GNSS Correction Services: SSR vs. OSR
The inherent limitations of raw Global Navigation Satellite System (GNSS) signals, such as atmospheric delays, satellite clock and orbit errors, and multipath effects, prevent standalone GNSS receivers from achieving the centimeter-level accuracy required for many advanced applications. To overcome these limitations, GNSS correction services have emerged as a critical component of high-precision positioning. These services provide real-time or post-processed data that allows GNSS receivers to mitigate errors and achieve significantly enhanced accuracy. The two primary approaches to delivering these corrections are **Observation Space Representation (OSR)** and **State Space Representation (SSR)**.
A. OSR (Observation Space Representation) / RTK Networks
Observation Space Representation (OSR) is the traditional approach for transmitting GNSS corrections, most notably employed in **Real-Time Kinematic (RTK)** positioning. OSR corrections group various errors together, providing total correction measurements rather than corrections for individual error parameters. This method requires a two-way communication channel for each user and has high bandwidth requirements, which can limit its scalability.
- Concepts of RTK Networks: Single-base RTK techniques are limited by the distance between the base receiver and the rover receiver due to distance-dependent biases, specifically orbit bias, ionosphere bias, and troposphere bias. To overcome these limitations, many commercial and government organizations have deployed dense networks of permanently operating reference stations (CORS).
B. SSR (State Space Representation) / PPP Corrections
**State Space Representation (SSR)** is an alternative to OSR that provides individual error components (e.g., satellite orbit, clock, and atmospheric biases) rather than a lumped correction. SSR is the basis for **Precise Point Positioning (PPP)**, a technique that allows a single receiver to achieve high-precision positions without the need for a local base station or network.
- Global Scalability: Because the corrections are location-independent, a single SSR stream can serve an unlimited number of users globally, delivered via satellite or cellular data.
- Lower Bandwidth: SSR requires significantly less bandwidth than OSR, making it ideal for satellite or cellular data delivery.
C. Hybrid Correction Approaches
The industry is moving toward hybrid solutions that combine the strengths of both OSR and SSR. Hybrid systems can provide the fast convergence of RTK with the global reach of PPP, offering the best of both worlds.
D. GNSS Correction Services Landscape
The GNSS correction services market is home to both private and public providers. Some notable players include:
- Hexagon | NovAtel: Offers a range of PPP and RTK correction services, including TerraStar-C PRO and TerraStar-X.
- Trimble: Provides Trimble CenterPoint RTX, a satellite-delivered SSR service.
- u-blox: Provides PointPerfect, a GNSS augmentation service.