Evolution of the United States National Airspace: The Move Towards Performance-Based Navigation
Satellite Navigations Contribution to RNAV/RNP
RNAV/RNP implementation depends on the availability of highly accurate navigation signals.. A number of available technologies are capable of supporting RNAV such as inertial navigation systems utilizing DME/DME corrections and GNSS. While inertial navigation systems provide amazing capability, they are primarily limited to high-end commercial and business jets due to their costs. For the remainder of the aviation fleet, GNSS remains the most viable option.
One of the most basic RNAV procedures was lateral navigation/vertical navigation procedures (LNAV/VNAV). LNAV/VNAV procedures were initially created to provide area navigation (RNAV) to users of GPS. This in of itself was a giant leap forward. Unlike other non-precision approaches, LNAV/VNAV provides vertical guidance. Numerous studies of aviation accidents have determined that vertically guided approaches are inherently safer than approaches without vertical guidance. Approaches without vertical guidance require the pilot to fly at a prescribed altitude until they cross a certain point, usually a navigation aid. They are then cleared to descend further until the next point. This “dive and drive” method is much more likely to cause an accident compared to the stabilized descent of a vertical approach.
Upon commissioning of WAAS in the United States, pilots in the continental United States and portions of Alaska were able to use all of the existing lateral navigation/vertical navigation (LNAV/VNAV) approaches. LNAV/VNAV approaches let pilots fly stabilized vertical descents into airports down to approximately 350 feet above the threshold of the runway without the need for an instrument landing system (ILS). Previously, pilots had to fly aircraft equipped with a GPS receiver and a barometric altimeter; WAAS receivers eliminated the need for the altimeter.
As RNAV procedures evolved, it quickly became apparent that LNAV/VNAV procedures failed to take advantage of the greater accuracy provided by SBAS. LNAV/VNAV approaches were created with GPS tolerances in mind; consequently, the protected areas (a bubble in space around the plane that must be free of obstacles) are rather large. A new procedure was developed that could take advantage of the accuracy gains provided by WAAS. The LPV procedure was born. LPV procedures are very similar to LNAV/VNAV approaches with one great benefit; they can get you much lower. Pilots can descend to as low as 250 feet using a LPV approach, only 50 feet shy of the current gold standard in navigation, the Category I precision approach. LPV approaches are able to shrink the size of this protected area without compromising safety. The LPV procedures remains a great example of how new technology can be leveraged to provide additional capabilities at a low cost.