Dealing with Rebels

The GPS units in differential mode (DGPS), which are to be used in a Hydrographic Survey are usually proved to the clients prior to the commencement of survey. Even though there are  several proven methods to perform this verification, the most common and simple method is to log the position of a known station using the equipment to be used, for a certain period of time and average the fixes to compare it with the established station co-ordinates.  With negligible mean difference and standard deviation of the observed and Known coordinates, we prove that the DGPS Unit has the prescribed accuracy level.

 But anyone who has worked with GPS receivers in the field knows that they are prone to occasional position jumps due to various reasons, even if the user has obtained commercial codes for precise positioning. A positional jump (Also referred as Bad coordinates) means the GPS receiver shows a different geographical position rather than its own position due to an algorithm error, and by naming it as a “Jump” it is evident that on the subsequent solution, it reverts back to its original Position. Often these odd position jumps are far apart, even to be on the opposite side of globe. If these occasional jumps are not removed from the observed data set to prove the positional integrity of an equipment against a known coordinate, it is going to affect the average positional value, sometimes in a more profound way than you can ever imagine.

In a world, that believe in statistics, where we are in a constant pressure to remain inside the threshold limits, we can’t afford to have a high standard deviation value in our reports. Certain GPS receivers are provided with filters, that allows one to omit values if subsequent observation differs in certain value parameters, like you can define that any position shift of more than 5 meters in two subsequent fixes should be discarded. But almost all the calibration procedures insist in removing all the filters from GPS equipment prior to calibration.

So the only solution to this problem is to remove these positional jumps from the observed data by filtering it manually before considering it in the report.

For this purpose, in SURge, we have a data cleaning utility to single out and clear these positional jumps

That’s why we show the following warning before opening the data cleaning utility…

In SURge, we understand the value of our profession and never allows any kind data manipulation as almost all the reports are directly generated from the raw observation files, which gives the user no room to manipulate and helps to attain the confidence of the client.

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…

We value your views....

Always be Proactive in your Profession.....

Whenever you come across any situation in your Profession, where you feel that there is a better way of doing it, or is being practiced wrongly, be proactive and report the irregularity to the concerned authority immediately.

Let it be a malfunctioning equipment, a bug in the program you use or a wrong practice of procedure, Voice out your opinions – Report an irregularity – Suggest a better way ….

Because that’s the only way to achieve Professional excellence, Make a correction and continue pursuing for professional excellence.

That’s why in SURge, we value your suggestions and feedbacks. We have a proper procedure of Reporting a Bug or suggesting a change.

Choose to be Proactive and be a true professional  

More SURge thoughts...

SURge - Survey Report generator is a software which has simplified the task of Hydrographic Survey Reporting.

It consolidates all the reports a Hydrographic Surveyor requires in the field (Not all.... ;), well almost.... ) into a single utility and reduces the work stress of the surveyor, at least in Reporting, allowing him to absorb all along his operational front.

SURge does not follow a universal pattern and can be customised as per the client's requirements.

What is SURge

SURge - A Novel idea to consolidate Hydrographic Survey Reporting in to a single platform.

Hydrographic Survey Reporting has always been a headache to Surveyors at field, where they have to spend a lot of time formatting the reports to be presented, finding the correct logos and diagrams, the schematic representations of observations, the irrelevant calculations - the result of which has to be neatly represented in the report - but has nothing to do with the operational part of the job.

Considering the usual cramp working environment onboard, these long and tedious Reporting exercises exhaust all the energy of the Surveyor and will make his life miserable.

Being an active surveyor, I came across this situation a number of times, which made me think about working on a project to prepare a consolidated software platform for survey reporting, where one just has to input the parameters on a Graphic User Interface and select the observation data files to receive a well manicured Survey Report.

I began to work on that idea during my free offshore days, and after almost 2 years, I have managed to produce a decent product after a lot of trial and errors.

'SURge' as I call it is a "Survey Report Generator", where you have a single platform solution for all the Hydrographic Survey Reports during the Calibration and Field phases of Survey is presently being used by the company where I am presently employed and the users agree that it has made their life easier.

I am planning to propagate this utility on a commercial basis and interested parties can inbox me regarding any technical or commercial queries.