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Aviation Learning Center Document GPS Approach Minima - How Low Can You Go?
Author: Martin Heller Date: July 2006
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GPS - Vertical Guidance
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The next system improvement added a calculation derived glide path. While not an electronic glideslope, vertical navigation (VNAV) guidance is displayed as a glideslope on the pilot's vertical deviation indicator. This capability came from a combination of Barometric (Baro) VNAV and GPS equipment. These GPS approaches provide both LNAV based on GPS and VNAV based on barometric sensing. (Note: Wide Area Augmentation System (WAAS) avionics approved for LNAV/VNAV can also fly these procedures without the Baro-VNAV temperature restrictions and local altimeter setting requirements.) VNAV allows for a more stabilized approach, flown like an ILS approach (but to higher minimums).


The ROC on final varies with distance from runway (minimum 250 feet) because the obstacle clearance is evaluated by a sloping obstacle surface rather than a set ROC value. While this occasionally results in minima higher than the LNAV minima, the added safety benefit of a stabilized descent outweighs the difference in minimums. Additionally, a glide path qualification surface (GQS) underlying the glide path from the threshold to the Decision Altitude (DA) point is evaluated to determine if the controlling obstacle's position will allow a vertically guided (LNAV/VNAV) approach to be constructed. The alerting process also uses Receiver Autonomous Integrity Monitoring (RAIM), or the WAAS avionics' integrity function. WAAS uses a complex integrity function based on information transmitted from the ground stations to the Telesat Geostationary Satellite (GEO) to the aircraft avionics.

LNAV/VNAV procedures require an approach certified barometric vertical guidance (Baro-VNAV) system; and a GPS or a WAAS system approved for LNAV/VNAV. This equipment must comply with (TSO)-C129, (http://www.airweb.faa.gov/Regulatory_and. . .6ACF8BA186256DC700717E0F?OpenDocument)Airborne Supplemental Navigation Equipment Using the Global Positioning System (GPS), or TSO-C145, (http://www.airweb.faa.gov/Regulatory_and. . .537BBC6286256DAD00643CE3?OpenDocument)Airborne Navigation Sensors Using the GPS Augmented by the Wide Area Augmentation System, or TSO-C146, (http://www.airweb.faa.gov/Regulatory_and. . .2EE5750A86256DAD00643D48?OpenDocument)Stand-Alone Airborne Navigation Equipment Using the GPS Augmented by the Wide Area Augmentation System. In addition, AC 20-130A, (http://www.airweb.faa.gov/Regulatory_and. . .AEE3F233862569AF006ABA6B?OpenDocument)Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors or equivalent provides guidance.

More information is available in Advisory Circular 90-97, (http://www.airweb.faa.gov/Regulatory_and. . .21CE439D862569EE005FD63E?OpenDocument)Use of Barometric Vertical Navigation (VNAV) for Instrument Approach Operations Using Decision Altitude.

Pilots flying aircraft equipped to fly LNAV/VNAV approaches may use the LNAV/VNAV or LNAV minima lines. There are almost 900 approaches with LNAV/VNAV minima.

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