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What is GAST in Aviation? (Gbas Approach Service Type)

Updated: February 26, 2024

What is GBAS Approach Service Type (GAST)?

The GBAS Approach Service Type (GAST), also known as GBAS Approach Service, is a navigation system used in aviation that provides precision approach and landing guidance for aircraft. GBAS stands for Ground-Based Augmentation System, and it is designed to enhance the accuracy, integrity, and availability of the Global Navigation Satellite System (GNSS) signals. GAST is a specific category within the GBAS system that defines the types of approaches available to aircraft.

GBAS is an advanced technology that improves the reliability and performance of satellite-based navigation systems. It relies on a network of ground-based stations to transmit correction messages to aircraft, enabling them to determine their precise position in real-time. GAST, as a part of the GBAS system, offers different approach types that can be used by aircraft during their final approach and landing phases.

Types of GBAS Approach Service Type (GAST)

1. Non-Precision Approach (NPA)

The Non-Precision Approach (NPA) is the basic type of approach available within the GBAS system. It provides lateral guidance for aircraft during the approach phase but does not provide vertical guidance. In other words, it aids the pilot in maintaining the correct flight path laterally but does not provide information on the aircraft's descent angle.

During an NPA, the pilot uses the lateral guidance provided by the GBAS system to align the aircraft with the runway centerline. The pilot then uses other instruments, such as the altimeter and vertical speed indicator, to control the descent rate and ensure a safe landing. NPA approaches are typically used when the weather conditions are favorable and the runway has visual aids, such as runway lights and markings, to assist the pilot.

2. Precision Approach (PA)

The Precision Approach (PA) is a more advanced type of approach available within the GBAS system. It provides both lateral and vertical guidance for aircraft during the approach phase, allowing pilots to perform highly accurate and controlled descents. PA approaches are often used when the weather conditions are poor or when the runway does not have visual aids.

During a PA, the pilot uses the lateral and vertical guidance provided by the GBAS system to align the aircraft with the runway centerline and control the descent rate. This allows for a more precise and stable approach, reducing the risk of accidents or runway incursions. PA approaches can be particularly useful in low-visibility conditions, such as fog or heavy rain, where visual references may be limited.

3. Category II/III Approaches

Category II (CAT II) and Category III (CAT III) approaches are specialized types of precision approaches available within the GBAS system. These approaches are designed for use in extremely low visibility conditions, such as dense fog or heavy snowfall, where visual references are almost non-existent.

CAT II approaches provide guidance for aircraft to descend to a decision height (DH) of 100 feet above the runway, while CAT III approaches allow for even lower decision heights, sometimes as low as zero feet. These approaches rely on highly sophisticated instrumentation and autopilot systems to ensure the safe landing of the aircraft.

It is worth noting that CAT II and CAT III approaches require both the aircraft and the airport to meet specific criteria and certifications to ensure the highest level of safety. Airports must have the necessary infrastructure, including runway lighting systems and monitoring equipment, to support these advanced approaches.

Conclusion

The GBAS Approach Service Type (GAST) is an integral part of the GBAS system, providing different types of approaches for aircraft during their final approach and landing phases. Non-Precision Approaches (NPA) offer lateral guidance, while Precision Approaches (PA) provide both lateral and vertical guidance. Category II/III Approaches are specialized types of precision approaches designed for use in extremely low visibility conditions. The GBAS system, including GAST, plays a crucial role in enhancing the safety and accuracy of aircraft operations, especially during critical phases of flight.

For more information on GBAS and GAST, you can visit the Federal Aviation Administration (FAA) website.

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