What are GNSS corrections?

GNSS corrections are simply any external corrections applied to your GNSS receiver to improve the accuracy of your position.

To understand the importance and complexity of GNSS corrections, you first need to understand GNSS errors. Simply put, errors are the factors that make it difficult for a GNSS receiver to calculate an exact position. Chapter 4 of our An Introduction to GNSS goes in-depth on where these errors originate. At a high level, these include:

• Satellite clocks – Atomic clocks in the GNSS satellites are very accurate, but they do drift a small amount. Small inaccuracies in the satellite clock results in a significant error in the position calculated by the receiver.
• Orbit errors – GNSS satellites travel in very precise, well-known orbits. However, the orbits do vary a small amount. A small variation in the orbit results in a significant error in the position calculated.
• Ionospheric delays – Ions, electrically charged particles in the atmosphere, alter the transmission time of satellite signals and can cause significant position errors. Ionospheric delays vary with radio frequencies, solar activity, time of year, season, time of day and location. Check out our ionospheric scintillation resources to learn more.
• Tropospheric delays – Variations in tropospheric delay are caused by changing humidity, temperature and atmospheric pressure. Since tropospheric conditions are very similar within a local area, base station and rover receivers experience a very similar delay.
• Receiver noise – Receiver noise refers to the position error caused by the GNSS receiver hardware and software. High-end GNSS receivers tend to have less receiver noise than lower-cost GNSS receivers.
• Multipath – Multipath error occurs when a signal from the same satellite reaches a GNSS antenna via two or more paths. This is often caused by the signals bouncing off obstructions like buildings in urban environments.

Now that we know how and where GNSS errors occur, we can look at how they are resolved.

Using GNSS corrections to resolve errors

Resolving errors is fundamental to the performance of a GNSS receiver. There is no one best GNSS correction method to resolve GNSS errors, only a method that best suits the intended application. The methods employed depend on the unique requirements of each application, such as level of accuracy, system complexity, solution availability, reliability and cost. Techniques to resolve errors fall within one of two camps:

1. Modelling of the phenomena that are causing the errors and estimating the correction values.
2. Reducing or removing the error sources by differencing between receivers.

A GNSS receiver cannot correct errors by itself. It requires data from an external source. This is where Correction Services come into play. While our An Introduction to GNSS book covers many different approaches to resolving errors, Hexagon | NovAtel is an expert in providing Correction Services, including precise point positioning (PPP) and real-time kinematics (RTK)  solutions for land, airborne and marine applications. Let's take a brief look at both.

GNSS corrections through real-time kinematics

RTK is a technique that uses fixed base station(s) to reduce the user’s (rover) position error, often to just centimeters. It uses carrier-based ranging, which provides ranges (and therefore positions) that are more precise compared to code-based positioning. The calculated ranges still include errors caused by the sources noted previously. Because the base stations in this network are in precise locations, they can calculate the data errors they receive from the satellite signals with a high degree of accuracy. From there, the base stations transmit in real-time this corrected data to RTK enabled rovers/receivers. Rovers will compare their own signal measurements with the additional measurements they receive from the base station and use these corrected measurements to determine their position.

For large geographic areas, RTK requires significant infrastructure of base stations, making it an expensive option. While highly accurate, there are still limitations to RTK corrections including the number of and distance between base stations, system set-up, signal obstructions and environmental conditions. There are subscription services that streamline setup and management of RTK infrastructure, like HxGN SmartNet+ or networks managed through governmental agencies.  

Learn more about RTK.

GNSS corrections through precise point positioning

PPP is a positioning technique that models GNSS errors to provide a high level of position accuracy from a single receiver. The error sources impacting PPP are mitigated by modelling, estimating and applying external corrections.

A PPP solution depends on a set of corrections generated from a network of global reference stations. Once the corrections are calculated, they are delivered to the end-user via satellite or over the Internet. These corrections are used by the receiver to improve position accuracy.

The drawback to PPP is a slower signal correction time, known as convergence time, although recent advancements have decreased this time significantly. PPP is ideal for applications that can wait for positioning information or in locations that cannot support the base station infrastructure required for RTK. 

Learn more about PPP.

GNSS corrections play a very important role in ensuring the delivery of accurate, available, efficient, repeatable, scalable, secure and fast positioning. That is why NovAtel's Correction Services transmit GPS and GNSS correction data in real-time, with minimal latency, directly to your NovAtel receiver via satellite link or Internet connection to provide assured, reliable positioning anywhere, anytime.