Skip to page content

GNSS Frequently Asked Questions - NAS Implementation

GNSS - Frequently Asked Questions - GNSS Operational Implementation

Q. FAA certified GPS for use over the ocean yet interference has been experienced precluding the use of GPS. Why is such a system certified?

"GPS avionics used for primary means of oceanic operations have built in Receiver Autonomous Integrity Monitoring (RAIM) functions to provide integrity of navigation. Integrity is the notification of Hazardously Misleading Information (HMI). In the remote circumstance of a loss of the integrity function, flight crews can use backup procedures to insure safe navigation until the RAIM function is restored."   Arrow up

Q. How much will it cost to design approach paths for ILS, MLS, and GPS? Will it be more expensive or more difficult with one system over another?

A. It should cost about the same to develop and test approach procedures regardless of the type of navigation aid used for the approach; roughly $20K per approach path. Even though the approach path design costs are basically the same for GPS and ILS/MLS, the real cost benefits of using GPS lie in the associated user equipment costs and the expected cost benefits from reduced flight times and fuel consumption possible with the use of GPS/WAAS. In addition, operations and maintenance costs of ground-based systems will be significantly reduced or eliminated when GPS approaches are approved for use in the NAS.   Arrow up

Q. Given that many NAS users have direct routing capability needed for Free Flight, to what extent are direct routing benefits being allocated to WAAS?

A. WAAS implementation and WAAS equipage will allow the NAS routes to be restructured, resulting in shorter flight times, saving aircraft operating time and passenger time. In a study commissioned by the FAA's Air Traffic Organization and sponsored by the GPS Product Team titled, New England GPS Alternate Route Study, the average reduction in flight time for the routes analyzed was one (1) minute. Savings due to universal WAAS equipage were further analyzed through an Air Traffic study, The Southern Region Multi-Center Study. This one-minute value was used to calculate benefits attributed to WAAS as a result of restructuring NAS routes. For the lower bound of the benefits to cost ratio the assumption was made that only 30% of all flights would recognize this benefit. This figure was used because it is recognized that some aircraft already have direct route capability. The benefit calculations for savings from reduced flight times begin in 2006, when it is anticipated overall equipage is great enough to begin flying the new routes. The benefits for reduced flight times are only those that will be attributed to future restructured NAS routes.   Arrow up

Q. How does the FAA plan to use GPS in the future? What are the key decision factors? When will decisions be made?

A. GPS is an essential element of the future navigation architecture. Existing ground-based navigation aids have limited coverage due to terrain, and the majority of instrument approaches do not have vertical guidance due to the cost of siting and operating an Instrument Landing System (ILS). A satellite-based navigation system is needed that can provide global coverage, regardless of user altitude, and support precision approach operations at every runway without the need for costly ground infrastructure. Note that providing vertical guidance on an approach reduces the risk of controlled flight into terrain (CFIT) significantly. This satellite-based navigation system must also be global, so that the level of safety can be raised not only within the U.S. but also in the rest of the world.

Without augmentation, GPS is already used as a supplemental means of navigation, in addition to conventional ground-based navigation. Augmented by the Wide Area Augmentation System (WAAS), satellite navigation will provide en route, terminal area, and approach services, down to GLS precision approach. Augmented by the Ground Based Augmentation System (GBAS), satellite navigation will serve all categories of precision approach.

A second civil frequency is the next logical step in modernization of the navigation architecture and providing improved satellite navigation services. Within the U.S., it will improve the reliability of satellite navigation, particularly against the potential threats of interference and ionosphere. Outside of the U.S., it will make precision approaches possible with even less infrastructure than is achievable with WAAS alone. In addition, additional operational benefits will be defined as the service is defined and applications for that service are developed.   Arrow up

Q. How does WAAS and GBAS fit into the FAA's future NAS architecture?

A. When the final WAAS architecture is in place, users in the NAS will be able to use GPS/WAAS as a stand-alone navigation aid for en route through GLS precision approach operations. Like WAAS, GBAS relies on GPS for basic navigation signals. GBAS will operate independently from WAAS, while at the same time complementing WAAS by providing additional GPS augmentation to support airport requirements for CAT-II/III precision approach operations. GBAS will also provide a GLS capability at locations where WAAS cannot, as well as provide a signal, which could be used, for surface navigation in the terminal area.   Arrow up

Q. What is the difference between an IFR and VFR GPS receiver?

A. The main difference is that Visual Flight Rules (VFR) receivers do not have a FAA-approved method for detecting satellite failures. Instrument Flight Rules (IFR) receivers have a feature known as Receiver Autonomous Integrity Monitoring (RAIM) which can detect bad satellite signals and remove them from the position calculations. IFR receivers usually cost more than VFR receivers due to this advanced capability.   Arrow up

Q. Why do I have to get FAA approval to install my VFR-only GPS?

A. Even in VFR conditions, some GPS receivers can produce hazardously misleading information. The approval process ensures that the receiver in question will detect such problems and prevent a user from believing they are somewhere they are not.   Arrow up

Q. How much does it cost to equip an aircraft with GPS for use in the National Airspace System (NAS)?

A. The cost to equip an aircraft with a GPS TSO C129A receiver varies depending on the type of the receiver, the degree of difficulty of the installation, the sophistication of the aircraft and existing avionics, and the level of services desired. Also, the cost of equipping will vary depending on whether GPS is used as a stand-alone system or as part of a Flight Management System (FMS).

Most dealers provide a package price when they sell a receiver. This package includes installation and airworthiness inspection, as well as the actual equipment. These packages can range anywhere from approximately $3,000 (US) for a basic general aviation IFR GPS receiver installation/certification, to $10,000 (US) for a more sophisticated GPS receiver installation/certification on a commercial aircraft. When comparing prices of GPS avionics versus existing ground-based avionics, one must consider that one GPS receiver will eventually replace all avionics associated with these other ground navigation aids (VOR/DME, NDB, Category I ILS).   Arrow up

Q. What is the role of current ground-based navigation aids (ILS, VOR/DME, NDB, LORAN, etc.) in the future successful implementation of GPS technology?


A. The NAS currently provides several systems to support enroute and terminal area navigation, including non-precision approach. These include VOR with associated DME, TACAN, NDB, Loran-C, and GPS.

The operational benefits of GPS/WAAS, especially increased routing flexibility and many more precision approaches, will motivate most operators of aircraft used extensively for IFR operations to equip with GPS/WAAS in the five to six year period following the availability of services. At that point, the current ground based systems - VOR, DME, and ILS - will become back-up systems for these operators. Since most aircraft will be navigating using GPS/WAAS, substantial reductions can then be made in the number of VOR/DME and ILS ground facilities. Maintaining the current VOR/DME system is expensive, thus, there is a considerable financial incentive to reduce the number of VOR/DME. As soon as GPS/WAAS avionics are available, operators are anticipated to begin equipping with it to achieve the associated operational benefits and convenience.   Arrow up


Q. Are certified GPS avionics available?

A. Certified GPS receivers have been available since 1993 for use of the basic GPS system. The FAA is working with industry and user groups through the RTCA to insure that WAAS and GBAS avionics are being developed that are both cost effective and user friendly.  Arrow up

Page last modified:

This page was originally published at: