 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. In fact, some services now provide centimeter level performance, in under a few minutes. This has brought PPP forward as an option for precision applications such as agriculture and automotive.
Choosing a GNSS Corrections Service
How do you choose which correction type to use?
When comparing correction types, you want to ask, what corrections are available near you? What are your hardware and infrastructure requirements? What kind of correction source reliability you need?
Each correction type has benefits and limitations, and their use ultimately depends on your application requirements. Some users choose to use multiple correction sources to provide extra coverage, redundancy and reliability.
Regardless of which service you choose, the additional information supplied by corrections all improve the GNSS position calculated by your receiver.
That concludes our section on reducing errors through GNSS equipment and correction services. Our next episode will describe how to use additional sensors like inertial navigation systems to enhance your position even further. If you’re ready to start learning these GNSS concepts, you can download our book An Introduction to GNSS.
Hexagon | NovAtel positioning product manager, agriculture, Haley Lawrance describes how inertial navigation systems can be combined with traditional GNSS for enhanced positioning in episode five of our on-demand webinar series. She demonstrates how INS use inertial measurement units to understand motion, and outlines several methods of sensor fusion.
Hello, and welcome to episode five of NovAtel’s webinar series on An Introduction to GNSS. My name is Haley Lawrance; I’m the positioning product manager, agriculture at NovAtel, part of Hexagon. Thanks for joining me!
So far, the series has demonstrated and explained calculations and technologies behind GNSS, so you now should have solid foundational understanding of satellite positioning. Last episode, Jenn reviewed how we resolve positioning errors through GNSS equipment and infrastructure. Anothe