What are the lateral sensitivities of GPS approaches?

Question

What are the lateral sensitivities of each of the WAAS and LNAV approach? Is the lateral sensitivity of WAAS the same as that of the localizer, which is 3 to 6 degrees? What about the LNAV approach?

Answer 1

Check out AIM 1-1-9 for localizer information. Basically, the localizer is positioned so to have a course width of 700' at the threshold. So full deflection to either side is 350'. A CDI has 5 dots on each side, so at the threshold each dot is 70'. Near the Middle Marker, one dot means you’re off course by 150 ft. Near the Outer Marker, a one-dot deviation puts you about 500 ft. from the centerline.

GPS (non-WAAS) approaches have a constant full scale deflection of .3 NM after the final approach fix.

AIM 1−1−17 5 (e) (5) When within 2 NM of the Final Approach Waypoint (FAWP) with the approach mode armed, the approach mode will switch to active, which results in RAIM and CDI changing to approach sensitivity. Beginning 2 NM prior to the FAWP, the full scale CDI sensitivity will smoothly change from ±1 NM to ±0.3 NM at the FAWP. As sensitivity changes from ±1 NM to ±0.3 NM approaching the FAWP,…

WAAS on the other hand is similar to the localizer.

AIM 1-1-18 (d) 4 Both lateral and vertical scaling for the LNAV/VNAV and LPV approach procedures are different than the linear scaling of basic GPS. When the complete published procedure is flown, ±1 NM linear scaling is provided until two (2) NM prior to the FAF, where the sensitivity increases to be similar to the angular scaling of an ILS. There are two differ- ences in the WAAS scaling and ILS: 1) on long final approach segments, the initial scaling will be ±0.3 NM to achieve equivalent performance to GPS (and better than ILS, which is less sensitive far from the runway); 2) close to the runway threshold, the scaling changes to linear instead of continuing to become more sensitive. The width of the final approach course is tailored so that the total width is usually 700 feet at the runway threshold. Since the origin point of the lateral splay for the angular portion of the final is not fixed due to antenna placement like localizer, the splay angle can remain fixed, making a consistent width of final for aircraft being vectored onto the final approach course on different length runways. When the complete published procedure is not flown, and instead the aircraft needs to capture the extended final approach course similar to ILS, the vector to final (VTF) mode is used. Under VTF, the scaling is linear at ±1 NM until the point where the ILS angular splay reaches a width of ±1 NM regardless of the distance from the FAWP.