CASE STUDY

Surveying the depths: the impact of real-time GNSS

Hydrographic surveying – the process of measuring and mapping underwater features to understand the physical characteristics of oceans, seas, rivers and lakes – is the quiet powerhouse behind safe navigation, coastal development and marine ecosystem protection. 

From bustling harbours to the open seas, hydrographic surveyors face challenges as unpredictable as the waters they work in. Waves, swells and currents constantly challenge the stability of sensitive equipment that includes sonar systems and GNSS receivers. Environmental factors such as sudden shifts in wind, tide changes and strong currents add layers of complexity to operations. The corrosive effects of saltwater wear down both equipment and the surveyors themselves. Safety is an ever-present concern, with unpredictable weather and equipment failures posing significant risks. In this demanding environment, the pursuit of improved efficiency and safety is paramount.      

The importance of enhanced accuracy in hydrographic surveying

The demand for accurate data is high – marine navigation and construction hinge on precise depth measurements and seafloor mapping. While onboard sensors deliver relative accuracy, integrating satellite technology and employing GNSS (global navigation satellite systems) provides absolute accuracy, ensuring the precise real-world positioning necessary for detailed measurements. Any reduction in accuracy within corrections or post-processing can propagate through the system, compromising survey results. 

This article explores how employing precise point positioning (PPP) technology and leveraging real-time data can simplify survey workflows, accelerate operations and ensure performance on water.

Beyond boundaries: real-time GNSS corrections for surveying excellence

Trimble RTX is a real-time PPP technology that uses Trimble’s global network of reference stations to accurately generate state space representation models of the ionosphere and troposphere, as well as accurate GNSS satellite ephemeris data to mitigate clock and orbit biases. Trimble RTX is available over the internet or via L-band from geostationary satellites. The independent Trimble RTX Integrity Network checks and ensures the availability and accuracy of the Trimble RTX correction data. The result is a dependable real-time correction product that you can count on for your precision surveying needs.

How Trimble's PPP solution works

 

Impact of real-time accuracy in hydrographic surveying

One key feature of a satellite-delivered PPP solution is the delivery of data in real time. This capability goes beyond merely meeting the demands of hydrographic surveying; it anticipates and addresses these needs as they arise, ensuring a proactive approach to challenges. 

Consider a typical project surveying a seabed for new port construction. Any inaccuracies during the survey could lead to costly errors in project design and implementation. Relying solely on post-processing is risky: missed areas or losses in RTK precision could lead to data rejection later, forcing surveyors to redo portions of the work. Using dependable real-time corrections helps ensure data acceptance in the office, reducing the risk of having to resurvey areas and thereby saving time and resources. 

Numerous maritime applications benefit from access to immediate data: 

  • Charting and hazard reporting: Some commercial companies, as part of their operations, submit updated navigation data to agencies such as the Maritime and Coastguard Agency (MCA) and the Civil Hydrography Programme (CHP) in the UK. When potential hazards to navigation are identified, precise real-time data allows surveyors to report them to authorities within 24 hours, supporting safer and more accurate chart updates. 
  • Marine construction: Projects involve cable and pipeline route surveys, turbine and subsea assets installations. Real-time positioning ensures decisions can be made swiftly and that assets are placed with high precision, directly impacting safety on-site. 
  • Dredging operations: Dredging relies on constant updates to accurately track progress. Real-time corrections enable dredging machines to receive the latest progress data, ensuring operations move forward without delay. Precision here is also crucial for volume calculations and meeting specifications for factors such as standard deviation and sounding density. 
  • Seismic and geophysical applications: Real-time data is critical for activities such as seismic mapping, resource exploration and environmental monitoring, ensuring that geophysical data is accurately aligned with spatial coordinates, which improves the reliability of the results and supports timely decision-making


Post-processed vs. real-time data: what’s the difference? 

The difference between post-processed and real-time data lies primarily in the timing and method of data correction. In hydrographic surveying, raw data collected during the survey that is analysed, edited or cleaned afterwards is called post-processed data. A workflow that integrates post-processing offers the ability to enhance accuracy by overcoming issues associated with real-time operations, such as correction telemetry outages. This step ensures that the multibeam sonar data meets stringent quality standards before final analysis and is essential for achieving high-accuracy results in hydrographic surveys. 

Real-time data, on the other hand, often requires directly georeferenced data at the centimetre-level provided using an external correction source. While real-time integration aligns multiple data streams immediately during collection, data often requires post-processing for stringent quality assurance. 

Real-time and post-processing methods are often combined, with the choice depending on specific survey requirements. When telemetry is interrupted – due to loss of internet, radio or line of sight – post-processing maintains data accuracy. Notably, as a satellite-delivered solution, PPP technology enables untethered operations, requiring no base station or connectivity and making it highly resilient to such interruptions. 

The ability to use real-time data, while also having the option for post-processing, ensures that data meets the highest standards of accuracy.

From land to sea: Trimble’s entry into the marine sector

With over a decade of expertise in high-accuracy PPP solutions on land, Trimble Positioning Services has established itself as a leader in GNSS corrections. Recently, Trimble leveraged its extensive experience and expanded this proven technology to the marine realm – introducing the CenterPoint RTX Marine correction service. Although a later entrant to the market, CenterPoint RTX Marine quickly distinguished itself and set new standards with industry-leading horizontal and vertical accuracy, fastest convergence times, global availability, unparalleled reliability and advanced security features. 

Paired with the trusted Applanix POS MV system, Trimble now offers hydrographic surveyors a comprehensive, integrated positioning solution that is easy to use, efficient to deploy and built on a foundation of dependable precision. 

For more information