Satellite Based Augmentation Systems

Description

Satellite Based Augmentation Systems (SBAS) are regional or wide-area systems that enhance the performance of core Global Navigation Satellite System (GNSS) constellations like GPS, GLONASS, or Galileo. They operate by deploying a network of precisely located ground reference stations that monitor GNSS satellite signals. These stations collect data on errors caused by ionospheric disturbances, satellite clock drift, and ephemeris inaccuracies. The data is processed at a central master station to generate differential correction messages and integrity information. These correction messages are then uplinked to geostationary (GEO) satellites, which broadcast them over a wide coverage area to user equipment (UE). The UE receives both the standard GNSS signals and the SBAS correction signals, applying the corrections in real-time to compute a more accurate and reliable position fix.

In the context of 3GPP standards, SBAS is integrated as a supported positioning method, particularly for Assisted GNSS (A-GNSS). The network can provide assistance data to the UE, which may include SBAS-specific information such as the identities of available SBAS satellites (e.g., WAAS, EGNOS, MSAS) and their signal characteristics. This assistance helps the UE acquire SBAS signals faster and with lower power consumption. The UE's location measurement unit processes the combined GNSS and SBAS signals to produce positioning measurements, which are reported back to the network via protocols like LTE Positioning Protocol (LPP) or NR Positioning Protocol (NRPPa).

The role of SBAS in 3GPP networks is primarily to meet stringent requirements for positioning services, especially for regulatory mandates like emergency caller location. SBAS significantly improves horizontal and vertical accuracy, often bringing it down to the meter-level range. It also provides vital integrity information, alerting the user if the system should not be used for safety-critical applications due to detected errors. This makes SBAS a key enabler for advanced location-based services, vehicular communications, and applications requiring high reliability, complementing other 3GPP positioning methods like Observed Time Difference of Arrival (OTDOA) and uplink Time Difference of Arrival (UTDOA).

Purpose & Motivation

SBAS was created to address the inherent limitations of standalone GNSS, which can suffer from significant errors due to atmospheric effects, satellite clock inaccuracies, and orbital errors. These errors can degrade positional accuracy to tens of meters, which is insufficient for safety-critical applications like aviation, maritime navigation, and increasingly, terrestrial applications such as autonomous driving and precise emergency services. Prior to augmentation systems, users had to rely on local differential correction stations, which offered high accuracy but only over a very limited geographic area. SBAS solves this by providing wide-area or regional correction signals from geostationary satellites, making high-accuracy, integrity-assured positioning available over entire continents.

The integration of SBAS into 3GPP standards, starting from Release 8, was motivated by the growing regulatory and commercial demand for highly accurate and reliable mobile positioning. Regulations in regions like the United States (FCC E911) and Europe (E112) mandated increasingly accurate location information for emergency calls. While network-based and assisted-GNSS methods existed, SBAS offered a way to meet and exceed these accuracy requirements without dense infrastructure deployment. It provided a standardized method to leverage existing civil aviation and navigation infrastructure for telecommunications, enhancing the capabilities of mobile devices for location-based services, logistics, and public safety applications.