Abstract
Hydrographic surveys, in accordance with the International Hydrographic Organization (IHO) S-44 standard, can be carried out in the following five orders: Exclusive, Special, 1a, 1b and 2, for which minimum accuracy requirements for the applied positioning system have been set out. They are as follows, respectively: 1, 2, 5, 5 and 20 m, with a confidence level of 95% in two-dimensional space. The Global Navigation Satellite System (GNSS) network solutions (accuracy: 2–3 cm (p = 0.95)) and the Differential Global Positioning System (DGPS) (accuracy: 1–2 m (p = 0.95)) are now commonly used positioning methods in hydrography. Due to the fact that a new order of hydrographic surveys has appeared in the IHO S-44 standard from 2020—Exclusive, looking at the current positioning accuracy of the DGPS system, it is not known whether it can be used in it. The aim of this article is to determine the usefulness of GNSS/Inertial Navigation Systems (INS) for hydrographic surveys. During the research, the following two INSs were used: Ekinox2-U and Ellipse-D by the SBG Systems, which were supported by DGPS and Real Time Kinematic (RTK) receivers. GNSS/INS measurements were carried out during the manoeuvring of the Autonomous/Unmanned Surface Vehicle (ASV/USV) named “HydroDron” on Kłodno lake in Zawory. The acquired data were processed using the mathematical model that allows us to assess whether any positioning system at a given point in time meets (or not) the accuracy requirements for each IHO order. The model was verified taking into account the historical and current test results of the DGPS and RTK systems. Tests have confirmed that the RTK system meets the requirements of all the IHO orders, even in situations where it is not functioning 100% properly. Moreover, it was proven that the DGPS system does not only meet the requirements provided for the most stringent IHO order, i.e., the Exclusive Order (horizontal position error ≤ 1 m (p = 0.95)). Statistical analyses showed that it was only a few centimetres away from meeting this criterion. Therefore, it can be expected that soon it will be used in all the IHO orders.
Highlights
Hydrographic surveys are among the navigation applications that commonly use Global Navigation Satellite Systems (GNSS)
Each of them was assigned a number of requirements, primarily including the following two defined navigational parameters for the positioning systems: the maximum permissible positioning accuracy and its confidence level, i.e., the availability of a specified position error value that is identical for all the International Hydrographic Organization (IHO) orders, and amounts to 95%
Differential Global Positioning System (DGPS) system has not once reached the positioning availability required for the Exclusive system has not once reached the positioning availability required for the Exclusive Order, Order, i.e., 38.48% (Ellipse-D (Route no. 1)), 15.54% (Ellipse-D (Route no. 2)) and 87.57%
Summary
Hydrographic surveys are among the navigation applications that commonly use Global Navigation Satellite Systems (GNSS). Exclusive Order—sets down the highest requirements in the field of positioning accuracy According to this order, hydrographic surveys are carried out on shallow waterbodies for which the under-keel clearance is minimal, and the bottom topography poses a potential hazard to the safety of navigation. Hydrographic surveys are carried out on shallow waterbodies for which the under-keel clearance is minimal, and the bottom topography poses a potential hazard to the safety of navigation Examples of such areas include harbours, berthing areas and critical areas of fairways and channels. The maximum permissible position error is 5 m + 5% of depth with a confidence level of 95%; Order 2—imposes requirements on the positioning systems (and ) used for hydrographic surveys of waterbodies with depths of more than 100 m, where the general depiction of the bottom is sufficient for the expected type of navigation. The maximum permissible position error is 20 m + 5% of depth with a confidence level of 95%
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