Sea Technology

MAY 2016

The industry's recognized authority for design, engineering and application of equipment and services in the global ocean community

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www.sea-technology.com May 2016 / st 37 P recise point positioning (PPP) provides very precise posi- tioning using measurements from a single GNSS receiver together with precise satellite orbits and clock products. Other corrections must also be applied, including relativ- istic effects, carrier-phase wind-up, satellite antenna offsets and variations, solid earth tide and ocean tide loading. In this technique, the user's coordinates can be obtained with centimeter-level accuracy in static mode and decimeter- level accuracy in kinematic mode on a global scale. PPP is now commonly used in different scientifc and practical studies, including surveying and engineering applications, marine applications, agriculture, precise time transfer, and crustal deformation monitoring. Although PPP gives several advantages to users, such as operational fexibility, ease of use and cost-effectiveness, the long convergence time for the PPP solution has limited its use in some applications. PPP-derived coordinates can be obtained through scien- tifc GNSS processing software packages, such as Bernese; in-house software packages coded by researchers; and on- line processing services, such as CSRS-PPP, GAPS, APPS and magicGNSS, developed by institutions, research cen- ters and organizations. Scientifc software packages mostly require a license fee and need advanced GNSS knowledge and experience to obtain reliable results. In contrast, online PPP services en- able users to estimate 3D coordinates with free and unlimit- ed access. The users of online services do not need detailed knowledge of GNSS methodology, experience, training or investment in GNSS processing software. These services are generally available 24/7/365 (except due to Internet out- age, maintenance, etc.). Users just need to upload or submit GNSS observation fles by choosing options through the ser- vice's website, FTP server or email. After the server receives the fle, the process starts automatically, and the results not only include estimated coordinates but also useful informa- tion about the process sent to the users via email or placed on the service's FTP site. The advantages of online services mean usage has been increasing very rapidly all over the world, making them a signifcant alternative to complex, high-cost, scientifc and commercial GNSS data processing packages. A number of institutions, research centers and organi- zations worldwide have developed online GNSS data pro- cessing services for estimating coordinates using differential (relative) and/or PPP techniques. The most known are: AUS- POS from Geoscience Australia; Scripps Coordinate Update Tool (SCOUT) from Scripps Orbit and Permanent Array Cen- ter; Online Positioning User Service (OPUS) from the U.S. National Geodetic Survey; Canadian Spatial Reference Sys- tem-Precise Point Positioning (CSRS-PPP) from the Geodetic Survey Division of Natural Resources Canada (NRCan); GPS Analysis and Positioning Software (GAPS) from University Accuracy Comparison Of Online GNSS PPP Analyzing Web Positioning Services' Performance in Marine Environment By Dr. Reha Metin Alkan • Î. Murat Ozulu • Veli Îlçi Test measurements and vessel trajectory.

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