Code-Carrier Divergence Monitoring for GAST-F GBAS

The ground-based augmentation system (GBAS) includes a ground monitor designed to protect against a code carrier divergence (CCD) fault originating from the satellite payload. The current single-frequency GBAS solutions known as GBAS approach service types (GAST) C and D which support Category I and Category II/III precision approaches, respectively, both utilize this monitor, but it has been noted that the test metric is subject to non-Gaussian tails as a result of nominal ionospheric errors. It has been observed that the ionospheric delay seen at low elevations can trigger alarms which are not distinguishable from the payload fault and thus impact the continuity and availability of GAST-D. The future GAST-F concept is designed to meet Category II/III precision approach using multi-frequency measurements which allows the CCD monitor proposed to be free of ionospheric influence. In order to address the full threat space, the combination of three ionospheric-free statistics is needed to form the test metric. This test metric is characterized through a combination of empirical multi-frequency data analysis and theoretical derivations leading to an approximately diagonal covariance matrix consisting of standard deviations 0.0017, 0.0050 and 0.0046 m/s, compared to 0.00399 m/s for the current single-frequency GAST-D design. Results from extensive simulations assessing the monitor’s integrity performance are then provided which show superior performance to the existing design. The proposed GAST-F monitor detects a divergence less than half the size of the GAST-D one, with the same probability of missed detection. Under the GAST-F concept, the aircraft may be operating in ionospheric-free smoothing mode which leads to inflation of the divergence impacts for much of the threat space and degrades the performance of all potential CCD monitors. It is shown that a longer delay is required for the incorporation of a smoothed ranging measurement into the solution. The worst-case fault mode requires a delay of 132 s over the current value 50 s.