FAQ

Q: 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.

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 "pseudo range") 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.

Q: Who uses GPS?

 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.

Q: What is the status of GPS?

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.

Q: How accurate is GPS?

 With the elimination of Selective Availability, GPS accuracy is typically within 10 to 20 meters, and with the help of WAAS, accuracy can be well below 10 meters. SA is an intentionally introduced degradation in the accuracy of civilian GPS by the United States Department of Defense. SA errors can degrade the GPS accuracy up to 100 meters. However, this error is adjustable and random. Typically, errors of 30 meters or more are not unusual. Now that SA has been removed, typical GPS position accuracy is approximately 15 meters (50 feet).

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.

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.

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.

Q: How can private and civil users depend on a system that is controlled by th US Military?

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.

Q: How many GPS Satellites are in orbit in the GPS Constellation at any given time?

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. 

Q: What type of orbits are GPS satellites in?

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.

Q: Are there plans to increase GPS capabilities in the future?

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.

Q: Can GPS be susceptible to jamming or interference?

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.

Q: Are there concerns about Radio Frequency Interference (RFI) ?

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.

Q: What is Differential GPS?

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 pseudo range measurements, just as any user receiver would. However, because the reference receiver knows its position accurately, it can determine “biases” in its pseudo range measurements. For each satellite in view of the reference receiver, these biases are computed by differencing the pseudo range measurement and the satellite-to-reference receiver geometric range. These biases incurred in the pseudo range 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 pseudo range 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.

Q: Will my unit work in bad weather?

GPS works 24 hours a day, anywhere in the world and is not affected by clouds, storms or weather. This is one of the advantages GPS has over Loran C.

Q: What is the difference between 12 channel and 16 channel?

A 12-channel receiver has 12 dedicated channels reserved for satellite communication. The additional channels on a 16-channel receiver provide quicker lock-on, more up to date information and the ability to stay locked-on when you are in a challenging area (a heavily wooded area or in locations where the horizon is blocked by mountainous terrain, cliffs, valley slopes or tall buildings).

Q: Why is my unit not locking on to any satellites?