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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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Overlays - 1994
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The first authorization for using GPS to fly approach procedures was known as GPS overlays. These procedures authorized use of approved GPS receivers to fly existing non-precision instrument approaches. The only difference was that course guidance could come from the GPS system. These procedures are identified with "or GPS" in the title. The advantage for these procedures was twofold. First, overlay approaches provide the aviator greater position awareness than that derived from using the ground NAVAID. Second, although they didn't provide lower minima, GPS overlays also introduced and validated GPS approaches to aviation. This initial validation was critical for future GPS improvements.


Since overlays were GPS approaches designed to overlay the ground-based NAVAID approach, the minimum Required Obstruction Clearance (ROC) and OEA was the same as for the underlying ground-based NAVAID. VOR (Very High Frequency Omni-directional Range) and VOR/DME (Distance Measuring Equipment) approaches have a ROC of 250 feet, while Non-directional Beacons (NDB) have a ROC of 300 feet. The approach chart minima line did not change; "S - (runway number)" identified straight-in approach minima.


Alerting for GPS approaches became more involved than the ground-based NAVAID system. Ground NAVAID failure results in cockpit warning flags for VORs and Instrument Landing Systems (ILS), Morse code identification removal, and triggering the remote status indicators in the air traffic control facility. GPS avionics alert via an internally calculated integrity alarm. One of the major differences between IFR-certified GPS avionics and other GPS systems is that IFR GPS avionics provide alerting by using Receiver Autonomous Integrity Monitoring (RAIM) algorithms to detect any system faults. Non-IFR certified GPS units do not have this alerting capability.

In order to fly an overlay GPS approach, neither the underlying conventional instrument procedure NAVAID(s) nor the associated aircraft avionics need be installed, operational, or monitored. However, flight planning is slightly different. In addition to checking RAIM availability and GPS NOTAMs, if an alternate airport is required, this airfield must have a non-GPS approach and the ground-based and associated aircraft navigation equipment installed and operational.

Equipment Availability

Several IFR GPS units are certified according to Technical Standard Order (TSO)-C129, (http://www.airweb.faa.gov/Regulatory_and. . .6ACF8BA186256DC700717E0F?OpenDocument)Airborne Supplemental Navigation Equipment Using the GPS. IFR GPS units must be either panel mounted or a sensor which provides data to an integrated navigation system, and must be installed in accordance with Advisory Circular (AC) 20-138A, (http://www.airweb.faa.gov/Regulatory_and. . .1C85226F86256E35004C638B?OpenDocument)Airworthiness Approval of Global Positioning System (GPS) Navigation Equipment for Use as a VFR and IFR Supplement Navigation System, or AC 20-130A, (http://www.airweb.faa.gov/Regulatory_and. . .AEE3F233862569AF006ABA6B?OpenDocument)Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors, as applicable.

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