WAAS Key Terms

Become familiar with common WAAS terminology with this index of key terms and abbreviations.

AC 43.13-2B – FAA Advisory Circular pertaining to Acceptable Methods, Techniques and Practices - Aircraft Alterations. This Advisory Circular contains guidance material pertaining to aircraft alterations.

AC 20-138A – Airworthiness Approval of GPS Navigation Equipment for Use as a VFR and IFR Navigation System. This Advisory Circular contains guidance for GPS installation certification.

AC 90-94 – Guidelines for Using GPS Equipment. This Advisory Circular contains guidance for GPS equipment operation.

Approach with Vertical Guidance (APV) – There are actually three approach categories now. Precision, Non-Precision and APV. “APV” approaches are an ICAO recognized term and have different criteria.

FAR Part 25 – Federal Aviation Regulations – Airworthiness Standards. These are the general regulations that apply for Transport Category Aircraft (generally over 12,500 pounds).

FAR Part 25.1309 – Equipment, Systems and Installations. This section covers the requirements for the addition of equipment and systems.

GBAS – Ground Based Augmentation System. A local area augmentation system (also know as LAAS) where the GPS correction signals are sent to the aircraft from ground based transmitters.

GPS NPA – refers to a Non-Precision Approach (NPA) procedure which uses GPS and/or WAAS for Lateral Navigation (LNAV).

GPS Overlay Approach – These procedures, available in 1994, 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.

HMI – Hazardously Misleading Information.

H/VPL – Horizontal/Vertical Protection Levels. There is one chance in 100 million of the GPS being in error by more than the H/VPL. You will be locked out of LPV if the required levels are not met. Other approaches will be offered if their H/VPL are met.

LAAS – Local Area Augmentation System, under development for more accurate GPS approaches. No completion date has been announced, pending funding by U.S. Congress. Refer to GBAS for more detail.

LNAV/VNAV – (Lateral Navigation/Vertical Navigation) approaches use lateral guidance (556m lateral limit) from GPS and/or WAAS and vertical guidance provided by either the barometric altimeter or WAAS. Aircraft that don’t use WAAS for the vertical guidance portion must have VNAV-capable altimeters, which are typically part of a flight management system (FMS). When the pilot flies an LNAV/VNAV approach, lateral and vertical guidance is provided to fly a controlled descent and a safer maneuver to the runway. The decision altitudes on these approaches are usually 350 feet above the runway.

LP Approach – A GPS approach with Localizer Performance with pilot guided vertical descent (2008).

From AIM 1-1-20: A new non-precision WAAS approach, called Localizer Performance (LP) is being added in locations where the terrain or obstructions do not allow publication of vertically guided LPV procedures. This new approach takes advantage of the angular lateral guidance and smaller position errors provided by WAAS to provide a lateral only procedure similar to an ILS Localizer. LP procedures may provide lower minima than a LNAV procedure due to the narrower obstacle clearance surface.

LPV Approach – Localizer Performance with Vertical Guidance Approach offering the lowest minimums of GPS systems. Requires WAAS GPS receivers.

RNP – Required Navigational Performance. It is a metric of system navigational capability. The FAA is moving toward a performance-based national airspace system.

SBAS – Satellite Based Augmentation System such as WAAS, EGNOS, etc.

SA – Selective Availability, military scrambling of GPS or no longer active.

STC – Supplemental Type Certification. The two objectives of Aircraft certification is to encourage and foster the development of civil aviation and to ensure aviation safety. One method used by the Federal Aviation Administration (FAA) to fulfill these objectives is the aircraft certification system through which aircraft design and modification must be approved. Title 14 of the Code of Federal Regulations (14 CFR) and the Civil Air Regulations (CAR) define the minimum required safety standards for FAA certification. By demonstrating compliance with these regulations, an aircraft modifier may obtain the necessary FAA approval for a modification.

Types of aircraft certification design approvals are determined by the magnitude and complexity of the change. Aircraft modifications can be subdivided into minor and major changes (14 CFR part 21, section 21.93). The type of FAA approval is applicable to a given modification. Supplemental Type Certificates (STC) are required for most major changes to existing Type Certificate (TC) products affected by a modification or installation when the change is not so intensive as to require a new TC (14 CFR part 21, section 21.19).

SAAAR – Special Aircraft and Aircrew Authorization Required. SAAAR Approaches are specially authorized and have stringent requirements. Refer to AC 90-101 for details.
VLF/Omega – A now defunct navigation product. The Very Low Frequency portion used U.S. Navy high-powered transmitters that were designed for submarine communications and navigation. The Omega system was a synchronized Low Frequency Pulsed transmitter system.

WAAS – The Wide Area Augmentation System or WAAS is a major improvement to GPS. A combination of 25 WAAS ground reference stations (WRS) monitor the GPS constellation signals and send corrections through two WAAS Master Stations (WMS) up to two geosynchronous satellites. These satellites then transmit the corrections to a WAAS–enabled FMS/GPS receiver.

WAAS Channel Number – A reference number assigned to each approach chart. If used by a particular manufacturer, the channel number provides a shortcut to the chart.

WAAS Containment – Similar to LNAV/VNAV and ILS approaches, LPV procedures evaluate the Glideslope Qualification Surface. Because of the smaller integrity limit and angular guidance, the size of the obstacle trapezoid is smaller than LNAV/VNAV. In 2003, the minimum height above touchdown (HAT) value was established at 250 feet. In March 2006, it was announced that the WAAS minimum HAT would be lowered to 200 feet if all other airport infrastructure requirements are met.

WAAS Alerting – Another major improvement is WAAS alerting. The WAAS horizontal integrity limit is 40 meters on final as opposed to 556 meters for basic GPS. More importantly, WAAS provides vertical integrity, which basic GPS does not. WAAS eliminates the requirements for RAIM predictions, but crews still must check WAAS NOTAMs. Additionally, on procedures with an inverse W, crews must plan using non-precision approach requirements since vertical NOTAMs are not provided. The inverse W symbols will be removed as the vertical signal availability improves at airports. Avionics equipment guidance is found in TSO-C145– V/VNAV and LNAV.

Why would one fly LNAV / VNAV or LNAV minima if they could fly LPV? The reason is that some GPS and RNAV (GPS) approaches have LNAV/VNAV, but not LPV minima. Also, if the WAAS system has an outage, the pilot can still fly the LNAV portion. Think of flying the localizer only approach when the ILS glideslope is out of service. There are approximately 1,800 LPV approaches already published with a production goal of 300 more LPV approaches each year.

Duncan Aviation is a FAA certified ODA and DAS with a full complement of qualified DERs working diligently to complete the list of required STCs. Please contact us with your aircraft information and we will give you our expected completion dates.

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