es, which your receiver uses to improve the quality of its GNSS position. This ultimately provides a more accurate and reliable solution.
The three commonly used correction services we’ll focus on here are: Satellite-Based Augmentation Systems, Real-Time Kinematic, and Precise Point Positioning.
How Corrections Services Work
By this point in our series, we’ve thoroughly discussed the basic relationship between GNSS satellites and transmission of data to user equipment. The signals travel from the satellites and are tracked by receivers on the ground. When a receiver is established on a fixed coordinate, this is called a base or reference station.
These base stations play a fundamental role in correction generation. First, we can get important information about error sources by comparing the measured base position against its known coordinate. Second, if you have many base stations, you can collect data across this network to get regional information and provide corrections over a large area.
Whether you’re using a single base station, or a network-based service, these corrections are delivered to the user’s equipment to improve the position accuracy.
Satellite-Based Augmentation System Corrections
Countries around the world have their own SBAS networks to supply corrections to regional users. As shown here, GNSS satellites are tracked by a network of reference stations on the ground, and this data is forwarded to a master station. The master station knows the coordinates of all the stations in the network, and so it computes corrections by comparing the measured positions to the known coordinates. This technique reduces atmospheric, satellite timing, and orbital errors.
The corrections are then uplinked to geo-stationary SBAS satellites and broadcasted back to SBAS-enabled receivers on the ground. The corrections are freely available to all users in the covered region, providing nearly immediate meter-level accuracy.
Real-Time Kinematic Corrections
RTK is a regional solution requiring a base station near to where the user’s receiver – or rover- is operating. The base uses a radio to broadcast its well-known coordinate and carrier phase measurements to the rover, which differences these measurements to eliminate common errors.
RTK uses carrier phase techniques to achieve cm-level accuracy. With complicated algorithms, the rover leverages the corrections to quickly perform carrier phase ambiguity resolution and precisely determine its location relative to the base station.
Because of this fast and accurate performance, RTK provides an effective solution for high accuracy applications. However, as this is a differencing technique, there is a basic assumption that the conditions at the base and rover are very similar. As the rover travels further from the base, atmospheric conditions begin to vary between the two locations. As a result, good RTK performance is limited to a 20-30 km baseline.
Many users set up their own local base station on a surveyed coordinate. To extend RTK coverage, you can also access services from Network RTK providers who maintain regional RTK networks and broadcast corrections over the internet.
Precise Point Positioning Corrections
Precise Point Positioning (PPP) services rely on an expansive network of base stations to gather GNSS data. This data is sent to a central server to model clock and orbit errors, as well as pseudorange and carrier phase biases. The resulting corrections are transmitted to users by geostationary L-Band satellites; some services also send these corrections over the internet.
Once the corrections are downloaded, the receiver requires time to properly compute any remaining errors; this is called the ‘convergence time’. Because of the low-density network required for PPP, most services are available globally. This provides a benefit to users covering large application areas, or who are operating in remote regions.
PPP has historically provided an accuracy in the decimeter range, and convergence times of over 45 minutes. But advances in technology are improving this performance