The National Executive Committee for Space-Based Positioning, Navigation, and Timing (PNT) is a U.S. Government organization established by Presidential directive to advise and coordinate federal departments and agencies on matters concerning the Global Positioning System (GPS) and related systems.
Frequently Asked Questions
* What is PNT?
* How many satellites are in the GPS constellation?
* How accurate is GPS?
* Is military GPS more accurate than civilian GPS?
* Isn't GPS under U.S. military control?
* Has the United States ever turned off GPS for military purposes?
* Doesn't the U.S. policy call for deactivation of GPS during national security crises?
* Will the United States ever reactivate Selective Availability?
* What is the potential for a gap in GPS service?
* Will civilian users experience any significant degradation in accuracy or availability in the future?
* Has the U.S. Government thought about privatizing GPS?
* Why does my GPS unit show my home/business in the wrong place?
* Where can I report GPS outages, interference, and other anomalies?
* Is it true the Census Bureau is recording the GPS coordinates of my home?
* How vulnerable is GPS to malicious jamming? Could a terrorist with a GPS jammer cause airplanes to crash?
* I read the Air Force doesn't want to use GPS in the future because of its vulnerabilities. Is this true?
* NEW! Does the 2010 National Space Policy call for shared control of GPS with other nations?
* Where can I download high-resolution images of the GPS satellites?
How many satellites are in the GPS constellation?
The U.S. Government is committed to provide a minimum of 24 operational GPS satellites on orbit, 95% of the time. The U.S. Air Force launches additional satellites that function as active spares to accommodate periodic satellite maintenance downtime and assure the availability of at least 24 operating satellites. As of March 19, 2010, there were 35 satellites in the GPS constellation, with 30 set "healthy" to users. The most current status information is available at the Navigation Center.
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How accurate is GPS?
The U.S. Government is committed to providing GPS to the civilian community at the performance levels specified in the SPS Performance Standard. For example, the GPS signal in space will provide a "worst case" pseudorange accuracy of 7.8 meters at a 95% confidence level. The actual accuracy end users can expect depends on factors outside the government's control, including atmospheric effects and receiver quality. Real-world data show that some high-quality GPS SPS receivers currently attain better than 3 meter horizontal position accuracy.
Higher accuracy is available today by using GPS in combination with various augmentation systems. These enable real-time positioning to within a few centimeters, and post-processed positioning to within millimeters. The U.S. Government is committed to modernizing the GPS constellation to enable higher civilian accuracy without augmentations. The first of many next-generation GPS satellites was fielded in 2005.
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Is military GPS more accurate than civilian GPS?
GPS offers two services -- the Precise Positioning Service (PPS) for U.S. and Allied military users, and the Standard Positioning Service (SPS) for worldwide civilian users. PPS is a more robust form of GPS that is encrypted and jam-resistant. It employs two signals to reduce radio transmission errors caused by the atmosphere, thus improving accuracy. SPS uses only one signal, but the ongoing GPS modernization program is adding several more civilian signals over the coming years. Detailed descriptions of PPS and SPS are available here.
Eventually, the accuracy difference between military and civilian GPS services will disappear. Using local or regional augmentations, today's civilian GPS users can already achieve much higher accuracy than PPS.
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Isn't GPS under U.S. military control?
Though acquired and operated by the Department of Defense, GPS is a multi-use system owned by the United States Government and paid for by the U.S. taxpayer. The outstanding performance of GPS over decades has earned the confidence of millions of civil and military users worldwide. The management improvements and modernization efforts directed by the U.S. Space-Based PNT Policy continue this support for multi-user applications. Any misperceptions will be overcome as they have in the past, by a demonstrated record of service and performance to all users.
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Has the United States ever turned off GPS for military purposes?
No. Since it was declared operational in 1995, the Global Positioning System has never been deactivated, despite U.S. involvement in wars, anti-terrorism, and other military activities.
Millions of users around the world have been monitoring and recording real-time GPS performance on a continuous basis since its inception. If the civilian GPS service had ever been interrupted by its operators, the evidence would be obvious and widespread. No such evidence exists.
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Doesn't the U.S. policy call for deactivation of GPS during national security crises?
No. The U.S. Space-Based PNT Policy does not include anything about deactivating GPS. In fact, the policy declares GPS to be an element of the nation's critical infrastructure that must be protected against disruption.
The policy reaffirms the U.S. intent to deny hostile uses of space-based PNT through Navigation Warfare. Navigation Warfare involves protecting U.S. and Allied use of GPS while simultaneously preventing hostile forces access to space-based PNT services and preserving peaceful civil GPS use outside of an area of military operations. The United States is committed to fielding a range of necessary denial options to prevent the hostile use of space-based PNT through regional denial of service, minimizing the impact to peaceful users outside the area of conflict.
Prior to the development of Navigation Warfare, GPS employed a feature called Selective Availability, which degraded civilian accuracy on a global basis. Selective Availability was discontinued in 2000 and the United States has no intent to ever use it again.
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Will the United States ever reactivate Selective Availability?
The United States does not intend to ever implement Selective Availability again and is committed to preventing hostile use of space-based PNT through regional denial of service, minimizing the impact to peaceful users. In September 2007, the U.S. Government announced its decision to eliminate the Selective Availability feature from future GPS satellites.
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What is the potential for a gap in GPS service?
The Air Force is confident GPS will continue to meet existing service commitments. Several years ago, the Air Force recognized the potential for an availability gap and took action to institutionalize procedures and processes to mitigate the potential gap or minimize any impact. Air Force Space Command developed key processes within the operational community as well as the acquisition community to extend the life of on-orbit assets and to ensure capability is delivered in a timely manner. Users can employ GPS with confidence today and continue to do so in the future. As identified by Ms. Chaplain from the General Accounting Office (GAO) in her testimony to Congress, "There have been times before where people have worried about gaps and the Air Force has managed them quite successfully."
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Will civilian users experience any significant degradation in GPS accuracy or availability in the future?
