The accuracy and integrity of GNSS can be greatly improved by using GNSS correction services. These services wirelessly transmit data containing real-time information on GNSS signal errors to GNSS receivers, enhancing their performance and precision, and enabling positioning accuracies down to and below the centimeter-level.
GNSS correction services generally fall into two categories, based either on the Observation State Representation (OSR) or a State Space Representation (SSR) of the errors. These groups use different techniques, delivery mechanisms, and core technologies to solve the same problem – the mitigation of key GNSS errors (e.g. clocks, orbits, biases, ionosphere, troposphere) in order to enable high precision GNSS performance.
Observation State Representation (OSR)
Observation Space Representation (OSR) correction services are supplied by the legacy GNSS correction services providers. The term observation is used as the core techniques are centered on transferring corrected GNSS observations from the nearest reference station to the rover.
OSR services typically use a standardized format in RTCM v 3.x over IP and the offerings are mainly B2C models focusing on a single geographic region (i.e. a country or state) targeting existing low volume, niche high precision markets (such as surveying, agriculture, machine control).
While OSR services deliver centimeter level accuracy provided the rover is within approximately 30 kilometers of the nearest reference station, they require two-way communication with high bandwidth, making them difficult to scale up to serve mass market applications.
List of companies providing OSR services:
- HxGN SmartNet
- Trimble VRSNow
- TopCon TopnetLive
- QXWZ FindCM
State Space Representation (SSR)
State Space Representation (SSR) correction services are the latest generation of GNSS correction services provided by new entrants and the larger legacy providers. The term state is used as the core techniques are centered on using the reference station network to model the key errors over large geographical regions and transferring them to the rover. The rover uses the incoming data to create a local model of the GNSS errors, which it applies directly to the GNSS observations. The performance of SSR correction services depends on what errors are transmitted to the rover device, which is detailed in the table below.
Furthermore, the broadcast nature of SSR corrections enables the service to be more easily distributed over IP and even L-Band satellite communication channels. In addition, providers of SSR services are focusing on scalable B2B models to meet the demands of high precision mass market applications such as in automotive field.
SSR services are able to achieve centimeter-level positioning accuracies over large geographic areas requiring only low bandwidth. Because all roving GNSS receivers rely on the same GNSS correction data stream, SSR services are a good fit for mass market applications.
List of companies providing SSR services:
- HxGN Terrastar X
- Sapcorda SAPA Premium
- Swift Navigation Skylark
- Trimble RTX Fast
- QZSS CLAS