Most Frequently Asked Questions & Answers on GPS

How was GPS developed?
A: From an idea that took shape during the "race to space" between the United States and the Soviet Union in the late 1950's, scientists at John Hopkins University developed a way to use radio signals originating from a satellite in space to provide accurate position updates to navigation equipment located on the U.S. Navy's ships and submarines. By the mid-1960's, the Air Force initiated a program consisting of several satellites with very accurate clocks onboard that could give off timing signals of their location in space which would accurately determine the position of a vehicle moving on land or in the air. In 1973, the Navy and the Air Force programs combined to form the Navigation Technology Program, which later evolved into the Navstar Global Positioning System (GPS) in operation today. 

Navstar GPS is the world's premier position, navigation and timing information service. The U.S. military and its allies are not the only beneficiaries of Navstar GPS however; civilian users around the globe rely upon Navstar GPS as well. In fact, the system serves millions of civil users. Over 1.4 million civilian GPS receivers have been produced each year since 1997. Perhaps very few individuals realized early on the financial impact that GPS would have in the global financial landscape. This is not the case now. The rapidly growing GPS market, including equipment and applications, reached $6.2 billion in 2000 and is expected to easily surpass $50 billion by 2010. 

Can I get a good layman's description of how it works?
A: Managed by the Navstar GPS Joint Program Office at the Space and Missile Systems Center at Los Angeles Air Force Base, California, Navstar GPS is a space-based radio navigation and time distribution system. The GPS constellation consists of 24 or more satellites, which orbit the Earth in six distinct orbital planes at an altitude of 10,900 nautical miles. GPS satellites circle the Earth twice per day and continuously transmit signals, which provide navigation and timing information to military and civilian users worldwide. GPS consists of three main elements, or "segments." In addition to the satellites themselves -- called the "Space" segment --the system includes a worldwide satellite control network -- the "Control" segment -- and GPS receiver units -- called the "User" segment. GPS receivers use the signals from the satellites to compute position and time information for users. The receivers do not send out any signals, or communicate back to the satellites.

The fundamental concept of GPS is to use simultaneous distance measurements from four satellites to compute the position and time of any receiver. The GPS satellites broadcast signals on two different frequencies so that sophisticated user receivers can correct for distortion effects due to the ionosphere, a layer of the atmosphere several hundred miles above the Earth. It takes between 65 and 85 milliseconds for a signal to travel from a GPS satellite to a receiver on the surface of the Earth. The signals are so accurate that time can be figured to much less than a millionth of a second, velocity can be figured to within a fraction of a mile per hour, and location can be figured to within a few meters. Typical horizontal positioning accuracy for military users is 5 to 10 meters, while for single frequency users is 10 to 20 meters.

What type of timing device is used on the GPS satellites and how accurate are they?
A: The GPS constellation presently (June 2005) consists of 1 Block II and 15 Block IIA satellites built by Boeing, and 7 Block IIR satellites built by Lockheed Martin. 8 Block IIRs remain to be launched. Each Block II or IIA satellite has two cesium atomic clocks and two rubidium atomic clocks, while each Block IIR has three rubidium atomic clocks. The stability of these clocks is estimated to be approximately 1 second per 300,000 years. (Atomic clocks utilize the vibration of atoms to achieve their accuracy. They are not nuclear powered.) Only one clock is in use on each satellite at a time. The others are backups.

What powers the satellites?
A: Each Block II/IIA satellite has two solar arrays, each 67" by 130". Each Block IIR solar array consists of two panels (four total on each satellite), with each panel 70" x 75". In addition to providing power to the satellite, the solar arrays are also used to charge batteries, which the satellite uses for power whenever it is in darkness. The Block II/IIA satellites have three nickel-cadmium batteries and the Block IIR satellites have two nickel-hydrogen batteries.

How much does it cost to operate?
A: The approximate cost to operate and maintain the constellation, including research and development, as well as, procurement for replenishment satellites, is $750M annually.

Can you describe what it takes to operate the system, and what they have to do to keep the satellites and signals working, etc.
A: The GPS worldwide satellite network consists of six monitor stations and four ground antenna stations. The monitor stations (located at Ascension Island, Cape Canaveral, Colorado Springs, Diego Garcia, Kwajalein and Hawaii) use specially designed GPS receivers to passively track the navigation signals of all of the satellites. Data from the monitor stations is continually sent to the GPS Master Control Station (MCS), located at Schriever Air Force Base, Colorado, for processing. The MCS computes precise, updated information on the satellites' orbits and clock status every 15 minutes, 24 hours a day, seven days a week. Updated navigation information is sent from the MCS to the ground antenna stations (located at Ascension Island, Cape Canaveral, Diego Garcia and Kwajalein) and then to the satellites. These "uploads" are done once or twice per day for each satellite. This is sufficient to maintain the high accuracy of the GPS constellation. 
In addition to uploads, the ground antennas are also used to transmit commands to the satellites and to receive the satellites' telemetry data. In addition to the MCS, the Air Force currently maintains an interim backup Master Control Station near Washington, DC, while a permanent alternate Master Control Station is under construction at Vandenberg AFB, California.

Can you tell me how many civilian companies make products that use GPS?
A: Although the Air Force does not keep track of the number of civilian companies that make GPS-related products, it is commonly known that civil users outnumber military users by 100 to 1 and the ratio is increasing. The Compound Annual Growth Rate of the GPS market is growing by approximately 22%. The Department of Commerce conducted a comprehensive market study in September 1998. The study expected worldwide sales to reach $4B by the end of 1998. Sales continued to grow by a little over $1B per year through 2000 to $6.2B.

What does a civilian company have to do to use the signal?  Don't they have to get some sort of code? 
A: To access GPS for general use, a person may purchase a handheld receiver or a vehicle equipped with a GPS navigation system. The signal is free to all users worldwide. No subscription, license, fee or registration is required.

What plans are underway to ensure the reliability of GPS in the future?
A: To ensure the best GPS system for the next 30 years, the future of GPS includes increased power and accuracy, as well as increased civil navigation safety with the addition of a new civil signal on the L2 link and a new civil-only signal on a new link, L5, to be broadcast at 1176.45 MHz. A new military-only signal (M-code) on the L1 and L2 links is programmable for completion in 2010. It will have increased power and reduced vulnerability to signal jamming. In addition to the improved signals, the reliability of the GPS navigation message will be improved by adding more monitor stations. These additional monitor stations will ensure that each satellite is simultaneously monitored by no less than two monitor stations. The data collected by these additional monitor stations will be combined with the data from the existing monitoring stations, and sent to the MCS for processing. The result is improved accuracy of the navigation message broadcast by the satellite.

For your further research:
As a service to GPS users, the DoT has established the �Navigation Information Service� (formerly �GPS Information Service�) as a point of contact for GPS users.  Operated and maintained by the U.S. Coast Guard, the NIS can be reached at (703) 313-5900 seven days a week, 24 hours a day and at www.navcen.uscg.gov.

For more detailed overviews of GPS, log onto: www.aero.org/publications/GPSPRIMER

There are also many books about GPS available today, with more published all the time.  They cover topics ranging from recreational and commercial applications of GPS to highly technical engineering volumes for scientists and engineers.

(Current as of June 2005)