Sea Topography of the Ionian and Adriatic Seas Using Repeated GNSS Measurements
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water Article Sea Topography of the Ionian and Adriatic Seas Using Repeated GNSS Measurements Sotiris Lycourghiotis 1,2 1 Department of Civil Engineering, University of Peloponnese, 1 M. Alexandrou Str., Koukouli, 26334 Patras, Greece; [email protected] 2 School of Science and Technology, Hellenic Open University, Tsamadou 13-15, 26222 Patras, Greece Abstract: The mean sea surface topography of the Ionian and Adriatic Seas has been determined. This was based on six-months of Global Navigation Satellite System (GNSS) measurements which were performed on the Ionian Queen (a ship). The measurements were analyzed following a double-path methodology based on differential GNSS (D-GNSS) and precise point positioning (PPP) analysis. Numerical filtering techniques, multi-parametric accuracy analysis and a new technique for removing the meteorological tide factors were also used. Results were compared with the EGM96 geoid model. The calculated differences ranged between 0 and 48 cm. The error of the results was estimated to fall within 3.31 cm. The 3D image of the marine topography in the region shows a nearly constant slope of 4 cm/km in the N–S direction. Thus, the effectiveness of the approach “repeated GNSS measurements on the same route of a ship” developed in the context of “GNSS methods on floating means” has been demonstrated. The application of this approach using systematic multi-track recordings on conventional liner ships is very promising, as it may open possibilities for widespread use of the methodology across the world. Keywords: kinematic GPS; ellipsoid; geoid; GNSS; Global Navigation Satellite System; coastal mapping Citation: Lycourghiotis, S. Sea Topography of the Ionian and Adriatic Seas Using Repeated GNSS Measurements. Water 2021, 13, 812. 1. Introduction https://doi.org/10.3390/w13060812 For many years the precise determination of the geoid has been one of the basic problems of physical geodesy. The geoid is the equipotential surface that describes the Academic Editor: Simone Marini irregular shape of the Earth and is used as the zero reference for elevation measurements. This would coincide with the Mean Sea Level (MSL), provided that dynamic factors (wind, Received: 22 January 2021 atmospheric pressure, tide, sea currents, etc.) were to be excluded—that is, if oceans and Accepted: 12 March 2021 Published: 16 March 2021 atmosphere were to be considered in equilibrium. If we extended the MSL below the continents by means of hypothetical canals, we could obtain the whole image of the geoid Publisher’s Note: MDPI stays neutral surface. The precise knowledge of the geoid is very important in several applications, e.g., with regard to jurisdictional claims in in investigating petroleum deposits. It is worth noting that the altitudes on the geoid are published maps and institutional affil- estimated with respect to the ellipsoid Earth. The methods for determining the geoid can iations. be classified into indirect and direct ones. Traditionally, the geoid has been determined indirectly either by gravitational or by astro-geodetic measurements in combination with various mathematical transformations (such as the Stokes equations). However, these calculations involve a great amount of uncertainty that can be up to several meters. Copyright: © 2021 by the author. Licensee MDPI, Basel, Switzerland. The more recent direct methods for geoid determination include marine surface This article is an open access article measurements by altimetry satellites [1] and airplane laser scanners [2], and “GNSS on distributed under the terms and floating means” measurements [3]. The first two methods face double uncertainty, since conditions of the Creative Commons the calculation of the Global Navigation Satellite System (GNSS) position is required in Attribution (CC BY) license (https:// addition to the distance estimation of the altimetry satellite or plane from the sea surface. creativecommons.org/licenses/by/ Furthermore, they face significant problems in coastal areas and closed seas due to land 4.0/). obstacles. To address these problems, a new method based on direct GNSS measurement Water 2021, 13, 812. https://doi.org/10.3390/w13060812 https://www.mdpi.com/journal/water Water 2021, 13, x FOR PEER REVIEW 2 of 23 Water 2021, 13, 812 2 of 22 Furthermore, they face significant problems in coastal areas and closed seas due to land obstacles. To address these problems, a new method based on direct GNSS measurement on the surface of the sea has been developed. During the decade 2003–2013, a number of on the surface of the sea has been developed. During the decade 2003–2013, a number of research groups have proposed “on-ship” GNSS measurements for the determination