 0.05°. As a result, station-keeping performance will be degraded.
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Vessel manoeuvres and startup time
A combination of transiting to site with the SPRINT-Nav DP deployed (and thus lever arms are correct) and box manoeuvres as part of standard DP checks is typically enough to improve heading to under 0.05°.
As such, dedicated manoeuvres should not be required unless a reset has been performed on the system. If a rest has been performed we recommend, if possible, sailing a ~250m box and completing a 360 degree spin to collect sufficient data. Otherwise, the INS position may degrade quickly.
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Offset and mounting angle errors
Offset errors can also arise from misread measurements or sign errors during setup. Ideally a dimension control survey should be carried out to calculate the lever arms between the GNSS receiver to centre reference point (CRP) and the SPRINT-Nav DP to the CRP.
The SPRINT-Nav DP is designed as a position reference sensor for dynamic positioning, with the purpose of sending position aiding to the DP desk. If the sensor is to be used as a heading, pitch and roll sensor this would require further rotational alignment to the vessel frame using a third party survey suite to calculate the corrections. Differences in pitch values between systems (e.g. external gyro vs. Sonardyne) are often due to sign convention changes, where pitch may appear inverted.
Using incorrect offsets or mounting angles can degrade the system performance as the wrong position will be fed into the INS. We recommend performing the lever arm calibration process described within our user manual or contacting Sonardyne support to verify your offsets
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Updating latitude and aiding in GNSS-denied areas
When relocating to a new site without an absolute position reference, always manually update the latitude and perform a gyrocompass (GC) reset before continuing operations. Typically, this is required if latitude has changed by more than 0.1°, latitude can be updated in the user interface or by providing GNSS during system startup.
In GNSS-denied environments, only provide aiding to the SPRINT-Nav when you are confident you have a valid GNSS position. If the SPRINT-Nav is operating with an incorrect or outdated latitude, it will initialise around that value. Introducing aiding with a different position later can create inconsistencies between the known position at startup and a correct current position, degrading overall navigation accuracy.
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Sound speed configuration
It is easy to forget sound speed input as SPRINT-Nav DP functions happily with DVL derived input, especially for station keeping operations.
As sound velocity (SV) applies as a scaling error it is most obvious when linear trajectories are followed (pipelay etc). For these kind of operations to achieve datasheet performance a sound velocity sensor is required. An invalid SV value will add error into the system and cause the position error to grow as it’s not compensating the doppler correctly.
If using external SV sensors consideration of their placement should be taken into account to prevent navigational issues occurring from false readings. These can be caused by aeration or poorly mixed water. The best practice for mounting a sound velocity sensor is to ensure the SV sensor is within the same water layer (depth), ideally within 30cm of vertical distance and fitted as close as possible to the DVL head.
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Error estimates
SPRINT-Nav DP produces an estimate of its error based on its internal sensor models and the quality of the aiding it has or is receiving. SPRINT-Nav DP publishes this as a 1DRMS. The web user interface displays this and it is available in output messages.
The confidence interval here is important, 1DRMS means SPRINT-Nav DP will be within this estimate approximately 66% of the time. The difference between current GNSS position and SPRINT-Nav DP position the range rings on the navigation chart can be used to see true “error”. Alternatively, the true error between SPRINT-Nav DP and another positioning sensor (PRS) can be viewed in the DP system.
Once the error estimate grows beyond the user defined 1Drms quality limit, the $INGLL output telegram will provide a ‘data invalid’ flag and the INS solutions will be rejected from the DP desk. Monitoring the system, will help the user understand when the INS solution will need to new aiding from GNSS.
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Understanding INS resets and system integrity
When an INS reset is performed, the systems state returns to its initial conditions, meaning that any estimates of IMU or DVL biases are lost. Until sufficient new data is collected to reacquire these estimates, the navigation system will not achieve its ultimate performance.
To help maintain optimal system performance, INS resets should only be used when necessary. Unnecessary resets can compromise navigation integrity while the system reconverges.
If the INS has been reset, you will need to perform dynamic manoeuvres for the sensor model to establish the bias estimates, including heading, pitch and roll changes, for at least 15 minutes. Resetting the INS and staying stationary is the worst thing for the SPRINT-Nav DP, as it will result in the INS position degrading quickly.
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DVL bottom lock
In order to maintain high accuracy positioning, SPRINT-Nav DP should have DVL bottom lock which generates earth referenced velocities to constrain the INS drift. Without DVL bottom lock a SPRINT-Nav DP will drift 1.2m in the first 60 seconds and typically hit one nautical mile of position error after 45 minutes.
Occasionally fish or thruster wash may exist near the DVL, in some cases setting the DVL to ignore the first section of water may improve bottom tracking performance. This setting can be configured in the advanced “operational” settings area.
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Time synchronisation
If externally time synchronising the SPRINT-Nav DP, the system will require 1Pulse Per Second (1PPS) and ZDA (time of day) to be configured. Attention should be focused on the incoming signal and if it is a rising and falling edge to ensure this is configured in the Web UI correctly. Alternatively, ZDA only or time synchronisation from a local NTP server are appropriate for station keeping.
For survey work where timing is critical ZDA and 1PPS should be used.
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Sensor placement near thrusters or heat sources
Avoid mounting sensors near thrusters, heat sources or in positions where they may be exposed to localised temperature gradients. Ensure external sound speed sensors are representative of the water column where the DVL is operating to maintain accurate velocity compensation.
It is recommended that a SPRINT-Nav DP is mounted where there is sufficient clearance below the hull of the vessel to prevent aeration over the head of the DVL and ensure a clear view of the seabed.
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Knowing your performance
The performance of a SPRINT-Nav DP will based on the use case of the system.
SPRINT-Nav DP when in a static and following our above tips can achieve a station keeping performance of <2m in 24 hours.
If the vessel is moving in a straight line, the expected performance could be 0.12% distance travelled from origin, if there have been no changes in heading.
If the vessel has been dynamic with variation in heading, pitch and roll it could expect typical survey performance of 0.04% distance from origin.
If any of the above tops are not taken into consideration this will have an adverse effect on system performance and result in the system not achieving the expected accuracy.
Recommended initialisation process for GNSS denied areas
- Check system is configured correctly
- Provide SPRINT-Nav with a good position to initialise the INS, using GNSS if you are confident in its position.
- Perform at least 15 minutes of manoeuvres (heading, pitch and roll changes) to improve estimates of bias and heading performance
- Ensure heading quality is approximately (secant latitude) 0.05°.
- Ensure GNSS / external position is removed to prevent feeding bad data into the INS
- Ensure SPRINT-Nav has good bottom lock
- Monitor station keeping over time