Frequently Asked Questions
- What is PNT?
- How many satellites are in the GPS constellation? NEW
- How accurate is the Global Positioning System? NEW
- How is military GPS different from civilian GPS?
- Isn't the Global Positioning System controlled by the U.S. military?
- Has the Global Positioning System ever been turned off by the military?
- Doesn't the 2004 policy call for deactivation of GPS during national security crises?
- Will the United States ever reactivate Selective Availability, since there is a war?
- Has the U.S. Government thought about privatizing GPS like the Europeans want to do with Galileo? If so, what would that mean for the existing free access to GPS?
- Where can I download high-resolution images of the GPS satellites?
- Where can I report GPS outages, interference, and other anomalies?
- Why does my GPS unit show my home/business in the wrong place? NEW
- Additional questions about Selective Availability
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 November 1, 2008, there were 33 satellites in the GPS constellation, with 31 set "healthy" to users. The most current status information is available here.
How accurate is the Global Positioning System?
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.
How is military GPS different from 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.
Isn't the Global Positioning System controlled by the U.S. military?
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.
Has the Global Positioning System ever been turned off by the military?
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.
Doesn't the 2004 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.
Will the United States ever reactivate Selective Availability, since there is a war?
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.
Has the U.S. Government thought about privatizing GPS like the Europeans want to do with Galileo? If so, what would that mean for the existing free access to 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.
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.