Frequently Asked Questions

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.

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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.

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