What is GNSS Augmentation?

GNSS Augmentation is used by GNSS receivers to provide more accurate positioning and timing results. GNSS receivers can determine their location (longitude, latitude, and altitude) with an accuracy of a few meters using real-time signals transmitted by GNSS satellites. However, these signals can be further supported by SBAS (Satellite-based Augmentation System), Ground-based Augmentation System (GBAS) or Differential GNSS (DGNSS). These augmentation systems can be used to improve positioning, timing and navigation accuracy.

Augmented GNSS can be classified into various categories depending on the augmentation provided from the ground, satellite, or using a network of reference stations.

Satellite-based Augmentation System (SBAS)

A satellite-based augmentation system (SBAS) enhances GNSS performance by providing ranging, integrity, and correction information. In SBAS, the ground infrastructure consists of accurately surveyed and geographically distributed sensor stations that receive the data from the primary GNSS satellites and a Central Processing Facility (CPF) that computes integrity, corrections, and ranging data forming the SBAS signal-in-space (SIS). Geosynchronous Earth Orbit (GEO) satellites then relay the SIS to the receiving devices which determine their position and time information by using the measurements and satellite positions from both the primary GNSS constellation(s) and the SBAS GEO satellites. 

Almost all developed and developing countries have an SBAS of their own. 

Some of them are as follows:

1. The Wide Area Augmentation System (WAAS) – USA
2. The European Geostationary Navigation Overlay Service (EGNOS) – European Space Agency Program
3. The Multi-functional Satellite Augmentation System – Japan
4. GPS-aided GEO augmented navigation (GAGAN) – India
5. The System for Differential Corrections and Monitoring – Russia
6. The Satellite Navigation Augmentation System (SNAS) – China
7. The Southern Positioning Augmentation Network (SouthPAN) – New Zealand and Australia. The program is expected to be fully functional in 2022. 

Ground-Based Augmentation System (GBAS)

GBAS is primarily used in aviation, and it enhances GNSS service levels during approach, landing, departure phases, and also for surface operations. GBAS aims to enhance the accuracy and integrity of location data. It is usually installed near an airport and consists of two or more GNSS receivers, GNSS antennas, a central processing facility, a Very High Frequency (VHF) antenna, and a supporting periphery. The GBAS ground facility broadcasts an updated correction message two times every second. The message contains the corrections, integrity parameters, GBAS ground facility characteristics, and approach path guidance for all installed approaches.

GBAS receives the signal from GNSS satellites through reference antennas. The receivers measure the time of transmission between the GPS satellite and reference antennas to determine the distance of the signal traveled. The GBAS Ground Facility then compares the estimated distance with the actual distance based on the broadcasted satellite position and the true GPS reference receiver’s position and determines the error in the measurement. This exchange of information is done in the VHF band. 

When the ground facility of GPS finds out any discrepancy in the communication from the GPS satellite, it stops broadcasting corrections for that particular satellite, thus preventing the GBAS avionics from using the satellite. These systems are so precise that the likelihood of the actual error being larger than the computed protection level is less than 1 in 10 million.

Differential GNSS (DGNSS)

DGNSS aims to enhance the correction of any errors and inaccuracies in the GNSS system. A DGNSS receiver has the potential to achieve accuracies of up to 10 centimeters. The base receiver is set up at a precise, known location and the other receiver used for positioning is called a roving receiver. The base receiver calculates the difference between its position as calculated by GNSS satellites and its actual, known position. This difference is the correction factor which is then broadcasted to the roving receivers to update its measurements.

There are many DGNSS models such as:

Real-Time Kinematic (RTK): RTK is based on code and carrier phase measurements from the primary GNSS satellites. Although it requires complex and sophisticated machinery, it can reach centimeter-level accuracy of positioning.

Precise Point Positioning (PPP): PPP requires real-time data from reference GNSS orbits and clocks. PPP combines the precise satellite positions and clocks with a dual-frequency GNSS receiver to achieve pinpoint positioning solutions ranging from centimeter to decimeter level.