GPS is powerful, but raw GPS alone is not always accurate or reliable enough for the most demanding phases of flight. Aviation uses Satellite-Based Augmentation Systems, or SBAS, to improve position accuracy, integrity, availability, and continuity. In the United States, the Wide Area Augmentation System, or WAAS, is the SBAS that helps aircraft fly precise lateral and vertical guidance to many runways.
This matters because accurate guidance can improve safety, increase airport access, and reduce the need for expensive ground-based landing equipment.
SBAS works by comparing GPS signals received at accurately surveyed ground reference stations with the stations' known positions. The system computes corrections for satellite clock errors, orbital errors, and ionospheric delay, then sends those corrections to geostationary satellites. Aircraft receive both the normal GPS signals and the SBAS correction message, allowing the receiver to calculate a more accurate and trustworthy position.
This corrected navigation can support LPV approaches, which feel similar to an instrument landing system even though the guidance comes from satellites.
Key Facts
- SBAS = Satellite-Based Augmentation System, a system that improves GPS accuracy and integrity for aviation.
- WAAS = Wide Area Augmentation System, the SBAS used in the United States and nearby regions.
- Corrected position = GPS position + SBAS error corrections.
- Major GPS errors corrected by SBAS include satellite clock error, satellite orbit error, and ionospheric delay.
- Position error can be described as error = measured position - true position.
- LPV approaches use SBAS to provide lateral and vertical guidance, but they do not require a ground-based glide slope transmitter.
Vocabulary
- SBAS
- A Satellite-Based Augmentation System is a regional system that improves GNSS navigation by broadcasting corrections and integrity information.
- WAAS
- The Wide Area Augmentation System is the United States SBAS that improves GPS performance for aircraft navigation and approaches.
- Reference station
- A reference station is a precisely surveyed ground receiver that measures GPS errors by comparing received signals with its known location.
- Integrity
- Integrity is the ability of a navigation system to warn the pilot quickly if the position information should not be used.
- LPV approach
- An LPV approach is a satellite-guided instrument approach that provides localizer-like lateral guidance and vertical guidance using SBAS.
Common Mistakes to Avoid
- Treating WAAS as a separate replacement for GPS is wrong because WAAS augments GPS by adding correction and integrity data.
- Assuming SBAS only improves horizontal accuracy is wrong because it can also support vertical guidance for approved approaches such as LPV.
- Forgetting the role of ground reference stations is wrong because SBAS corrections come from comparing GPS measurements with precisely known ground locations.
- Thinking every GPS receiver can fly WAAS approaches is wrong because the aircraft must have approved SBAS-capable avionics and the approach must be authorized.
Practice Questions
- 1 A raw GPS position has a horizontal error of 12 m. After WAAS corrections, the error is 2 m. By what factor did the error decrease, and what percent reduction is this?
- 2 A GPS signal travels about 20,200 km from a satellite to an aircraft. If an uncorrected timing error causes a range error of 9 m, what timing error caused it? Use c = 3.0 x 10^8 m/s and time = distance/c.
- 3 An aircraft on final approach receives GPS satellite signals and an SBAS correction message from a geostationary satellite. Explain why surveyed ground reference stations are still essential to the system.