The Air Force currently foresees no loss of service in the future, near or far. The Air Force has high confidence it will continue to sustain at least the 24 satellites required to maintain the current GPS performance standards. As of October 8, 2009, there were 29 operational satellites orbiting the earth actively broadcasting positioning, navigation, and timing messages to users, 24/7, 365 days a year, around the globe. In addition, the Air Force currently maintains four older satellites on orbit in residual status that can be brought back to operational status if required.
The Air Force is also actively modernizing the constellation, which should enhance the performance and capabilities of the system. The Air Force launched eight GPS Block IIR-M satellites during 2005-2009 and launched the first of 12 GPS Block IIF satellites in May 2010. The next generation of satellites, GPS III, is currently in development and on schedule for a first launch in 2014.
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Has the U.S. Government thought about privatizing GPS?
There are no plans to privatize GPS. The U.S. Space-Based PNT Policy establishes the management framework for GPS and its augmentations and reaffirms that the U.S. Government will provide on a continuous, worldwide basis civil space-based, positioning, navigation, and timing services free of direct user fees for civil, commercial, and scientific uses, and for homeland security through the Global Positioning System and its augmentations, and provide open, free access to information necessary to develop and build equipment to use these services.
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Why does my GPS unit show my home/business in the wrong place?
GPS satellites do not provide any information about things on the ground. They only provide reference points, like lighthouses, so people can determine where things are in relation to the satellites. (See how GPS works.)
The maps and other information about roads, homes, businesses, etc., are loaded onto consumer GPS units by whoever manufactures them. Typically, the manufacturers get their maps and data from digital content suppliers such as NavTeq and Tele Atlas. The same applies for online mapping tools such as Google Maps.
Erroneous map information should be reported directly to the digital content suppliers. Most of the them allow the public to submit data corrections through their websites. Here is a link to a private website that explains how to do it. This link is provided for information only; the government does not endorse or control the contents of external websites.
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Is it true the Census Bureau is recording the GPS coordinates of my home?
The Census Bureau website includes several Q&A's about its use of GPS technology to support the 2010 Census. These are linked below for your convenience.
* Why is the Census Bureau using Global Positioning Systems (GPS)?
* Why is the accuracy of our address critical for the success of the 2010 Census?
* Are the GPS coordinates collected during the 2010 Census operation kept confidential?
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How vulnerable is GPS to malicious jamming? Could a terrorist with a GPS jammer cause airplanes to crash?
Like all radio-based services, GPS is subject to interference from both natural and human-made sources. A civilian GPS unit can lose reception in the presence of a device designed for intentional radio jamming. This can also occur during a solar flare. For this reason, the U.S. government strongly encourages all GPS users to maintain backup capabilities for positioning, navigation, and timing. In addition, the government is currently fielding new GPS signals that are more resistant to jamming.
Commercial airliners that use GPS are required to maintain alternative means of navigation. If intentional jamming were directed against aircraft, the pilots would revert to other sensors and ground-based navigation aids.
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I read the Air Force doesn't want to use GPS in the future because of its vulnerabilities. Is this true?
The Air Force is fully committed to continuing its operation and use of GPS in the future. The ongoing GPS modernization program will enhance the jam resistance of the military GPS service, making it more robust. At the same time, the Department of Defense is making prudent investments in alternative PNT technologies to supplement GPS in times when satellite services are unavailable. This will ensure that future troops have continuous, uninterrupted access to PNT under the most challenging conditions.
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NEW! Does the 2010 National Space Policy call for shared control of GPS with other nations?
No. The National Space Policy is clear that the United States will continue to operate and maintain the GPS constellation in accordance with published standards and interface specifications. The policy reaffirms principles of international cooperation that already existed in the U.S. Space-Based Positioning, Navigation, and Timing Policy of 2004. The United States has been pursuing compatibility and interoperability (at the user level) between GPS and other satellite navigation systems for over a decade. Such cooperation promotes GPS standards and will improve service for GPS users worldwide. For more information, see the International Cooperation section of this website.
GPS FREQUENTLY ASKED QUESTIONS
* What is GPS?
* How is GPS used?
* Who uses GPS?
* Will GPS be free in the future?
* What is the status of the GPS?
* What is the Standard Positioning Service?
* What is the status of Selective Availability (SA)?
* What is GPS Rollover?
* How do I report a GPS Mapping Data error such as an incorrect address for a home or a business?
What is GPS? (From Dept of Homeland Security)
GPS is a satellite-based radionavigation system developed and operated by the U.S. Department of Defense (DOD). GPS permits land, sea, and airborne users to determine their three-dimensional position, velocity, and time 24 hours a day, in all weather, anywhere in the world with a precision and accuracy far better than other radionavigation systems available today or in the foreseeable future.
GPS consists of three segments: space, control, and user.
* The Space Segment, consists of a minimum of 24 operational satellites in six circular orbits 20,200 km (10,900 NM) above the earth at an inclination angle of 55 degrees with a 12 hour period. The satellites are spaced in orbit so that at any time a minimum of 6 satellites will be in view to users anywhere in the world. The satellites continuously broadcast position and time data to users throughout the world.
* The Control Segment consists of a master control station in Colorado Springs, with five monitor stations and three ground antennas located throughout the world. The monitor stations track all GPS satellites in view and collect ranging information from the satellite broadcasts. The monitor stations send the information they collect from each of the satellites back to the master control station, which computes extremely precise satellite orbits. The information is then formatted into updated navigation messages for each satellite. The updated information is transmitted to each satellite via the ground antennas, which also transmit and receive satellite control and monitoring signals.
* The User Segment consists of the receivers, processors, and antennas that allow land, sea, or airborne operators to receive the GPS satellite broadcasts and compute their precise position, velocity and time.
The GPS concept of operation is based upon satellite ranging. Users figure their position on the earth by measuring their distance from the group of satellites in space. The satellites act as precise reference points.
Each GPS satellite transmits an accurate position and time signal. The user's receiver measures the time delay for the signal to reach the receiver, which is the direct measure of the apparent range to the satellite. Measurements collected simultaneously from four satellites are processed to solve for the three dimensions of position, velocity and time.
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How is GPS used?
GPS receivers collect signals from satellites in view. They display the user's position, velocity, and time, as needed for their marine, terrestrial, or aeronautical applications. Some display additional data, such as distance and bearing to selected waypoints or digital charts.
