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GNSS Frequently Asked Questions - GPS

GNSS - Frequently Asked Questions - GPS

  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?

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?

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)?

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?

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?

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  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   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?

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?

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)?

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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