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SBAS Fundamentals
| Fundamentals | |
|---|---|
| Title | SBAS Fundamentals |
| Author(s) | GMV |
| Level | Basic |
| Year of Publication | 2011 |
A [Satellite-based Augmentation System (SBAS) is a civil aviation safety-critical system that supports wide-area or regional augmentation – even continental scale - through the use of geostationary (GEO) satellites which broadcast the augmentation information[1][2]. A SBAS augments primary GNSS constellation(s) by providing GEO ranging, integrity and correction information. While the main goal of SBAS is to provide integrity assurance, it also increases the accuracy with position errors below 1 metre (1 sigma).
The ground infrastructure includes the accurately-surveyed sensor stations which receive the data from the primary GNSS satellites and a Central Processing Facility (CPF) which computes integrity, corrections and GEO ranging data forming the SBAS signal-in-space (SIS). The SBAS GEO satellites relay the SIS to the SBAS users which determine their position and time information. For this, they use measurements and satellite positions both from the primary GNSS constellation(s) and the SBAS GEO satellites and apply the SBAS correction data and its integrity.
The augmentation information provided by SBAS covers corrections and integrity for satellite position errors, satellite clock – time - errors and errors induced by the estimation of the delay of the signal while crossing the ionosphere. For the errors induced by the estimation of the delay caused by the troposphere and its integrity, the user applies a tropospheric delay model.
SBAS Performances
The SBAS performances are defined with respect to the level of service that the system is designed to. The main source for SBAS performances comes from civil aviation navigation safety requirements and they are different for each civil aviation operation (see Table ICAO GNSS performances requirements [3]).
| Typical Operation | Horizontal Accuracy (95%) | Vertical Accuracy (95%) | Integrity | Time-To-Alert (TTA) | Horizontal Alert Limit (HAL) | Vertical Alert Limit (HAL) | Continuity | Availability |
|---|---|---|---|---|---|---|---|---|
| En-route | 3.7 km (2.0 NM) | N/A | 1 – 1 × 10-7/h | 5 min | 1 – 1 × 10-4/h to 1 – 1 × 10-8/h | 0.99 to 0.99999 | ||
| En-route Terminal | 0.74 km (0.4 NM) | N/A | 1 – 1 × 10-7/h | 15 s | 1 – 1 × 10-4/h to 1 – 1 × 10-8/h | 0.99 to 0.99999 | ||
| Initial approach, Intermediate approach, Non-precision approach (NPA), Departure | 220 m (720 ft) | N/A | 1 –1x10-7/h | 10 s | 556 m (0.3 NM) | N/A | 1 – 1x10-4/h to 1 – 1x10-8/h | 0.99 to 0.99999 |
| Approach operations with vertical guidance (APV-I) | 16 m (52 ft) | 20 m (66 ft) | 1 – 2 × 10-7 per approach | 10 s | 1 – 8 × 10-6 in any 15 s | 0.99 to 0.99999 | ||
| Approach operations with vertical guidance (APV-II) | 16 m (52 ft) | 8 m (26 ft) | 1 – 2 × 10-7 per approach | 6 s | 1 – 8 × 10-6 in any 15 s | 0.99 to 0.99999 | ||
| Category I precisión Approach | 16 m (52 ft) | 6.0 m to 4.0 m (20 ft to 13 ft) | 1 – 2 × 10-7 per approach | 6 s | 1 – 8 × 10-6 in any 15 s | 0.99 to 0.99999 |
Notes