Link: walter.bislins.ch/bloge/?page=Knowledge+Database&qs=%23131
#131 | 3/21/2020 | Author: Wolfie6020 | Type: Youtube | Keywords: Airplane, Approach, Calculations, Curvature, ILS, GPS, GNSS, Glide Slope
Some airports use GNSS (GPS) non precision approach (NPA) and ILS (radio beacon) precision approach (PA) vertical guidance for the final approach to the runway. ILS is a straight line approach, while GNSS is a curved line approach following the curvature of the earth on a glide slope angle. In the calculations of the distance from the runway to the final approach point the curvature of the earth is taken into account for both approach variants.
The United States Standard for Area Navigation (RNAV) (pdf); page 66
United States Standard for Terminal Instrument Procedures (TERPS) (pdf); page 79 and paragraph 98:
98. Precise final approach fix (PFAF). The PFAF is a calculated WGS84 geographic position located on the final approach course where the designed vertical path (NPA procedures) or glidepath (APV and PA procedures) intercepts the intermediate segment altitude (glidepath intercept altitude). The PFAF marks the beginning of the FAS. The calculation of the distance from LTP to PFAF includes the earth curvature.