Learning Center Library Contents

Down Arrow
Welcome Guest
Aviation Learning Center Document GPS Approach Minima - How Low Can You Go?
Author: Martin Heller Date: July 2006
used for alignment
used for alignment
Viewing Options: View Document as Chapters Chaptersused for alignment View Full Document Full Documentused for alignment View Print-Friendly Document Printer Friendlyused for alignment Search Inside this Document Search Insideused for alignment View PDF Version of Document PDFused for alignment

< Previous Chapter used for alignment Next Chapter > used for alignment

WAAS - 2003
used for alignment

The Wide Area Augmentation System or WAAS is a major improvement to GPS. A combination of 25 WAAS ground reference stations (WRS) monitor the GPS constellation signals and send corrections through two WAAS Master Stations (WMS) up to two geosynchronous satellites. These satellites then transmit the corrections to a WAAS enabled GPS receiver. More WRS are being installed in Alaska (4), Canada (4) and Mexico (5) to improve Northern Hemisphere coverage.

WAAS provides several advantages. First, the geosynchronous satellites provide additional ranging signals into the WAAS enabled receiver, increasing GPS system coverage and availability. Since WAAS monitors and corrects variations in the GPS positioning, the system is much more accurate with smaller alert limits. This smaller integrity limit supports the current generation of GPS approaches, Localizer Performance with Vertical guidance (LPV). Another advantage is that it allows WAAS-equipped users to be able to flight plan and file for alternate airfields with GPS-based approaches. (Note: This includes any procedure with GPS in the title.)


Similar to LNAV/VNAV and ILS approaches, LPV procedures evaluate the Glideslope Qualification Surface. Because of the smaller integrity limit and angular guidance, the size of the obstacle trapezoid is smaller than LNAV/VNAV. In 2003, the minimum height above touchdown (HAT) value was established at 250 feet In March 2006, it was announced that the WAAS minimum HAT would be lowered to 200 feet if all other airport infrastructure requirements are met. The first procedures to the lower minima should appear in 2007.


Another major improvement is WAAS alerting. The WAAS horizontal integrity limit is 40 meters on final as opposed to 556 meters for basic GPS. More importantly, WAAS provides vertical integrity, which GPS does not. WAAS eliminates the requirements for RAIM predictions, but crews still must check WAAS NOTAMs. Additionally, on procedures with an inverse W, crews must plan using non-precision approach requirements since vertical NOTAMs are not provided. The inverse W symbols will be removed as the vertical signal availability improves at airports. Future improvements will result from the planned addition of WAAS Reference Stations which will extend and improve WAAS service. Avionics equipment guidance is found in TSO-C145 (http://www.airweb.faa.gov/Regulatory_and. . .537BBC6286256DAD00643CE3?OpenDocument) and TSO-C146. (http://www.airweb.faa.gov/Regulatory_and. . .2EE5750A86256DAD00643D48?OpenDocument)

Pilots can fly the following minima with an appropriately certified WAAS receiver: LPV, LNAV/VNAV, and LNAV. Why would one fly LNAV/VNAV or LNAV minima if they could fly LPV? The reason is that some GPS and RNAV(GPS) approaches have LNAV/VNAV, but not LPV minima. Also, if the WAAS system has an outage, the pilot can still fly the LNAV portion. Think of flying the localizer only approach when the ILS glideslope is out of service. There are approximately 400 LPV approaches already published and a production goal of 300 more LPV approaches each year.

used for alignment