Refining the usefulness and safety of satellite navigation

More accurate GPS flightpaths, curved RNP approaches, lower minimums—all coming soon to an airport near you.

Approach symbols. A white W in a black box (AeroNav format—Jeppesen uses clear text) on an approach plate means WAAS may not be available and Notam service is not available. If IFR, plan for no lower than LNAV minima at destination or your filed alternate. However, you may fly the vertically guided approach if WAAS is available.

RNP SAAAR approach segments

SAAAR approaches are GPS based, with a requirement for advanced features for onboard navigation systems and approved training and crew procedures. RNP SAAAR procedures can vary in required performance sensitivity between 0.1 and 1.0 RNP, depending on approach obstructions and available airspace.

An RNP SAAAR approach may have an RNP requirement the same as an RNAV (GPS) approach at 0.3—however, the SAAAR requirement is necessary for higher levels of integrity, availability and continuity. Consider the obvious requirement for accurate navigation and tighter alarm limits for RNP SAAAR approaches and missed segments to DCA.


The implementation in 2009 of a ground-based augmentation system (GBAS) called local area augmentation system (LAAS) provides further inputs for accuracy and reliability. The currently developed nonfederal LAAS system meets Cat I minima but further development promises minima to Cat III.

LAAS uses precisely located GPS receiver–transmitters on the airport to correct the GPS signal and transmit this directly to the aircraft line-of-sight.

This straightforward method for error correction and system monitoring is more accurate than WAAS but has a smaller service volume. The LAAS system for EWR extends 23 nm from the airport, limited only by line-of-sight.

This approach type is not in the family of RNAV approaches but is a specific precision GPS landing system (GLS) whose name is similar to ILS. The LAAS channeling information is for reference—when the approach is loaded in a WAAS/LAAS enabled navigator the WAAS/LAAS linkup is automatic.

With a higher degree of accuracy, the airspace footprint FAA must protect is smaller, offering the lowest possible minima (200 and 1/2) in a very challenging environment.

What you need

LNAV and circling approaches do not require WAAS. LNAV/VNAV, LPV and LP approaches do. RNP SAAAR approaches are GPS WAAS based, but require additional on­board equipment and training. GLS ap­proaches require WAAS and LAAS.

A white W in a black square for AeroNav approach documents or a note on Jeppesen approach documents indicates that loss of WAAS signals may occur without Notam warning. Planning for non-WAAS dependent approaches is required. Fly the WAAS approach if the signal is present but be prepared to fly a non-WAAS approach if necessary.

Runway environment

When considering approach minima, the runway environment plays a significant part in the complex minimum determination process. The pilot's argument often goes, "The WAAS GPS signal is so accurate, why not low minimums for all airports?"

In addition to the obstruction along the approach path, much of the determination of any airport's final approach minima will be based on cost, surrounding land use and activity level at the airport.

A GPS approach to minima as low as 200 and 1/2 requires a full-length parallel taxiway offset sufficiently from the runway, precision runway markings, an approved approach lighting system to the runway end, medium or high-intensity runway lights, and clear areas around the runway (precision obstacle-free zone, runway protection zone, etc).

All new approaches require an obstruction survey to as far out as 20 nm from the airport and a current engineering study for the airport (airport layout plan). Each of these standards requires land ownership or easements and levels of funding many smaller airports cannot afford.

A non-vertically guided approach to about 400 and 1 depending on approach path obstructions is often the best available for smaller airports or urban airports in busy terminal areas that have obstructions around the runway or insufficient airspace for the missed approach segment.

Some older airports do not have sufficient separation between the runway, taxiways, ramps or hangars/ buildings to meet precision standards, even if all other requirement can be met.

Looking at the future

Pilots who have grown up with GPS realize the tremendous advances in onboard equipment as well as the steady development of types of GPS approach. FAA has set an ag­gres­sive goal of requiring GPS-based positioning systems to replace radar as the primary means of traffic separation for Jan 1, 2020.

An astute observer of the growth of GPS for air navigation can see that many of the levels of approach we have today are ripe for development and that technology can alter the paradigm we now hold—if technology provides head-up displays with an equivalent visual display, what is the role of ceilings and visibility limits as part of the approach? The future is tantalizing, the path is possible and technically challenging.

Bill Gunn is the compliance manager for the State of Texas Aviation Division. He lectures nationally for a private aviation advocacy group and flies for work and pleasure.


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