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Who uses GPS?
GPS is used to support land, sea, and airborne navigation, surveying, Geophysical exploration, mapping and geodesy, vehicle location systems, and a wide variety of additional applications.
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Will GPS be free in the future?
GPS will be free as stated in the Presidential Decision Document (29 March 1996) and by Congress in the 1998 Public Law (105-85)
Both state that the U.S. "will continue to provide the GPS Standard Positioning Service for peaceful civil, commercial and
scientific use on a continuous, worldwide basis, free of direct user fees.
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What's the status of the GPS?
The Global Positioning System reached full Operational Capability (FOC) on July 17, 1995.
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What is the Standard Positioning Service?
GPS provides two levels of service -- a Standard Positioning Service (SPS) for general public use and an encoded Precise Positioning Service (PPS) primarily intended for use by the Department of Defense. SPS signal accuracy is intentionally degraded to protect U.S. national security interests. This process, called Selective Availability (SA), controls the availability of the system's full capabilities. The SPS accuracy specifications, given below, include the effects of SA.
SPS provides accuracy's of (for position, the accuracy with respect to geographic, or geodetic coordinates of the Earth) within:
100 meters (2 drms) horizontal 156 meters (2 Sigma) vertical 300 meters (99.99% prob.) horizontal 340 nanoseconds time (95% prob.)
SPS Coverage is continuous and worldwide, with a position dilution of precision (PDOP) of 6 or less.
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What is the status of Selective Availability (SA)?
Effective as of Midnight 01 May 2000, Selective Availability has been set to zero.
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What is GPS Rollover?
This document contains information about GPS Week 1024 Rollover.
How do I report a GPS Mapping Data error such as an incorrect address for a home or a business?
If you wish to report an incorrect address, report or request a change to the address for a business or home, or add an address, please submit your corrections to the Tele Atlas website.
If you are experiencing GPS issues relating to positioning, navigation, timing or signals please proceed to and fill out a GPS outage report.
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From GNSS Library FAA Admin (.gov.)
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1. What is GPS?
2. How is GPS used?
3. Who uses GPS?
4. What's the status of the GPS?
5. What are the service levels provided by GPS?
6. What is Selective Availability (SA)?
7. Why was SA necessary?
8. What is the status of Selective Availability (SA)?
9. Will SA ever be turned back on?
10. How can civil users depend on a system controlled by the U.S. military?
11. How many GPS satellites are there at a given time in the GPS constellation?
12. What kind of orbits are the GPS satellites in?
13. How do GPS accuracy and integrity compare to that of existing ground-based navigation systems such as VOR/DME?
14. Are there plans to increase the capabilities of GPS?
15. How vulnerable are GPS satellites to jamming and interference?
16. What concerns are there regarding Radio Frequency Interference (RFI)?
17. Is the basic GPS signal sufficient to meet all the needs of civil aviation?
18. What augmentations to the basic GPS service is the FAA working on and why?
19. What is DGPS (Differential GPS)?
Q. What is GPS?
A.GPS consists of three segments - the satellite constellation, ground control network, and user equipment. The satellite constellation comprises satellites in low earth orbit that provide the ranging signals and navigation data messages to the user equipment. The ground control network tracks and maintains the satellite constellation by monitoring satellite health and signal integrity and maintaining the satellite orbital configuration. Furthermore, the ground control network also updates the satellite clock corrections and ephemerides as well as numerous other parameters essential to determining user position, velocity and time (PVT). The user equipment receives signals from the satellite constellation and computes user PVT. More details on each of the aforementioned GPS segments are provided below.
GPS Satellite Constellation:
The baseline satellite constellation consists of 24 satellites positioned in six earth-centered orbital planes with four operation satellites and a spare satellite slot in each orbital plane. The system can support a constellation of up to thirty satellites in orbit. The orbital period of a GPS satellite is one-half of a sidereal day or 11 hours 58 minutes. The orbits are nearly circular and equally spaced about the equator at a 60-degree separation with an inclination of 55 degrees relative to the equator. The orbital radius (i.e. distance from the center of mass of the earth to the satellite) is approximately 26,600 km.
With the baseline satellite constellation, users with a clear view of the sky have a minimum of four satellites in view. It’s more likely that a user would see six to eight satellites. The satellites broadcast ranging signals and navigation data allowing users to measure their pseudoranges in order to estimate their position, velocity and time, in a passive, listen-only mode.
Ground Control Network:
At the heart of the Ground Control Network is the Master Control Station (MCS) located at the Schriever (formerly named Falcon) Air Force Base near Colorado Springs , Colorado . The MCS operates the system and provides command and control functions for the satellite constellation.
The satellites in orbit are continuously tracked from six USAF monitor stations spread around the globe in longitude: Ascension Island , Diego Garcia, Kwajalein , Hawaii , Cape Canaveral and Colorado Springs . The monitor stations form the data collection component of the control network. A monitor station continuously makes pseudorange measurements to each satellite in view. There are two cesium clocks referenced to GPS system time in each monitor station. Pseudorange measurements made to each satellite in view by the monitor station receiver are used to update the master control station’s precise estimate of each satellite’s position in orbit.
User Equipment:
The user equipment, often referred to as “GPS receivers”, captures and processes L-band signals from the satellites in view for the computation of user position, velocity and time. Arrow up
Q. How is GPS used?
A. GPS receivers collect signals from satellites in view. They display the user's position, velocity, and time, as needed for their marine, terrestrial, or aeronautical applications. Some display additional data, such as distance and bearing to selected waypoints or digital charts.
The GPS concept of operation is based upon satellite ranging. Users determine their position by measuring their distance from the group of satellites in space. The satellites act as precise reference points.
