What is SBAS?
The production of a user position by a GNSS receiver requires determining, at each instant, the satellite position and clock synchronization of each GNSS satellite. This is performed through prediction parameters which are broadcasted by the GNSS satellites to all user receivers through messages encoded into the GNSS signals. Such prediction parameters are in essence subject to inaccuracies, increasing over time.
Furthermore, the signals received from the GNSS satellites provide the user receivers with a measurement of the distance to each GNSS satellite, which is also necessary to compute the user position. However, as GNSS signals travel from space down to Earth and pass through the ionosphere (part of the Earth’s upper atmosphere), they can become delayed and distorted. If left uncorrected, this delay can significantly alter the accuracy of the measurements, resulting in positioning errors.
Satellite-based Augmentation Systems (SBAS) help resolve these GNSS positioning errors.
How SBAS works
SBAS improves the accuracy and reliability of GNSS positioning by correcting signal measurement errors and by providing integrity information allowing each user to get a highly reliable bound of its residual positioning error. In case such residual positioning error becomes too large, the user is alerted within a few seconds.
To do so, these regional systems use GNSS measurements taken by accurately located reference stations deployed across a country, region or continent.
All measured GNSS errors are transferred to a central computing centre, where differential corrections and integrity messages are calculated. These calculations are then broadcast over the covered area using geostationary satellites that serve as an augmentation, or overlay, to the original GNSS message. For integrity alert messages, this process is performed in less than 6 seconds.
Critical for safety-critical applications
SBAS is essential for applications where accuracy and integrity are critical.
In particular, SBAS is indispensable in situations where people’s lives are at stake or where a form of legal or commercial guarantee is required and GNSS is being used – such as is the case with aviation.
In the aviation sector, GPS does not satisfy the strict operational requirements set by the International Civil Aviation Organisation (ICAO) for use in such critical flight stages as final approaches. The addition of SBAS satisfies these requirements.
Beyond the aviation sector, SBAS brings an important added layer of accuracy to such applications as precision farming, on-road vehicle fleet management and geodesy – to name only a few.
SBAS near you
Many countries and regions have implemented their own Satellite-based Augmentation System. EGNOS is the European Union SBAS, covering the EU territory along with some neighbouring countries and regions.
In addition to EGNOS, several other SBAS are currently operational such as WAAS in the USA, GAGAN in India, MSAS in Japan or KASS in South Korea.
Each of these systems complies with a common global standard, meaning they are both:
- Compatible: the systems do not interfere with each other
- Interoperable: a user with a standard receiver can benefit from the same level of service and performance regardless of what coverage area they are in
Other systems are in different stages of development. These include BDSBAS (China), SDCM (Russia), ANGA (Africa) and SouthPAN (Australia and New Zealand).