Apologies for the delay in submitting, internet issues over the holidays…
Below is my proposed thesis topic and abstract.
Computation of the survey reference frame to vessel reference frame alignment using multiple GNSS sensors.
With increasing costs, it is becoming common to use vessels of opportunity to conduct survey operations. These vessels are often mobilised and calibrated with the vessel either afloat or underway. Though this is not ideal, a high degree of accuracy in the lever arm measurements within the Vessel Reference Frame (VRF) is achievable. With the correct application of a Patch Test the Survey Reference Frame (SRF) (Motion Reference Unit (MRU) reference frame) to multi-beam echo sounder (MBES) reference frame alignment can be accurately determined. (Hughes Clark, 2003)
Demands for increased resolution and accuracy in bathymetric data has resulted in the need for higher grade systems and the elimination of potential error sources. Hughes Clark (2003) spoke on the inaccuracies in lever arm computations resulting when VRF to SRF rotational alignments are not considered or are inaccurate. He postulated that a dual GPS system can be used compute these alignments, one axis at a time. This would be processed using the “Dutch Method” described in the 2009 Hydro International two-part article, by de Hilster. This only takes the “Z” (vertical) component into account.
MRU equipment conduct the Tait-Bryan angle or the 3D Euler angle computation, resolving all 3 rotational axes simultaneously (generally intrinsic rotations). The current “Dutch Method” for afloat MRU calibration conducts the calibration on a single axis at a time. This method works well for vessels with minimal movement, but an experienced team is required to achieve high accuracy, repeatable results.
This paper will attempt to determine if all 3 rotational axes of a MRU can be calibrated simultaneously using 3D coordinates from GNSS sensors mounted on the vessel and to what accuracy can the SRF to VRF alignment be computed in this way. The following will be covered:
1. Why is VRF/SRF alignment so critical and what is the impact of neglect to apply these on Patch Test results and survey accuracies?
2. What is the maximum tolerance for VRF/SRF misalignments of these sensor when considering various survey conditions and accuracy requirements?
3. What grade of GNSS sensor is needed to accomplish the required accuracies (i.e. RTK, PPP, WAAS, Stand-alone)
4. Does the simultaneous computation of all rotational alignment offsets provide a more repeatable/consistent result to the present “Dutch Method”.
Experimental data will be gathered in cooperation with the South African Navy Hydrographer aboard the SAS PROTEA. Data will be modelled mathematically to determine results. Graphical comparisons may be used to verify individual data point. Various grade GNSS systems (RTK, Veripos DGNSS etc) and software will be provided by Underwater Surveys (Pty) Ltd.
This experiment seeks to provide evidence (for or against) the use of the 3D Euler computation method of SRF/VRF alignment computation using simultaneously gathered 3D coordinates from multiple GNSS sensors.
I hope that this is acceptable. It has been quite challenging attempting to find a topic and then narrow it down sufficiently to be appropriate. It is much too easy to get distracted by interesting tit-bits (papers not directly related to the topic) if room is left for it. Please comment and advise on any changes/recommendations.
Awaiting your response.
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