Each GPS satellite transmits an accurate position and time signal. The user's receiver measures the time delay for the signal to reach the receiver, which is the direct measure of the apparent range (called a "pseudorange") to the satellite. Measurements collected simultaneously from four satellites are processed to solve for the three dimensions of position (latitude, longitude, and altitude) and time. Position measurements are in the worldwide WGS-84 geodetic reference system, and time is with respect to a worldwide common U.S. Naval Observatory Time (USNO) reference. Arrow up
Q. Who uses GPS?
A. GPS is used to support land, sea, and airborne navigation, surveying, geophysical exploration, mapping and geodesy, vehicle location systems, farming, transportation systems, and a wide variety of other additional applications. Telecommunication infrastructure applications include network timing and enhanced 911 for cellular users. Global delivery of precise and common time to fixed and mobile users is one of the most important, but least appreciated functions of GPS. Arrow up
Q. What's the status of the GPS?
A. The Global Positioning System reached Full Operational Capability (FOC) July 17, 1995. Per U.S. Policy and Law, the GPS Standard Positioning Service is available to civil users worldwide for their peaceful transportation, scientific, and other uses free of direct user charges. Arrow up
Q. What are the service levels provided by GPS?
A. GPS provides two levels of service:
* a Standard Positioning Service (SPS) for general civil use; and
* a Precise Positioning Service (PPS) primarily intended for use by the Department of Defense and U.S. allies.
There are no restrictions on SPS usage and is available to users worldwide. With Selective Availability (SA) , SPS provides predictable accuracies of 100m (2drms, 95%) in the horizontal plane and 156m (95%) in the vertical plane. UTC (USNO) time dissemination accuracy is within 340 nanoseconds (95%) referenced to the time kept at the U.S. Naval Observatory. These accuracies reflect the last signal specification in the Federal Radio Navigation Plan which is in the process of being revised to reflect the accuracy obtained following the deactivation of Selective Availability. Without SA, SPS accuracy would be of the order of 25m (2 drms, 95%) in the horizontal plane and 43m (95%) in the vertical plane.
PPS provides a predictable accuracy of at least 22m (2drms, 95%) in the horizontal plane and 27.7m (95%) in the vertical plane. PPS provides UTC (USNO) time transfer accuracy within 200 nanoseconds (95%) referenced to the time kept at the U.S. Naval Observatory.
PPS is primarily intended for military and select government agency users. Civilian use is permitted but only upon special U.S. Department of Defense approval. Arrow up
Q. What is Selective Availability (SA)?
A. SA was a technique implemented by the DOD to intentionally degrade a user’s navigation solution. The single largest source of error for SPS users was SA. The net result of SA was about a five-fold increase in positioning error. DOD achieved signal degradation by altering (also known as dithering) the satellite clock. Another means designed by DOD to degrade GPS performance was to broadcast less accurate ephemeris parameters.
The DOD-authorized users were able to undo SA. However, due to the fact that SA is spatially correlated, civil users were able to eliminate SA through the implementation of Differential GPS (DGPS), albeit an additional expense on the part of the users. Arrow up
Q. Why was SA Necessary?
A. SA was used to protect the security interests of the U.S. and its allies by globally denying the full accuracy of the civil system to potential adversaries. Arrow up
Q. What is the status of Selective Availability (SA)?
A. By order of the President of the United States, the use of Selective Availability was discontinued on May 1, 2000. Arrow up
Q. Will SA ever be turned back on?
A. It is not the intent of the U.S. to ever use SA again. To ensure that potential adversaries to do not use GPS, the military is dedicated to the development and deployment of regional denial capabilities in lieu of global degradation through SA.Arrow up
Q. How can civil users depend on a system controlled by the U.S. military?
A. GPS is owned and operated by the U.S. Government as a national resource. DOD is the "steward" of GPS, and as such, is responsible to operate the system in accordance with the signal specification. The National Space-Based Positioning, Navigation, and Timing (PNT) Executive Committee was established by Presidential directive in 2004 to advise and coordinate federal departments and agencies on matters concerning the Global Positioning System (GPS) and related systems. This Committee replaced the Interagency GPS Executive Board (IGEB), which oversaw GPS policy matters from 1996 to 2004. The Executive Committee is chaired jointly by the Deputy Secretaries of Defense and Transportation. Its membership includes equivalent-level officials from the Departments of State, Commerce, and Homeland Security, the Joint Chiefs of Staff, and NASA. Components of the Executive Office of the President participate as observers to the Executive Committee, and the FCC Chairman participates as a liaison.
DOD is required by law to "maintain a Standard Positioning Service (SPS) (as defined in the Federal Radionavigation Plan and the Standard Positioning Service Signal Specification) that will be available on a continuous, worldwide basis," and, "develop measures to prevent hostile use of GPS and its augmentations without unduly disrupting or degrading civilian uses." These strict requirements and current augmentation systems should actually make DOD use of the system transparent to the civil user. (Note: There will, necessarily, be localized testing of the system by military and development teams but the testing will fall under strict notification guidelines of safety-of-life users such as Coast Guard and FAA).
U.S. transportation, public safety, economic, scientific, timing, and other users rely on GPS extensively. In aviation and maritime transportation, GPS is used for "safety of life" navigation and it is a critical system for these applications. DOD is the steward of the system, responsible to maintain the signal specification; the PNT provides management oversight to assure that civil and military needs are properly balanced. Arrow up
Q. How many GPS satellites are there at a given time in the GPS constellation?
A. The exact number of satellites operating at any one particular time varies depending on the number of satellite outages and operational spares in orbit. For current status of the GPS constellation, please visit http://tycho.usno.navy.mil/gpscurr.html. Arrow up
Q. What kind of orbits are the GPS satellites in?
A. The GPS satellites operate in circular 10,900nm (20,200km) 12-hour orbits at an inclination of 55 degrees. They are not in geo-stationary orbit. Arrow up
Q. How do GPS accuracy and integrity compare to that of existing ground-based navigation systems such as VOR/DME?
A. The basic GPS signal is accurate on a worst-case basis to within approximately 100 meters lateral and 140 meters vertical everywhere on earth. GPS, as provided to civil users, appears to be just as accurate as the most accurate service being provided by the VOR/DME, i.e., non-precision approaches. It should be noted that VOR accuracy degrades as you move farther away from the navigation aid. GPS accuracy is space-based, and thus not constrained by ground equipment. The basic GPS signal is not as accurate as the existing ILSs; however, augmented by WAAS and GBAS, GPS will be able to supply a precision approach capability (CAT-I with WAAS and progressing to CAT-II/III with GBAS). Arrow up
Q. Are there plans to increase the capabilities of GPS?
A. Yes, one of the main components of GPS modernization is the addition of two new navigation signals for civil use. These signals will be in addition to the existing civilian service broadcast at 1575.42 MHz (L1). The first of these new signals will be a new civil code, called L2C, which will be added on the existing L2 carrier, located at 1227.60 MHz. It will be available for general use in non-safety critical applications. The Block IIR-M satellite, the first to add his capability was launched September 25, 2005. A third civil signal, located at 1176.45 MHz (L5), will be provided initially on GPS Block IIF satellites beginning in 2007, and continuing with the Block III satellites scheduled for launch beginning in 2012. This new L5 signal is protected worldwide for aeronautical radionavigation use, and will support aviation safety-of-life applications. The addition of L5 will make GPS a more robust radionavigation service for many aviation applications, as well as all ground-based users (maritime, railways, surface, shipping, agriculture, recreation, etc.)
