Application of LIDAR-based F-factor in windshear alerting
The use of windshear hazard factor, also called F-factor, in the alerting of low-level windshear at the Hong Kong International Airport (HKIA) is studied in this paper, by calculating the F-factor using the headwind profiles measured by Doppler Light Detection And Ranging (LIDAR) systems. The LIDAR headwind profiles are processed through a flight simulator. The resulting winds are used to calculate the headwind gradient term of F-factor. The quality of such LIDAR-based F-factor is first studied by comparing with the aircraft-based F-factor. It turns out that, as long as the headwind data from the LIDAR and the aircraft have good correlation with each other, the F-factors from the two platforms are correlated well at the same time. Among the two commonly used arrival runway corridors of HKIA, the arrival corridor to the north runway of HKIA (location in Figure 1) from the west has better correlation for the F-factor. The performance of LIDAR-based F-factor in the alerting of low-level windshear is then studied by using the pilot windshear reports as the sky truth. The optimum threshold for F-factor alerting is established by striking a balance between the probability of detection (POD) of pilot windshear reports and the percentage of time on alert (POTA). It turns out that the negative maximum F-factor has better performance in the alerting of windshear than the positive F-factor value. The POD could reach 87 % wind a POTA of less than 10 %, which is suitable for operational use in the alerting of low-level windshear. The optimum F-factor threshold is about -0.05, which is smaller than the conventional value of -0.105 commonly adopted for onboard reactive windshear alerting system. The performance of LIDAR-based F-factor in the alerting of stronger windshear, namely, pilot reports of windshear magnitude of 25 knots (12.9 m/s) or 30 knots (15.4 m/s) or more, is also discussed in this paper.