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
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.