At the current GPS satellite replenishment rate, all three civil signals (L1-C/A, L2C, and L5) will be available for initial operational capability by 2012, and for full operational capability by approximately 2015. For more information on GPS modernization activities, please visit our GPS Modernization page and http://pnt.gov. Arrow up
Q. How vulnerable are GPS satellites to jamming and interference?
A. GPS satellite signals, like any other navigation signals, are subject to some form of interference. The FAA is actively working with the U.S. Department of Defense and other U.S. Government Agencies to detect and mitigate these effects and make sure that the GPS and any related augmentation systems are available for safe aviation operations. As with all navigation aids, interference, whether intentional or unintentional, is always a concern. A number of methods for minimizing interference have been identified and tested and others are being investigated. The FAA is also working to make sure augmentation systems detect and mitigate these effects. Arrow up
Q. What concerns are there regarding Radio Frequency Interference (RFI)?
A. As with all navigation aids, Radio Frequency Interference (RFI), unintentional or intentional, is always a concern. The FAA is evaluating several GPS interference detection systems, which will determine the direction and source of the GPS interference. The FAA is also working with DOD and other agencies to make sure that GPS augmentation systems detect and mitigate the effects of interference. Arrow up
Q. Is the basic GPS signal sufficient to meet all the needs of civil aviation?
A.This is not a simple yes/no answer. The answer is that it depends on the service requirements of each user or aviation authority. For many countries, GPS supplies a better capability than the existing ground-based systems or lack thereof. Yet for other countries with large infrastructures, the GPS signal does not meet the accuracy, integrity, availability, and continuity requirements critical to safety of flight. Enhancements to the Global Positioning System (GPS) such as the Wide Area Augmentation System (WAAS) and Ground Based Augmentation System (GBAS) provide the necessary corrections for meeting safety-of-life flight requirements. Arrow up
Q. What augmentations to the basic GPS service is the FAA working on and why?
A. The FAA is developing the Wide Area Augmentation System (WAAS) and the Ground Based Augmentation System (GBAS). Both augmentation systems focus on the same concerns: integrity, availability, and accuracy.
Integrity is the ability to alert users, within a prescribed number of seconds (depending on the type of flight operation), when GPS should not be used for navigation. Availability is needed to assure users that the basic GPS civil service is accessible nearly 100% of the time. Accuracy enhancements are necessary to conduct precision approach and terminal navigation operations.
The WAAS will cover the Continental U.S. and provide a navigation signal capable of supporting navigation from enroute through Category I precision approach. GBAS will cover approximately a 30-mile radius and will provide up to a Category III precision approach. WAAS and GBAS will work together to provide users a navigation capability for all phases of flight. Arrow up
Q. What is Differential GPS (DGPS)?
A. DGPS achieves enhanced accuracy since the reference and user receivers both experience common errors that can be removed by the user. Position errors less than 10 meters are typically realized.
In the basic form of DGPS, the position of a reference receiver at a monitoring or reference station is surveyed in, that is, its position is known accurately. The user receiver should be no more than about 300 miles away from the reference receiver which makes pseudorange measurements, just as any user receiver would. However, because the reference receiver knows its position accurately, it can determine “biases” in its pseudorange measurements. For each satellite in view of the reference receiver, these biases are computed by differencing the pseudorange measurement and the satellite-to-reference receiver geometric range. These biases incurred in the pseudorange measurement process include errors arising from ionospheric delay, tropospheric delay, and satellite clock offset from GPS time. For real-time applications, the reference station transmits these biases, called differential corrections, to all users in the coverage area of the reference station. Users incorporate these corrections to improve the accuracy of their position solution.
For the basic local area DGPS (LADGPS) the position solutions of users further away from the reference station are less accurate than those closer to the monitoring station because pseudorange measurement errors tend to be spatially correlated. This loss of accuracy due to spatial decorrelation can be improved with more sophisticated techniques that fall under the heading of wide area DGPS (WADGPS) such as WAAS. Arrow up
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****2. Wide Area Augmentation System (WAAS)
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1. What is WAAS?
2. How does WAAS know that the correction it sends is valid to my particular location?
3. Will the WAAS provide a performance comparable to ILS? How will the FAA respond to users who claim to be getting poorer performance than ILS?
4. Does the FAA plan to live up to its original commitment to deliver the WAAS program with the capability that was envisioned when initial funding was requested?
5. What is the data collecting method for ionospheric data, solar activity, etc. to evaluate performance?
6. Who manages (and how) the safety analysis of the WAAS system including the ground component, RF Uplink system, and satellite component?
7. What is the six-second time to alarm?
8. When will a WAAS-like system be available in Europe?
9. What approaches can I fly with GPS? WAAS?
10. OK, so explain what these various TSOs and ACs relate to!
11. What are the differences in capabilities between the various WAAS TSO’s?
12. I already have GPS. Does WAAS require a separate antenna?
13. Can I use GPS to fly RNAV approaches or doI need WAAS?
14. Wait—there’s another new term…what is RNP?
15. What is an LPV approach? Can I fly LPV approaches with WAAS today?
16. I see comments about WAAS “integrity.” What is meant by “integrity?” How does integrity differ from availability or continuity? What is “ephemeris?”
