The Sense of Knownsense....

 Of the late much has been discussed about the “Calibration Free” (as claimed by the company) underwater positioning Ultra Short Base Line Equipment GAPS. If we Inspect their claim in detail, we can conclude that their claim is legitimate in terms and by large misleading to some extent.

But is it really calibration free, when it is used onboard a Survey vessel, with positional feed from a Positioning Equipment and a referential frame referred to a CRP or Common Reference Point? I am afraid it is not… since the GAPS system is calculating the positions of underwater sensors (Beacons) using its own position, the heading sensor of the USBL system will play a major role in it.

It is true that if the equipment is used as a stand-alone system with the low-quality GPS receiver they provide, exactly placed above the GAPS head then the equipment does not require a calibration as it is receiving an absolute position and by making use of its in built heading and attitude sensors, it correctly calculates the position of underwater unit.

 But sadly, that will not be the case in most of the times, where GAPS has to be used with a combination of different sensors married together on a referential frame of the Survey Vessel. For this purpose, the GAPS head has to be mounted on a frame and placed on a suitable location on the Survey vessel’s coordinate frame and has to be positioned using it.

The USBL head thus mounted may have a mounting angle error, which means that the heading sensor which is being used by the USBL System to calculate the under water positions, may not be aligned with the heading sensor which is being used by the survey vessel to calculate the relative positions of the various equipment including the USBL head. 

The mounting angle error can be easily identified by comparing the Vessel heading and GAPS heading and has to be corrected for if GAPS is using positional input from the Vessel’s referential frame. But at this point, I have seen people ignoring this factor simply quoting the “Calibration Free” tagline provided by the manufacturer and end up inducing error in data collected. They simply measure the X and Y coordinates of the USBL head with respect to the Positional sensor and enter it in the USBL system (Surprisingly, the manufacture has kept the abnormal coordinate convention for the Lever Arms), ignoring the fact that the GAPS USBL system has its own Referential coordinates and the system may end up providing wrong coordinates of underwater units, if the GPS antenna coordinates has not been recalculated according to the GAPS Lever arm frame.

GAPS has not provided a provision to put a correction value for the heading probably because it will diminish the sheen of their claim as an equipment which does not require calibration, but tried to negotiate the factor with relatively new term – ‘The Lever arm Correction’, for which the relative position of USBL head with respect to GPS Unit has to be re calculated according to the lever arms of USBL system.

Let me illustrate the scenario with an example…

In the figure given above, consider the GPS positioned at Vessel Reference Point (the small circle on the lower edge) has an absolute coordinate value of 0,0. The Gaps head is measured as 2 meters to starboard and 6 meters to bow, (2,6) had the Lever arm correction not calculated and applied, the GAPS coordinates will remain as 2,6. Now consider that the angular difference between the vessel Gyro and GAPS Gyro is 10° (GAPS Gyro high) which can be a result of mounting angle (10 ° !!!, slightly exaggerated to prove the point in a conspicuous manner)

 Now we have an underwater object, which is in absolute coordinates 15,10. If we take a fix using Beacon, the GAPS will provide the values 14.1079,12.1967 by calculating the position in its own coordinate frame.

Now if we have calculated the Lever arm corrections exactly, ie recalculate the position of RP with respect to GAPS head in its Coordinate frame (Shown in red colour), we will get 0.93 and 6.26 as GAPS head coordinates instead of 2 and 6. This will result the USBL system in correctly calculating the coordinates of underwater object in position 15,10.

The problem in this scenario is the values 0.93 and 6.26 are hypothetical values and cannot be physically measured onboard. The values has to be derived….

But how… Well open Auto Cad (which takes hell lot of time… ) and play around with both coordinate frames or…..

In SURge, we have a utility to calculate the Lever arm coordinates for GAPS, where you have to just enter the measured coordinates of the GPS and USBL unit and angular difference of the unit with respect to Vessel. The utility will provide the coordinates to be entered.

The inclusion of this utility came as a result out of necessity, as they say – “It is the mother of …..”

We never drive looking sideways, and never allow our Sensors to do so…