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38 st / May 2016 www.sea-technology.com and clock products, positioning mode (as static or kinemat- ic), elevation cutoff angle, neutral atmosphere delay, user receiver antenna calibration, site displacement effects and so on. The service processes data in PPP mode (ambiguities are estimated as real numbers) through the use of precise orbit and clock products, depending on availability, from IGS and NRCan within centimeter- and decimeter-level accuracy in static and kinematic modes, respectively. The results are sent via email. The service is available at http:// gaps.gge.unb.ca. magicGNSS This free online multiconstellation GNSS PPP service was developed by the Spanish GMV Aerospace and Defense Co. in 2008. Currently, the system uses GPS, GLONASS and GALILEO observations in static or kinematic modes and allows the users to obtain positioning with PPP technique with centimeter-level accuracy depending on the observa- tion time. magicGNSS provides a post-processing service (for registered users), an email service and a real-time ser- vice. It is available at http://magicgnss.gmv.com. Test Measurement, Evaluation In order to assess the accuracy performance of the above three kinematic PPP online services in a marine environ- ment, a trial test was carried out at the Obruk Dam in Ço- rum, Turkey, in October 2015. The sea trial was conducted with a vessel equipped with a geodetic-grade GNSS receiver (ProFlex 500) and geodetic-grade antenna. The trial started with a static initialization for a short period, and then the receiver was moved to the vessel and several measurement profles were taken for about 1.5 hr. at 1-s intervals. Another GNSS receiver was set up on a reference point with known coordinates in ITRF on the shore, and data were simultaneously collected in static mode. This allowed us to calculate the reference trajectory of the vessel at centime- ter-level accuracy through relative GNSS positioning tech- nique. Data processing for relative positioning was carried out using Leica Geosystems AG commercial GNSS process- ing software, Leica Geo Offce (LGO). The reference trajec- tory was then used to compare the accuracy of kinematic PPP results obtained from the three online services being analyzed. of New Brunswick; magicGNSS from GMV Aerospace and Defense; and Automatic Precise Positioning Service (APPS) from Jet Propulsion Laboratory (JPL) at NASA. Some of them utilize relative positioning technique (i.e., AUSPOS, SCOUT and OPUS) to estimate coordinates by using reference sta- tion data from a nearby International GNSS Service (IGS) station or national CORS network points, while others uti- lize the PPP technique (i.e., CSRS-PPP, GAPS, magicGNSS and APPS). In our study, CSRS-PPP, GAPS and magicGNSS were selected to estimate kinematic PPP coordinates. The main goal of this study is to investigate the usability of the PPP technique in marine environments and to make an accu- racy comparison of these three commonly used online PPP processing services. A kinematic test was carried out at the Obruk Dam in Çorum, Turkey. CSRS-PPP This service has been operated by the Geodetic Survey Division of NRCan since 2003. The CSRS-PPP uses the PPP technique with the best available precise products, i.e., sat- ellite ephemeris, clock correction, etc. from NRCan or IGS. CSRS-PPP provides converged foat solutions at centimeter- to decimeter-level accuracy, depending on the data (i.e., single or dual-frequency GNSS data, GPS and, if available, GLONASS) and measurement modes (static or kinematic). CSRS-PPP provides unlimited access free of charge but requires registration to the system. Use of the service is very simple with minimal input requirement. The user just needs to register a valid email address, select the processing mode and desired reference frame, including additional option (i.e., vertical datum, select ocean tidal loading), and upload the RINEX observation fle via the service's user-friendly website. When the process is complete, the results will be returned to the user via email. The service is available at http://webapp.geod.nrcan.gc.ca/geod/tools-outils/ppp.php. GAPS This Web-based PPP service was developed in 2007 at the University of New Brunswick. Registration is not need- ed. The collected dual-frequency GPS observation fles are produced by a single receiver and can be submitted via the service's website by choosing some parameters, e.g., orbit (Top) The differences between known coordinates and online service results. (Bottom) Statistical com- parison of the results.