Understanding correction services for GNSS
By Towfique Ahmed
Real-time correction services were created to enhance the accuracy of Global Navigation Satellite Systems (GNSS), reducing errors from tens of meters to precise, centimeter-level measurements. While some applications can function with sub-meter accuracy—such as those supported by regional Satellite-Based Augmentation System (SBAS) solutions—surveying requires far greater precision.
When making a choice between which type of correction service to use, several factors must be considered. Accuracy required, satellite and signal availability, and other project conditions will help determine the best option. Keep in mind, there is no true one-size-fits-all and there may be times when having access to more than one correction service is the best approach.
The Evolution of GNSS Accuracy
Mount Etna is an active stratovolcano on the east coast of Sicily, Italy.
Initially, GNSS surveying relied on post-processing. This technique involved a labor-intensive workflow, requiring pre-planning, field data collection, and office-based post-processing to produce results. Although accurate, this method was inefficient and lacked the real-time capabilities needed for tasks like design stake-out.
The mid-1990s saw the introduction of Real-Time Kinematic (RTK) corrections, which revolutionized the field. RTK enabled surveyors to achieve highly accurate positioning directly in the field, cutting both field and office time compared to post-processing. However, RTK had its drawbacks: it required an on-site base station, a precise and time-consuming setup, and relied on line-of-sight radio communication. Additionally, the accuracy of RTK diminished with increasing distance from the base station, limiting its range.
Further innovations brought about network RTK, (also known as virtual reference stations (VRS) or real-time networks (RTN)) and precise point positioning (PPP). These advancements aimed to provide reliable high-accuracy corrections across larger areas with streamlined workflows.
The Importance of Correction Quality
Modern surveyors are generally familiar with the technologies underpinning real-time GNSS corrections, yet the nuances of quality control often receive less attention. When evaluating the three primary correction methods—RTK, network RTK, and PPP—understanding the factors that influence accuracy is crucial for delivering high-quality work.
RTK requires a base station positioned over a known control point. Accurate results depend on the stability of both the base station and the control point throughout the survey. This setup is particularly critical for revisiting a job site in the future.
Researchers mapping Mount Etna’s gravity points and elevation profiles relied on CenterPoint RTX to collect real-time data in a challenging environment with unreliable cellular connectivity. PPP is particularly advantageous in rugged terrains such as these.
RTK is best suited for high-accuracy tasks such as cadastral surveys within a limited area where radio line-of-sight to the base station is unobstructed.
Network RTK eliminates the need to set up individual base stations, simplifying workflows, and reducing the risk of setup errors. However, data quality now hinges on the performance of the network RTK provider. High-quality services rely on continuous monitoring, ensuring that reference stations function properly, corrections remain accurate, and any anomalies are promptly addressed. Performance depends on reference stations being installed in locations with excellent conditions, such as open sky, low multipath, and free from interference in the signal spectrum, as well as choosing reference stations that are capable of tracking all constellations and all signals.
In urban environments where cellular connectivity is strong, large buildings obstruct line-of-sight, and base stations face potential security risks, network RTK offers a reliable alternative.
Choosing the right GNSS correction service involves weighing several factors, including accuracy needs, client requirements, cellular availability, convergence time, and location. By evaluating the specific requirements of a project, surveyors can identify the method that best suits their needs.
PPP goes a step further by providing a global solution that removes the need for localized reference stations. Surveyors can access corrections from a single source, even in areas without network RTK or cellular coverage. Like network RTK, PPP providers handle network monitoring, allowing surveyors to focus on fieldwork.
However, challenges remain, particularly around managing coordinate systems and datums. This complexity can introduce inefficiencies, though innovations like Trimble’s CenterPoint RTX with Trimble Access software offer solutions. These tools streamline coordinate transformations using Time-Dependent Transformations and Local Deformation Modeling. In addition, choosing a trusted provider, who installs high-quality reference stations in locations with excellent characteristics, who monitors those stations for performance and integrity, and who continuously invests into the research and development of their solution, is critical.
PPP is particularly advantageous in rugged terrains. For instance, researchers mapping Mount Etna’s gravity points and elevation profiles relied on CenterPoint RTX to collect real-time data in a challenging environment with unreliable cellular connectivity.
Choosing the Right Correction Service
Selecting the appropriate correction service is critical for efficient and accurate work. Quality issues, delays, and rework caused by inaccurate corrections can be costly. Each correction method offers unique advantages, and understanding these pros and cons helps surveyors make informed decisions to optimize their workflows and deliverables (See graphic).
Making a Selection
Choosing the right GNSS correction service involves weighing several factors. By evaluating the specific requirements of a project, surveyors can identify the method that best suits their needs. Below is a framework of critical questions to guide this decision-making process.
Key Considerations
Accuracy: What degree of precision is required to meet project specifications?
Regulatory or client requirements: Is a specific correction method required to be used?
Availability: Is there a local correction network, and what type of connectivity (cellular, satellite, or radio) is accessible at the site? What satellite constellations and signals are available?
Initialization (or convergence time): How much downtime is acceptable for waiting on system initialization? Will frequent re-initializations be necessary if the connection is lost or the site location changes?
Location: Does the terrain or environment support the practical use of a base station? Can a clear line-of-sight to geostationary satellites be achieved in areas like urban canyons or dense forests? Will full GNSS constellation support be beneficial due to a compromised sky view?
Budget: Is the project better served by investing in equipment such as a base station or by subscribing to a correction service? Consider costs related to setup and teardown, security, transportation, and potential downtime.
Backup options: How will data integrity be maintained if the primary real-time correction source fails? Are alternative correction sources, post-processing options, or additional setups available to prevent rework?
Examples of Best-Fit Scenarios
Cadastral surveying: Some municipalities mandate RTK or network RTK for these high-precision surveys. In areas without such regulations, PPP offers a streamlined workflow while still meeting accuracy requirements.
Pipeline and electric network mapping: Projects in remote locations often lack cellular coverage, making PPP the optimal solution. Satellite-delivered corrections eliminate the need for base stations, reducing setup time and enabling surveyors to work untethered.
Drainage surveys: Vertical precision is crucial for drainage projects, making short-baseline RTK or traditional workflows with lasers the preferred method. Baselines must be short when utilizing GNSS receivers for this work, and the cost of additional control points can add up quickly.
Large-scale road and construction projects: Precision GNSS is well-suited for earthmoving activities. With clear lines of sight and often reliable cellular connectivity, RTK networks can extend coverage while maintaining accuracy comparable to short-baseline base/rover RTK. PPP solutions can reduce the cost of establishing base station control points on site during early phases of clearing and grubbing.
Multi-country or Multi-state Projects: For large-scale projects spanning multiple regions, PPP is ideal. It provides global corrections without the complexity of managing multiple RTK network subscriptions or repeatedly calibrating base stations to local geodetic systems.
Finding the Right Fit
By answering key questions related to accuracy, availability, and site conditions, surveyors can confidently select the correction service that aligns with their project’s unique demands. Whether prioritizing cost-efficiency, ease of use, or precision, the right method ensures smoother workflows, reliable results, and minimized rework.
Surveyors should not feel constrained to a single GNSS correction methodology but think of each correction as a tool in their toolbox, where they pick the right tool for the right job. Carefully consider the added cost of setup, teardown, densifying site control points, and balance those costs with subscription services from reputable providers if possible. 
Towfique Ahmed is a corrections solutions architect manager in the positioning services division at Trimble. He has more than a decade of experience in GNSS correction services, combining technical expertise with a passion for user experience and innovation. He graduated from the University of Calgary with a Bachelor of Science in geomatics engineering.