17. Can my aircraft be equipped only with GPS for navigation and be legal for flying in IMC?
18. I want to fly an RNAV (GPS) approach. I have an FMS, but no GPS. The approach says GPS or RNP-0.3 required. It also says DME/DME-0.3 NA. Can I fly this approach legally?
19. What do I need to know about RAIM?
20. Is my GPS-equipped aircraft considered RNAV-capable?
21. I have heard about flying an “overlay” approach, and that it is basically flying a VOR or other approach, but using the GPS instead of the VOR or ADF. Can I just use the GPS instead of the VOR?
22. What is the sensitivity of my CDI when using GPS?
23. Can I use an alternate with a GPS approach?
24. The GPS approach at my destination had a NOTAM saying “Unreliable.” Can I file based upon the NOTAM? Can I fly the approach upon arrival?
25. I’ve seen the acronyms TSO and STC used together. What’s the difference?
26. I’ve heard the term, “number of nines” what does that mean?
27. What is the problem with flying baro-VNAV in really cold weather?
28. I’ve seen the term APV approach. What does that mean?
29. What’s an MMR?
30. I’ve heard about Beta, Delta, and Gamma WAAS equipment. What’s the difference and which one will I need to buy for my single-engine aircraft? When will I be able to buy them?
31. I already have a GPS receiver—how do I upgrade it for WAAS? Do I need to buy a new box or can I upgrade with just a card or with better software? How much will it cost me?
32. Are LNAV/VNAV or LPV approaches considered RNP approaches?
33. Why do so many RNAV approaches say RNP-0.3 (DME/DME NA)?
34. I have a WAAS non-aviation receiver - Where can I use it? What types of accuracies will I get?
35. Can I use WAAS in my vehicle?
Q. What is WAAS?
A. The Wide Area Augmentation System (WAAS) uses a system of ground stations to provide necessary augmentations to the GPS SPS navigation signal. A network of precisely surveyed ground reference stations is strategically positioned across the country including Alaska, Hawaii, and Puerto Rico to collect GPS satellite data. Using this information, a message is developed to correct any signal errors. These correction messages are then broadcast through communication satellites to receivers onboard aircraft using the same frequency as GPS. The WAAS is designed to provide the additional accuracy, availability, and integrity necessary to enable users to rely on GPS for all phases of flight, from en route through GLS approach for all qualified airports within the WAAS coverage area. This will provide a capability for the development of more standardized precision approaches, missed approaches, and departure guidance for approximately 4,100 ends of runways and hundreds heliport/helipads in the NAS. WAAS will also provide the capability for increased accuracy in position reporting, allowing for more uniform and high-quality worldwide Air Traffic Management (ATM). In addition, WAAS will provide benefits beyond aviation to all modes of transportation, including maritime, highways, and railroads. Arrow up
Q. How does WAAS know that the correction it sends is valid to my particular location?
A. The WAAS supplies two different sets of corrections: 1) corrected GPS parameters (position, clock, etc) and 2) Ionospheric parameters. The first set of corrections is user position independent - they apply to all users located within the WAAS service area. The second set of corrections is area specific. WAAS supplies correction parameters for a number of points (organized in a grid pattern) across the WAAS service area. The user receiver computes ionospheric corrections for the received GPS signals based on algorithms which use the appropriate grid points for where the user is located. Further, the appropriate grid points may differ for each GPS satellite received and process by the user receiver as the GPS satellites are located at various positions in the sky relative to the user. The combination of the two sets of corrections allows for significantly increased user position accuracy and confidence anywhere in the WAAS service area. Arrow up
Q. Will the WAAS provide a performance comparable to ILS? How will the FAA respond to users who claim to be getting poorer performance than ILS?
A.Yes. WAAS has been designed and is being built to provide performance comparable to Category 1 ILS. The Satellite Operational Implementation Team, or SOIT, believes that WAAS will provide an equivalent level of precision approach service to that of existing Category 1 ILS when fully deployed.
When the WAAS signal is fully stabilized, and the FAA accepts the system, we believe actually performance will exceed system specifications. Preliminary tests using WAAS software indicate that this is the case. Arrow up
Q. Does the FAA plan to live up to its original commitment to deliver the WAAS program with the capability that was envisioned when initial funding was requested?
A. The FAA remains committed to the implementation of WAAS because of its safety benefits for the aviation community and the flying public, and because it is central to our overall efforts to modernize the NAS. Arrow up
Q. What is the data collecting method for ionospheric data, solar activity, etc. to evaluate performance?
A. WAAS will collect GPS data at the reference stations. The system will then be able to estimate the amount of signal delay and error that is the result of the ionospheric and/or solar activity. This information is then passed onto the user as a part of the WAAS navigation message. The GPS Product Team is currently developing requirements for a performance evaluation system to monitor how well WAAS is accounting for these and other sources of delay/error.
In addition, ionospheric data is collected and archived by the NSTB for analysis of scintillation and range delay effects by experts in the ionospheric field. Arrow up
Q. Who manages (and how) the safety analysis of the WAAS system including the ground component, RF Uplink system, and satellite component?
A. A Safety Working Group has been formed to continuously look into the safety performance/operation for WAAS (including all of the components - reference stations, uplink stations, and satellites). This working group is comprised of various representatives from the FAA, Raytheon, and Mitre Corporation. Arrow up
Q. What is the six-second time to alarm?
A. WAAS has six seconds to do one of two actions:
1. Correct user position outside the guaranteed accuracy protection limits to get back within the protection limits. If WAAS is able to correct misleading information within six seconds, there is no lapse in system integrity.
2. Shut-off connections and notify the user not to use. If the system is unable to correct misleading information in the six-second timeframe, it becomes Hazardously Misleading Information (HMI) and should not be used for navigation. Arrow up
Q. When will the WAAS system be available in Europe?
A. On August 2, 2010, the European Geostationary Navigation Overlay Service (EGNOS) began broadcasting satellite-based navigation service certified to meet the high standards of international aviation. EGNOS is the European equivalent of the U.S. Wide Area Augmentation System (WAAS). Both systems monitor and provide correction updates to Global Positioning System (GPS) signals and support aircraft flight operations including departure climb, enroute flight, arrivals and landing – using only guidance from satellites. This new operational capability will significantly boost the availability and utility of worldwide satellite based augmentation system (SBAS) service. SBAS is the international term for navigation systems such as WAAS and EGNOS. More information on EGNOS can be found on the European Satellite Services Provider (ESSP) website at http://www.essp-sas.eu. Arrow up
Q. What approaches can I fly with GPS? WAAS?
A. This answer is highly dependent upon your specific equipment and installation. Please check the Aeronautical Information Manual, Table 1-1-5 and compare this with the documentation provided with your GPS and installation. This table provides information on TSO-C129 units. LPV approaches require WAAS Class 3 equipment built to TSO-C145a or TSO-C146a and installed IAW AC 20-138A.
