GPS and satellite navigation

Your phone can show a dot on a map.

The dot is where you are. How does the phone know? It listens to satellites far up in space.

When a map app shows a dot where you are standing, it is using signals from space. NASA's Space Place explains that GPS, short for Global Positioning System, is a system of more than 30 navigation satellites circling Earth. A receiver in your phone listens for their signals.

Once the receiver works out its distance from four or more GPS satellites, it can figure out where you are. Long ago, sailors used the stars to find their way. Today, we use satellites instead of stars.

GPS.gov, the U.S. government's official GPS site, describes the Global Positioning System as a U.S.-owned utility that provides positioning, navigation and timing services. The civilian service is freely available to all users, continuously and worldwide, which is why GPS is now in everything from phones and wristwatches to bulldozers, shipping containers and cash machines.

The principle is simple to state. Satellites broadcast signals giving their position and the time; a receiver measures how far it is from several satellites and works out where it must be to be those distances from all of them. NASA's Space Place notes that sailors once used constellations to navigate; GPS satellites play the role of stars whose positions are known exactly.

GPS has three parts.

There are satellites in space. There are stations on the ground that look after them. And there is the receiver in your phone or car.

GPS is made of three parts that work together. The space part is the satellites, which send out signals. The control part is a network of stations on the ground that track the satellites, check their signals and send them updates. The user part is every GPS receiver, in phones, cars, planes and ships, that listens to the signals.

The U.S. Space Force runs the satellites and the ground stations. Receivers only listen; they never send anything back to the satellites.

GPS.gov describes three segments. The space segment is a constellation of satellites transmitting one-way signals that give each satellite's position and time. The control segment is a worldwide network of monitor and control stations that track the satellites, keep them in their proper orbits with occasional maneuvers, adjust their clocks and upload updated navigational data; it currently includes a master control station, an alternate master control station, 11 command and control antennas and 16 monitoring sites.

The user segment is the receiver equipment, which uses the signals to calculate the user's three-dimensional position and time. GPS.gov notes that billions of users worldwide rely on these signals. The U.S. Space Force develops, maintains and operates the space and control segments.

The GPS satellites are very high up.

Each one goes all the way around the Earth two times every day. There are about 30 of them working.

GPS satellites fly in what is called medium Earth orbit, about 20,200 kilometers (12,550 miles) above Earth. Each one circles the planet twice a day. They are spread out in six orbits around Earth, so that from almost anywhere you can pick up at least four of them.

The United States promises to keep at least 24 GPS satellites working. GPS.gov reported that, as of July 3, 2023, there were 31 operational GPS satellites, not counting spares.

GPS satellites orbit in medium Earth orbit at about 20,200 km (12,550 miles), each circling Earth twice a day. GPS.gov explains that they are arranged in six equally spaced orbital planes, each with four slots for baseline satellites; this 24-slot arrangement lets users see at least four satellites from virtually any point on Earth. A 2011 expansion turned it into an effective 27-slot constellation.

The U.S. commits to at least 24 operational satellites 95% of the time, and GPS.gov says the Space Force has flown 31 operational satellites for well over a decade. Its satellite table reports 31 operational satellites as of July 3, 2023, not counting decommissioned spares, a mix of generations from Block IIR (launched from 1997) to GPS III (first launched in 2018).

Each satellite has a very good clock.

It sends out the time again and again. Your phone checks how long the message took to arrive. That tells it how far away the satellite is.

The secret of GPS is time. NOAA explains that each GPS satellite carries one or more atomic clocks, which are amazingly precise. The satellites send out radio signals with exact time stamps.

Radio signals travel at a steady speed, the speed of light. So if a receiver knows exactly how long a signal took to arrive, it can work out how far away that satellite is. With distances to four or more satellites, it can work out where it is.

NOAA explains that GPS satellites carry atomic clocks accurate to within 40 nanoseconds (40 billionths of a second) and broadcast radio signals with precise timestamps. Because the signals travel at a constant speed, the speed of light, the travel time from satellite to receiver gives the distance between them.

Precision matters because light is fast: in one billionth of a second it travels about 30 centimeters, so tiny timing errors become position errors. A receiver uses at least four satellites because it solves for four unknowns, its three position coordinates and its own clock error. As a side effect, GPS.gov notes, receivers become synchronized to the satellites' atomic clocks and can tell the time to within 100 billionths of a second.

GPS can find you very closely.

Outside, a phone can usually find you within a few steps. Tall buildings and trees can block the signals and make it less sure.

GPS.gov says GPS-enabled smartphones are typically accurate to within about 4.9 meters (16 feet) under open sky. Near buildings, bridges and trees, accuracy gets worse, because they can block the signals or bounce them around. GPS also struggles indoors and underground.

Special receivers used by surveyors can do much better, finding positions to within a few centimeters. And sometimes when GPS seems wrong, the GPS is fine but the map in the app is out of date.

GPS.gov's answer to "how accurate is GPS?" is "it depends." Satellites broadcast with a known accuracy, but what a user gets depends on satellite geometry, signal blockage, atmospheric conditions and receiver quality. A GPS-enabled smartphone is typically accurate to within a 4.9 m (16 ft) radius under open sky; dual-frequency receivers and augmentation systems allow real-time positioning to a few centimeters and long-term measurements at the millimeter level.

Common causes of error are signals blocked by buildings, bridges or trees; use indoors or underground; and multipath, signals reflected off buildings or walls. The same list includes radio interference or jamming, major solar storms and satellite maintenance. GPS.gov adds that many "GPS errors" are really mapping-software errors: badly drawn maps, mislabeled businesses or missing roads.

GPS belongs to the United States.

Other places have their own satellites too. Europe has one called Galileo. China has one called BeiDou.

GPS is not the only satellite navigation system. The general name for all of them is GNSS, which stands for global navigation satellite system. GPS.gov lists the main ones: Galileo, run by the European Union; BeiDou, run by China; and GLONASS, run by Russia. India and Japan run regional systems that cover their own parts of the world.

These systems work in the same basic way: satellites broadcast their positions and the time, and receivers work out where they are.

GPS.gov describes the other main GNSS. BeiDou (BDS), owned and operated by China, was formally commissioned in 2020 with 35 satellites in the operational system. Galileo, owned and operated by the European Union, began Initial Services in 2016 and has 24-plus satellites. GLONASS, Russia's system, consists of 24-plus satellites when fully operational.

Two regional systems complement them. India's NavIC (formerly IRNSS) has 7 satellites covering India and about 1,500 km around its mainland. Japan's QZSS, which complements GPS over East Asia and Oceania, began official service in 2018 with 4 satellites and now has 7-plus. GPS.gov notes that these systems provide complementary, independent positioning, navigation and timing.