of research groups have proposed “on-ship” GNSS measurements for the determination of sea surface topography, contributing to the development of this method [4–14]. sea surface topography, contributing to the development of this method [4–14]. The installation of one or more GPS/GNSS receivers on a navigable medium was The installation of one or more GPS/GNSS receivers on a navigable medium was based on earlier research efforts involving GPS/GNSS placement on a buoy [15–17]. These based on earlier research efforts involving GPS/GNSS placement on a buoy [15–17]. These buoysbuoys used GPS/GNSS GPS/GNSS to to measure measure the the dynamic dynamic characteristics characteristics of the of waves the waves and the and aver- the averageage sea sealevel level of a of particular a particular point. point. Thus, Thus, th thee topography topography of of a a mari marinene area could notnot bebe assessed.assessed. TheThe GPS/GNSSGPS/GNSS buoy methodology waswas developeddeveloped mainlymainly forfor thethe constructionconstruction ofof earlyearly tsunamitsunami warningwarning systemssystems [[18,19].18,19]. TheThe goalgoal of of this this work work was was the the determination determination of of the the mean mean surface surface topography topography (MST) (MST) of theof the Ionian Ionian and and Adriatic Adriatic Seas. Seas. This This was was attempted attempted by developing by developing an alternative an alternative methodol- meth- ogyodology in the in context the context of the “GNSSof the “GNSS on floating on fl means”oating techniques.means” techniques. The methodology The methodology followed infollowed the present in the work present is highly work promising is highly forprom variousising for reasons. various Firstly, reasons. the exactFirstly, knowledge the exact ofknowledge the shape of of the the shape sea surface of the maysea surface provide ma quitey provide useful quite information useful information concerning concern- climate change.ing climate It is wellchange. known It is thatwell in known most cases that thein most investigation cases the concerning investigation the effectconcerning of global the warmingeffect of global on the warming sea level ison limited the sea tolevel certain is limited places to of certain the coast, places where of the tide coast, gauges where are installedtide gauges [20]. are However, installed this [20]. approach However, does this not approach allow investigating does not theallow important investigating effects the of globalimportant warming effects on of the global shape warming of the sea on surface. the shap Thosee of the effects, sea surface. to the extent Those of effects, the author’s to the knowledge,extent of the remain author’s for knowledge, the moment remain largely for unknown. the moment This largely issue could unknown. be investigated This issue throughcould be studies investigated like the through present studies one, repeated like the atpresent regular one, intervals. repeated Secondly, at regular an intervals. obvious advantageSecondly, ofan theobvious present advantage method withof the respect present to themethod methods with based respect on to altimeter the methods satellites based is thaton altimeter the hundreds satellites of thousands is that the of hundreds passenger of andthousands cargo ships of passenger traveling and worldwide cargo ships provide trav- aeling continuous worldwide grid provide of potential a continuous measurements. grid of Itpotential is actually measurements. simpler to place It is aactually GNSS onsim- a shippler thanto place to launch a GNSS an on altimeter a ship than satellite. to launch Moreover, an altimeter the satellites satellite. measure Moreover, the the sea satellites surface atmeasure specific the points, sea surface only along at specific their orbit points, (Figure only1 ),along whereas their theorbit method (Figure “GNSS 1), whereas on ship” the allowsmethod one “GNSS to identify on ship” possible allows small one to changes identify or possible anomalies small anywhere changes on or theanomalies sea surface any- andwhere to focuson the at sea specific surface points and to not focus covered at specif by satellites.ic points Furthermore,not covered by the satellites. methods Further- based onmore, satellites the methods fail to provide based on good satellites accuracy fail to in provide coastal areasgood [accuracy21]. In these in coastal areas, areas however, [21]. theIn these determination areas, however, of the the average determination marine topography of the average and marine the geoid topography is of great and scientific the ge- interestoid is of [22 great]. The scientific detection interest of significant [22]. The changes detection in specific of significant parts of changes the marine in specific topography parts couldof the revealmarine interesting topography