The short answer is that you cannot use any hand-held receiver for anything other than situational awareness. Older GPS receivers may be for VFR-only use, or may be sufficient to fly non-precision GPS approaches. LNAV/VNAV and LPV approaches require additional equipment beyond the basic GPS receiver. Arrow up
Q. OK, so explain what these various Technical Standard Orders and Advisory Circulars relate to.
A.
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TSO-C129 is “Airborne Supplemental Navigation Equipment Using the GPS.”
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TSO-C145a is “Airborne Navigation Sensors Using the GPS Augmented by the Wide Area Augmentation System.”
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TSO-C146a is “Stand-Alone Airborne Navigation Equipment Using the GPS Augmented by the Wide Area Augmentation System.”
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AC 20-130a is “Airworthiness Approval of Navigation or Flight Management Systems Integrating Multiple Navigation Sensors.”
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AC 20-138a is “Airworthiness Approval of GPS Navigation Equipment for Use as a VFR and IFR Navigation System.”
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AC 90-94 is “Guidelines for Using GPS Equipment.” Arrow up
Q. What are the differences in capabilities between the various WAAS TSOs?
A. First, the basic information for GPS. To use GPS for navigation, the equipment must be certified in accordance with TSO-C129 and the installation must be done in accordance with AC 20-138 or AC 20-130A. TSO-C115a does not meet the requirements of TSO-C129.
For WAAS, you must use either TSO-C145a or TSO-C146a. Most General Aviation WAAS receivers would comply with TSO-C146a, which applies to panel-mounted navigation equipment (as opposed to sensors that provide data to a flight management system). Arrow up
Q. I already have GPS. Does WAAS require a separate antenna?
A. No, the WAAS message is broadcast on the same frequency as the GPS signal. You will just need a WAAS-capable receiver provided your existing antenna is compatible with the WAAS equipment. Your current system may be upgradeable. Please contact your manufacturer directly for information on availability, installation and price. Arrow up
Q. Can I use GPS to fly RNAV approaches? WAAS?
A. To fly an LNAV or LNAV/VNAV approach, you must have either (1) WAAS avionics approved for LNAV/VNAV approaches, (2) a certified Baro-VNAV approach system with IFR-approved GPS, (3) a certified Baro-VNAV approach system with an IFR-approved WAAS, or (4) an RNP-0.3 certified system. Arrow up
Q. Wait—there’s another new term…what is RNP?
A. RNP stands for “Required Navigational Performance.” It is a metric of system navigational capability. The FAA is moving toward a performance-based national airspace system. In the future, your ability to fly in certain areas may be governed by your ability to achieve precise navigational performance within specific tolerances. RNP-0.3 will be used for approaches, and it refers to .3 nautical mile accuracy. This accuracy may be achieved through various means (GPS, WAAS, flight management system using automatic DME updates), but your aircraft will be certified to a particular RNP. There are other requirements beyond accuracy that will be defined for each RNP operation before it is implemented. Arrow up
Q. What is an LPV approach? Can I fly LPV approaches with WAAS today?
A. LPV is a new category of approach that uses the WAAS signal and provides vertical guidance. You can fly LPV with WAAS avionics approved for LPV approaches. WAAS avionics do not automatically mean that you can fly the LPV minima line. WAAS approaches do not require any special ground equipment at the destination airport—no localizer or glide slope transmitters are necessary. To fly an LPV approach, you must have WAAS Class 3 avionics certified to TSO-C145a or TSO-C146A, and installed IAW AC 20-138A. Arrow up
Q. I see comments about WAAS “integrity.” What is meant by “integrity?” How does integrity differ from availability or continuity? What is “ephemeris?”
A.
Integrity refers to usability of the satellite signal, and means that the signal has not been corrupted. Integrity is the ability of a system to provide timely warnings to users when the system should not be used for navigation as a result of errors or failures in the system. WAAS improves upon the integrity of the basic GPS signal and detects much smaller errors more quickly.
Availability refers to the percentage of time in a given period that the signal is expected to be received and usable.
Continuity differs from availability, in that it refers to the continuous reception of the signal. A signal could have high availability but numerous short outages.
Ephemeris is the term for the exact position of a GPS satellite in space at a given time, and it is necessary for GPS calculations. Arrow up
Q. Can my aircraft be equipped only with GPS for navigation and be legal for flying in IMC?
A. The FAA has authorized GPS as the primary means of navigation in certain areas (oceanic and remote). The FAA requires that the aircraft have the appropriate equipment necessary for the route of flight. Furthermore, a GPS approach cannot be used for the alternate if it is also used for the destination when using TSO-C129 equipment. Therefore, it may be legal to fly a GPS-only equipped aircraft in IMC, but the circumstances are quite rigorous. In Alaska , IFR-approved and installed WAAS avionics are legal as the only navigation equipment on board if the route and destination are identified for GPS/WAAS. Arrow up
Q. I want to fly an RNAV (GPS) approach. I have an FMS, but no GPS. The approach says GPS or RNP-0.3 required. It also says DME/DME-0.3 NA. Can I fly this approach legally?
