The Major Advantage of This Upgrade Is Ability to Generate a GPS Based Glideslope (LPV
What’s WAAS?
By
O.C. Hope
Reviewing some of the latest IAP changes will reveal new approaches to some of our common domestic airports. These areRNAV/GPS approaches which reflect the improved accuracy capabilities of WAAS (Wide Area Augmentation System) enhanced GPS signals. These signals,available over the continental U.S., are accurate to less than 1 meter horizontally and 1.5 meters vertically, and are certified for navigation performance of 10 meters horizontally and vertically. There is insufficient space here to go into the details of “how it works,” but for those of you interested, a Google search for “WAAS” will reveal more than can be digested in one sitting.
The major advantage of this upgrade is ability to generate a GPS based “glideslope” (LPV – Localizer Precision with Vertical guidance) anchored in space to the runway – any runway. These approaches are currently certified to take properly equipped aircraft to DAs as low as 250’ AGLand as low as 200’ AGL in the near future.
WAAS capable avionics such as the Garmin430/530 will perform an internal integrity check at the beginning of the approach and display the capability level for the approach selected as LPV, L/VNAV, or LNAV. Pilots select the appropriate minimums from the chart based on this indication.
When viewing the approach plates for these new approaches, a few differences become apparent. Reference the PROVO RNAV (GPS) RWY 13 chart.
Note the reference to the WAAS channel and identifier for this approach (Ch 97307 W-13A). This is tuned internally by our current systems. We simply select the proper approach from the PROC/APR menu of the GPS system.
The most important difference for pilots is in the Straight-in Landing Minimums portion of the Chart. Three sets of minimums are depicted with their associated meaning:
LPV 4770’(273’) DA –WAAS based glide slope is depicted on the HSI/CDI – in magenta for glass panel displays. This Precision approach is flown just like an ILS.
LNAV/VNAV 4940’(443’)DA -This mode is available in FMSs and otherWAAS capable GPS boxes. It provides an internally calculated descent path (sometimes called a gradient path) depicted by a glide slope indicator similar to the LPV glide slope.
LNAV 5040’ (543’) MDA - Used by the rest of the pilots withLNAV only (unenhanced GPS) steering.
Some of the earliest WAAS IAPs, such as the KOGD RNAV (GPS) Z RWY 03, were depicted as an entirely separate approach from the established RNAV approaches. These were labeled RNAV Z and had only LPV minimums. The LNAV version of the approach was labeled RNAV Y, and had only LNAV minimums. As this required twice as much space in the nav databases (and twice as many pages of charts), the standard was revised to list only one RNAV (GPS) approach and simply list different minimums as is done with ILS/LOC approaches. These older depictions will be updated as changes to the approach necessitate.