Global positioning system (GPS)

The Global Positioning System (GPS) is a satellite-based navigation system that allows users to accurately determine their location on Earth. It consists of orbiting satellites that transmit location and time data, ground control stations that ensure proper satellite operation, and user devices that receive radio signals from the satellites and use the data to calculate their position. The network of GPS satellites and control stations is overseen by the United States government. GPS receivers are available as standalone devices and are also built into many products, such as smartphones and automobiles.

GPS was originally developed for military applications and launched in the 1970s. In the 1980s, the US government made the system available for civilian use, initially with some limitations. Usage increased steadily through the 1990s and into the early twenty-first century, and the accuracy of the system improved as well as technology advanced. GPS is widely used as a navigation and positioning tool in numerous fields, from critical military operations to everyday consumer conveniences like online maps and directions. It has enabled efficiencies in many commercial endeavors, such as transportation, surveying, and outdoor recreation. In the scientific community, GPS plays an important role in geology, meteorology, wildlife studies, archeology, and many other areas.

History

GPS technology is rooted in the development of space satellites after World War II. Scientists developed methods to track satellites in orbit using radio signals. During the Cold War, the US military explored ways to use satellite signals to calculate the position of a mobile receiver device on the ground. Such a system would allow accurate navigation regardless of weather conditions or the availability of other communication systems. The US Department of Defense began the Navigation System with Timing and Ranging (NAVSTAR) project in 1973 and launched the first satellite in the system in 1978. Over the years, NAVSTAR evolved into the Global Positioning System (GPS). The network reached its full capability with the launch of a twenty-fourth satellite in 1993.

GPS was initially available only for US government purposes. In the 1980s, it was made available for civilian use as a way to help improve air traffic safety and other navigation needs. At first, however, civilian GPS devices were intentionally less accurate than military ones. This intentional degradation of the signal was known as Selective Availability (SA), and was intended to prevent military adversaries from using the highly accurate GPS signals. This limitation ended in 2000, as technology advances made it increasingly unnecessary for national security.

The US government also continued to work to upgrade GPS satellites over the years. By the 2020s, most GPS receivers were accurate within less than 20 feet, especially when combined with other positioning data such as through Wi-Fi signals. Some sophisticated professional devices were accurate down to less than an inch.

GPS Signal Transmission

GPS satellites transmit two low-power radio signals, designated “L1” and “L2.” Civilian GPS uses the L1 frequency of 1575.42 MHz in the UHF band. A GPS signal contains three different bits of information: a pseudorandom code, ephemeris data, and almanac data. The pseudorandom code is simply an identification code that identifies which satellite is transmitting information. Ephemeris data, which are constantly transmitted by each satellite, contain important information about the status of the satellite (healthy or unhealthy), current date, and time. The almanac data tell the GPS receiver where each GPS satellite should be at any time throughout the day. Each satellite transmits almanac data showing the orbital information for that satellite and for every other satellite in the system.

Trilateration

The satellites transmit signal information to Earth. GPS receivers take this information and calculate the user’s exact location through a process called trilateration. Each satellite continuously transmits a data stream containing orbit information, equipment status, and the exact time. GPS receivers contain computer chips that then calculate the difference between the time a satellite sends a signal and the time it is received. The unit multiplies this time of signal travel by the speed of travel to get the distance between the GPS receiver and the satellite. Since these are radio waves, the speed used is the speed of light.

One satellite gives a sphere on which the receiver sits. Two satellites give two spheres on which the receiver sits. The intersection of two spheres (and they must intersect) is a circle. Adding a third satellite gives the receiver one of two points at which the sphere will intersect the circle. Using the geoid of the Earth as the fourth solid, the receiver fixes the point of location.

Despite the power of trilateration, there is still some possibility for error if the clock on the receiver has a slight error. A clock error of only one-thousandth of a second causes a position error of almost 200 miles. The solution is to use geometry. If one more satellite is added, then even if the clock in the receiver is off, it is off for all of the satellites by the same amount. The receiver lies on a line from each of the satellites. If all clocks are exact, then the receiver will sit at the intersection of the lines. However, the error in the receiver clock will cause the lines to intersect in different points, resulting in a polygon surrounding the receiver. The receiver can be calculated to be at the center of this polygon.

GPS Capabilities and Accuracy

A GPS receiver must be locked on to the signal of at least three satellites to calculate the latitude and longitude and to track movement. With four or more satellites, the receiver can determine the user’s latitude, longitude, and altitude. Once the user’s position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset times, and more. Most GPS receivers are accurate to within 15 meters on average. Newer GPS receivers often come with wide-area augmentation system (WAAS) capability that can improve accuracy to less than three meters on average.

Users can also get better accuracy with differential GPS (DGPS), which corrects GPS signals to within an average of three to five meters. The US Coast Guard operates the most common DGPS correction service. This system consists of a network of towers that receive GPS signals and transmit a corrected signal by beacon transmitters. In order to get the corrected signal, users must have a differential beacon receiver and beacon antenna in addition to their GPS.

Possible sources of error include the following:

• Ionosphere and Troposphere Delays. Different layers of the atmosphere have different impacts on the speed of the satellite signal through those layers. Mathematicians have been working on creating better models of these atmospheric layers in order to give smaller errors.
• Geoid Error. The receiver uses a mathematical model of the surface of Earth, the geoid. Better mathematical models can improve the accuracy as long as they are relatively easy to use in computation.
• Signal Multipath. The GPS signal may be reflected off objects, increasing the travel time of the signal, thereby causing errors. Mathematicians are working on developing models to account for multipath based on the relative location of receiver.
• Orbital Errors. Inaccuracies in the satellite’s reported location are handled by the control segment, which tries to keep each satellite on track.
• Number of Satellites Visible. If only three satellites are visible, the receiver gives a position with a warning that it is likely to be very inaccurate.
• Satellite Geometry/Shading. Differences in the relative position of the satellites at any given time may cause errors. Ideal satellite geometry exists when the satellites are located at wide angles relative to each other. Poor geometry results when the satellites are located in a line or in a tight grouping.

Bibliography

Cooke, D. Fun with GPS. ESRI Press, 2005.

"GPS." National Geographic Education, education.nationalgeographic.org/resource/gps/. Accessed 18 Nov. 2024.

"GPS Overview." GPS.gov, www.gps.gov/systems/gps/. Accessed 18 Nov. 2024.

Kaplan, Elliot D., and Christopher Hegarty, eds. Understanding GPS: Principles and Applications. 3rd ed. Artech House, 2017.

Levitan, Ben. GPS Quick Course: Systems, Technology and Operation. Althos, 2007.

Manning, Catherine G. "GPS." NASA, 25 Sept. 2023, www.nasa.gov/directorates/somd/space-communications-navigation-program/gps/. Accessed 18 Nov. 2024.