|
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.
Containment
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
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.
|
