What Is WAAS / LPV?

January 2015

I get a lot of questions about WAAS and LPV. The following is what you need to know about WAAS/LPV for corporate aircraft.

What is WAAS?

WAAS is an extremely accurate navigation system that utilizes a combination of global positioning satellites and geostationary satellites to improve the GPS navigational service. It stands for "Wide Area Augmentation System."

Satellite-based navigation fits within the NextGen framework and provides the same capability as a 60-year old Cat-1 ILS type of approach but to more runways.

How Accurate Is WAAS?

WAAS has an accuracy to within one to two meters. That's about as accurate as you can get.

The WAAS Network

The WAAS Network uses over 25 precision ground stations to provide corrections to the GPS navigation signal. The network of precisely surveyed ground reference stations is strategically positioned across the country including Alaska, Hawaii, Puerto Rico, Canada and Mexico to collect GPS satellite data. Using this collected error information, a message is developed to correct any signal errors. These correction messages are then broadcast through communication satellites to the airborne GPS receiver using the same frequency as GPS.

The WAAS message improves the accuracy, availability and safety of the GPS.

What is WAAS Coverage?

WAAS coverage includes the United States, from Alaska all the way down to Latin America and part of the Caribbean.

Is WAAS Worldwide?

No.

WAAS is a regional augmentation. However, there are several countries working on similar space-based systems that will be compatible with WAAS avionics.

Region: Name of system
Europe: EGNOS
Japan: MSAS
India: GAGAN

All of those units should be compatible with what is in the United States.

What are LNAV/VNAV, LPV and LP approaches?

LNAV is a non-precision approach. It uses GPS and/or WAAS for lateral navigation, but with no vertical guidance. LNAV procedures achieve a minimum descent altitude of 400 feet above the runway.

LNAV/VNAV is also a non-precision approach. It provides lateral guidance from GPS and/or WAAS and vertical guidance from a barometric altimeter or WAAS. Aircraft without WAAS must have a VNAV altimeter. The decision altitudes on these approaches are usually 350 feet above the runway.

LPV is the most desired approach. It stands for Localizer Performance with Vertical Guidance and can only be used with a WAAS receiver. It is similar to LNAV/VNAV except it is much more precise enabling a descent to as low as 200-250 feet above the runway.

LP is an approach that uses the high precision of LPV for lateral guidance and a barometric altimeter data for vertical. These approaches are needed at runways where, due to obstacles or other infrastructure limitations, a vertically guided approach (LPV or LNAV/VNAV) cannot be published. LP approaches can only be flown by aircraft equipped with WAAS receivers. The minimum descent altitude for an LP approach is 300 feet above the runway.

If I install WAAS-capable equipment, can I fly LPV approaches?

No.

There is a lot more required to a WAAS installation than can be conducted under a straight field approval. After installation, all equipment in the airplane must be tested for proper operation, including the autopilot, scaling and anything else impacted. Most WAAS receivers are installed under an STC.

WAAS-capable avionics do not automatically mean you are able to fly to an LPV minimum. LPV minimums require dual WAAS receivers that are under TSO 145/146.

Can I upgrade my existing navigational system to a WAAS receiver?

Current systems have completely different criteria and are certified under TSO C129. Units certified under TSO C145 / 146 are certified as standalone receivers. That means no other signal needs to go into that box in order to give it the accuracy readings on your aircraft instruments. It also requires an antenna change. The antennas from a TSO 129 box are different from those certified on a TSO 145 / 146.

Installation is performed by STC and requires the following:

• Dual GPS receivers. This is not an FAA requirement. It is per the manufacturer's specifications.
• Other equipment mods, such as the scaling and autopilot.
• Annunciation, whether it's external or on an EFIS system.
• A flight test.

The WAAS/LPV Approach Plate

An LPV approach will be called out on an approach plate with the words "WAAS Approach." And just like any other approach (GPS, VOR or ILS), it calls for particular criteria. Therefore you are able to take the approach appropriate to that airport, channel it up in your receiver and fly the approach.