A traditional position reference sensor (PRS) system will consist of a sensor mounted on a vessel and connected to its DP system, and one or more co-operative targets which the sensor will track
The presence of co-operative targets as part of the system gives the sensor a clear signal onto which it can lock.
The co-operative targets also can cause specific difficulties which are well known to DPOs. These include lack of available targets on assets, target obscuration during DP operations and poor target placement causing difficulties with the availability and continuity of the position measurement.
Because of these difficulties there is a desire to have available targetless PRS systems which can mitigate these problems.
The performance testing of a targeted PRS can be characterised on land at a suitable location. Large open areas which are substantially flat are required, however locations such as test tracks or former airfields provide a good opportunity to characterise performance which can be subsequently ratified at sea.
Targetless local positioning sensors will generally utilise some aspect of the environment to allow the tracking of pose. This environmental dependence results in the characterisation of performance being a more complex task which also requires a more complex way of looking at the system performance.
The SceneScan sensor is a targetless laser position reference sensor. It utilises a simultaneous location and mapping (SLAM) algorithm.
To track off a targetless scene the sensor first takes a reference scan of the scene. This forms our initial map. Each subsequent scan is then matched to this initial reference scan and we can infer the sensor motion between the scans by taking the translation and rotation required to correctly overlay the scans.
This process is repeated until what is observed by the sensor no longer closely matches the reference scan data. The system then updates its map by incorporating new data from the different viewpoint. Subsequent scans are then matched to the updated reference scan and this process continues throughout the tracking session. This allows us to continue tracking as we navigate around an asset.
The SceneScan generates its map in a 2D plane however at the start of a tracking session it optimises which 2D slice tilt angle of the sensor to scan. This optimisation can be shown in the form of a colour coded 3D plot. In the plot above we have the following colour code:
Green: Acceptable or better tracking performance from this slice
Red: Poor tracking performance from this slice
Grey: Excluded (part of the sensor vessel)
Blue: Sensor tilt angle out of range for tracking should the vessel move in close to the asset.
Verification against other sensors
We can use data recorded from the CyScan and RadaScan to compare performance of the three sensors. Looking at the overall trend in range and bearing for the station keeping we can see that the three sensors remain consistent with each other.
Looking closer at a small period of this run we can see that the three sensors look to be in good agreement about the vessel motion. The SceneScan was reporting a smaller overall level of wave motion due to its lower mounting height. The locations of the reference frames for each sensor were slightly different as well.
Results against a drillship
We have shown techniques used to evaluate the performance of a targetless position reference sensor and shown these used to evaluate a laser position reference sensor.
MTS TechOp OPD 14 – PRS and DPCS handling of PRS, September 2017