A. Your system updates using ranging from multiple DMEs. Optimal DME geometry of multiple DME facilities and validation is required to achieve this performance. At this time, RNP-0.3 approaches using multiple DMEs are not common. Therefore, there is no way you can fly this approach without having GPS. Arrow up
Q. What do I need to know about RAIM?
A. Receiver Autonomous Integrity Monitor (RAIM) is a form of integrity monitoring performed within the avionics themselves. It ensures available satellite signals meet the integrity requirements for a given phase of flight. By comparing the pseudorange measurements of a number of satellites, the RAIM function can identify a satellite failure and issue an alert to the pilot. Many VFR GPS receivers and most hand-held receivers do not have RAIM capability. Without RAIM capability, the pilot has no assurance of the accuracy of the GPS position. A minimum of five satellites is required to detect a bad satellite; at least six satellites are required to detect and exclude a bad satellite from the navigation solution if your receiver has a fault detection and exclusion (FDE) RAIM algorithm. The GPS receiver should also tell you when its RAIM function is unavailable, at both present time/position and at any selected future time/position. You can get information on satellite outages through the NOTAM system, however the effect of an outage on the intended operation cannot be determined unless the pilot has a RAIM availability prediction program which allows excluding a satellite which is predicted to be out of service.
If you are using GPS to fly an approach and you receive a RAIM annunciation prior to the final approach waypoint, you may not have sufficient accuracy to complete the approach.
You can get information on satellite outages through the NOTAM system, however the effect of an outage on the intended operation cannot be determined unless the pilot has a RAIM availability prediction program which allows excluding a satellite which is predicted to be out of service. Arrow up
Q. Is my GPS-equipped aircraft considered RNAV-capable?
A. Aircraft operating by IFR-approved GPS are considered to be area navigation (RNAV) aircraft, and should file the appropriate equipment suffix. Arrow up
Q. I have heard about flying an “overlay” approach, and that it is basically flying a VOR or other approach, but using the GPS instead of the VOR or ADF. Can I just use the GPS instead of the VOR?
A. No. Overlay approaches can use GPS instead of the primary designated navigational aid, but the approach must be designated for GPS and be in the current aircraft database. For example, it must say “VOR or GPS RWY 16.” You cannot just use GPS in lieu of VOR, Automatic Direction Finder (ADF) or other navigational source naming the approach. You can, however, use GPS to determine waypoints during the approach. Arrow up
Q. What is the sensitivity of my CDI when using GPS?
A. En route, full-scale Course Deviation Indicator (CDI) deflection is typically 5 miles with an accuracy of +/- 2 miles. Within 30 miles of the arrival and departure airport, GPS CDI sensitivity typically transitions to one mile. When flying an approach (and the approach mode is armed), GPS CDI sensitivity transitions from 1 mile to 0.3 miles approximately 2 miles from the FAWP. Arrow up
Q. Can I use an alternate with a GPS approach?
A. If your system is a TSO-C129, any required alternate must have an approved instrument procedure other than GPS, and your aircraft must have the appropriate equipment to fly the approach. If you have approved WAAS avionics, you may plan to use any instrument approach authorized for use with WAAS avionics at a required alternate. You must use the LNAV minima line for planning purposes in case vertical guidance is not available. Arrow up
Q. The GPS approach at my destination had a NOTAM saying “Unreliable.” Can I file based upon the NOTAM? Can I fly the approach upon arrival?
A. You can file using the GPS, however you must file an alternate. You can fly the approach—even though listed as “Unreliable.” The term UNRELIABLE is an advisory to pilots indicating the expected level of service may not be available. You should use RAIM and all other sources to confirm the suitability of the GPS signal upon arrival. Unreliable and Unavailable have different meanings, and you must be certain of each. Unavailable indicates a loss or malfunction of the GPS signal in the specified area during the specified time period. Arrow up
Q. I’ve seen the acronyms TSO and STC used together. What’s the difference?
A. TSO is a Technical Standard Order, and it describes the minimum performance standard for a system or component. An STC is a Supplemental Type Certificate. An STC is a document issued by the Federal Aviation Administration approving a product (aircraft, engine, or propeller) modification. The STC defines the product design change, states how the modification affects the existing type design, and lists serial numbers of the component affected by the change. Safe flight using GPS equipment depends on airworthiness (does it have a TSO?), installation (was it installed per AC 20-138 or AC 20-130a), and signoff (was it done properly per TC/STC or Form 337).Arrow up
Q. I’ve heard the term, “number of nines” what does that mean?
A. The FAA uses metrics in the process of determining the safety of operations. Two nines would equate to 99%, or one occurrence in 100. Five nines would be .99999, or one occurrence in 100,000. Arrow up
Q. What is the problem with flying baro-VNAV in really cold weather?
A. Altimeters are susceptible to atmospheric-related error, and when the temperature is colder than standard, this error can become significant, with your altimeter reading higher than your actual altitude. Some charts list minimum temperatures for certain minimums, and the reason is that your actual altitude is below the altitude that would be displayed on your altimeter. Sometimes there are limitations associated with the availability of a local altimeter as well. The pilot is expected to account for this error in extremely cold conditions (as discussed in the AIM). Arrow up
Q. I’ve seen the term APV approach. What does that mean?
A. A. APV is the International Civil Aviation Organization (ICAO) term for an approach with vertical guidance, and it refers to specific ICAO criteria adopted in May 2000. This approach classification allows the use of stabilized descent using vertical guidance without the accuracy required for traditional precision approach procedures. The US has developed criteria for lateral/vertical navigation (LNAV/VNAV) and LPV approach procedures that meet this approach classification. The LNAV/VNAV and LPV approaches provide guidance in both the lateral and vertical planes. Arrow up
Q. What’s an MMR?
A. An MMR is a multi-mode receiver. It would receive the basic GPS signal and the WAAS signal. It may also include the Ground Based Augmentation System (GBAS) signal at a future date. It may also receive VHF, UHF, VOR or other signals. Arrow up
Q. I’ve heard about Beta, Delta, and Gamma WAAS equipment. What’s the difference and which one will I need to buy for my single-engine aircraft? When will I be able to buy them?
A. These designations were used in the engineering standards for WAAS equipment, but they are not used operationally. WAAS equipment that provides PVT (position, velocity, time) into a flight management system, where the flight management system provides the actual navigation function is marked with a TSO-C145a. Panel-mount navigation systems that include a sensor and a navigation unit are TSO-C146a receivers. A special class of TSO-C146a equipment is an MMR that includes the WAAS capability to conduct LPV approach procedures. It does not support en route navigation other than providing PVT into an FMC.
