Creation and Implementation of a Real-Time, High-Precision SSR-RTK Positioning Service within the North Sea Exclusive Economic Zone
Improving Offshore Positioning: A German Research Project
A research and development (R&D) project named SSR-RTK correction data service for highly accurate positioning in the German exclusive economic zone in the North Sea was conducted from 2020 to 2022 to address the issue of reliable, real-time, and highly accurate positions in the offshore area.
During the September and October 2021 test cruises, the reception performance of the VSAT system installed on board the survey, wreck search, and research vessel VWFS WEGA of the Federal Maritime and Hydrographic Agency of Germany (BSH) was investigated. The Federal Maritime and Hydrographic Agency of Germany is responsible for surveying tasks in German territorial waters and its exclusive economic zone (EEZ) in the North Sea and Baltic Sea.
To transmit SSR-RTK (State Space Representation Real-Time Kinematic) correction data with high performance in the North Sea survey area, the most effective method is using internet-based streaming protocols such as NTRIP (Networked Transport of RTCM via Internet Protocol). SSR corrections are designed to be transmitted as discrete components for entire regions, enabling efficient data delivery over wide areas like the North Sea.
Key points for high performance transmission include:
- Using SSR technology: SSR sends corrections as separate components optimized for regional coverage, which improves transmission efficiency and accuracy over large marine areas.
- NTRIP over reliable internet connections: NTRIP is currently the standard way to broadcast RTK and SSR corrections over cellular or satellite internet links to GNSS receivers in the field, including offshore platforms or vessels in the North Sea.
- Integration with professional GNSS receivers: Devices capable of receiving SSR corrections via NTRIP, such as Septentrio or u-blox-based receivers, can be configured to maintain centimeter-level accuracy even in challenging offshore antenna positions by continuous correction streaming via internet.
- Satellite communication as a backup: Where terrestrial internet is unavailable, satellite-based communication can relay SSR-RTK data using similar protocols, although latency and bandwidth need to be managed carefully.
During the test cruises, despite complete signal interruptions over several minutes, short restart times lasting only a few seconds were possible. However, at a north azimuth of 140° (approx. ±5°), the communication links were sometimes completely severed.
A new approach combining irregularly distributed GNSS CORS in a wide- and a narrow-meshed network called North Sea Network (NSN) was implemented for the R&D project. A GNSS antenna was mounted in such a way that the sky view was partially obstructed by the mast, and the data of all GNSS receivers were recorded to investigate the impacts of antenna mast shading on GNSS signal reception.
The deviations from a reference C/N model were limited to ±6dB-Hz; these were displayed as a sky plot and centred and oriented on the location of the GNSS antenna. The surveyed track lines and RTK status are both illustrated in Figure 13 and show that well over 90% of all solutions were in the RTK fixed status.
Kinematic tests on land were conducted to simulate ship borne conditions, achieving a standard deviation of 1.8cm for the height component even though the network configuration's spacing was highly heterogeneous. The receivers for which solutions are shown were almost completely (more than 99.9%) in RTK status fix during the test period.
The results from the monitoring station data showed that almost without exception, the differences were within the expected range of a few centimeters, confirming the complete modelling of the error budget for this period of investigation.
The quality objectives for the R&D project include a height accuracy for the position of the antenna in the reference system ETRS89 of better than 10cm at 95% level of confidence, a convergence time of less than two minutes upon first initialisation and less than 20s after signal loss, a high availability of correction data in the survey area of the German EEZ in the North Sea, and an open, license-free, and well-documented format for correction data transmission.
The R&D project was carried out in stages, including establishing data connections to GNSS CORS, renting hardware for operating the real-time SSR network in the Cloud, configuring and commissioning software components, establishing redundancies and quality assurance measures, conducting test measurements on land and offshore, and assessing the functionality and performance of the prototype.
Two test cruises were carried out with the survey, wreck search, and research vessel VWFS WEGA of BSH in September and October 2021. The setup of the GNSS receivers, computers, and communication equipment was completed in accordance to the given figures. The test cruise achieved a high-quality standard with an average of 87.4% of RTK fixed solutions during the entire cruise duration. In well over 90% of cases, internet connectivity was maintained via the on-board VSAT system.
The objective of the R&D project was to refine the model and format parameters of SSR-RTK technologies, test their full potential for highly accurate and reliable positioning in the German EEZ of the North Sea, and meet certain quality objectives. The installation of GNSS antennas at interference-free or at least low-interference sites on a vessel should therefore always be favoured.
References: [1] [Link to reference 1] [2] [Link to reference 2] [3] [Link to reference 3] [4] [Link to reference 4]
- The implementation of the North Sea Network, which combines irregularly distributed GNSS CORS in a wide- and a narrow-meshed network, is a testament to the integration of science and technology in the education-and-self-development sector, serving as a catalyst for advancements in hydrographic survey.
- The successful conducted R&D project, SSR-RTK correction data service for highly accurate positioning in the German exclusive economic zone in the North Sea, demonstrates the application of technology in general-news arenas, such as offshore positioning and maritime surveys.
- The use of internet-based streaming protocols like NTRIP for transmitting SSR-RTK correction data with high performance in the North Sea survey area showcases the integration of science, technology, and education-and-self-development, as knowledge in these areas allows for efficient and accurate data delivery in sports and other fields requiring precise positioning.
