WAAS and LPV
WAAS and LPV are two important navigation tools used to supplement GPS in aircraft. WAAS, or Wide Area Augmentation System, is an air navigation aid developed by the Federal Aviation Administration to improve the accuracy, integrity, and availability of the Global Positioning System. LPV, or localizer performance with vertical guidance, are the precision GPS aviation instrument approach systems available without specialized aircrew training requirements. This blog will discuss both WAAS and LPV in greater detail.
WAAS is a highly accurate navigation system that combines the use of global positioning satellites with geostationary satellites to improve the navigational services of GPS. WAAS has an accuracy within one to two meters, virtually as accurate as any system could hope to be. The WAAS network comprises more than 25 precision ground stations to provide corrections to the GPS navigation signal. The network of precisely surveyed ground reference stations are tactically positioned across the continental United States, as well as in Alaska, Hawaii, Puerto Rico, Canada, and Mexico, where they continually collect and process data.
After collecting this data, a message is developed to correct any signal errors. These correction messages are then broadcast through communication satellites to the airborne GPS receiver through the same frequency as GPS. WAAS messages improve the accuracy, availability, and reliability of the GPS. Coverage ranges from Alaska all the way down to much of Latin America and parts of the Carribean. Though WAAS is a regional augmentation (meaning it is not worldwide), many other regions have an equivalent system to WAAS avionics. For example, Europe has a system called EGNOS, Japan has MSAS, and India has GAGAN, all of which are compatible with WAAS.
The other main GPS systems are LNAV/VNAV, LPV, and LP. LNAV is a non-precise approach that utilizes GPS, WAAS, or both for lateral navigation, but lacks a means of vertical guidance. These procedures can obtain a minimum descent altitude of 400 feet above the runway. LNAV/VNAV is another non-precise system. This offers lateral guidance from GPS or WAAS as well as vertical guidance from a barometric altimeter or WAAS. aircraft without WAAS can never use a VNAV altimeter.
LPV is the most desired of all these approach systems. This can only be used in conjunction with a WAAS receiver. It is similar to the LNAV/VNAV procedure, but is much more precise; it enables 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 with barometric altimeter data for vertical. These approaches are needed at runways where a vertically guided approach cannot be done, usually due to obstacles or infrastructure limitations. LP approaches can only be done by aircraft equipped with WAAS receivers. 300 feet above the runway is the minimum descent altitude for an LP approach.
An LPV approach will be called out on an approach plate with the words "WAAS Approach." Similar to other approaches, (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.
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