How Does GPS Work?
The Short Answer:
GPS is a system of 30+ navigation satellites circling Earth. We know where they are because
they constantly send out signals. A GPS receiver in your phone listens for these signals. Once the
receiver calculates its distance from four or more GPS satellites, it can figure out where you are.
Earth is surrounded by navigation satellites. Credit: NOAA.
Humans have looked to the skies to find their way since ancient times. Ancient sailors used the
constellations in the night sky to figure out where they were and where they were going.
Today, all we need is a simple hand-held GPS (short for Global Positioning System) receiver to
figure out exactly where we are anywhere in the world. But we still need objects high in the sky
to figure out where we are and how we get to other places.
Instead of stars, we use satellites. Over 30 navigation satellites are zipping around high above
Earth. These satellites can tell us exactly where we are.
What is GPS?
The Global Positioning System (GPS) is made up of satellites, ground stations, and receivers.
GPS is a system. It’s made up of three parts: satellites, ground stations, and receivers.
Satellites act like the stars in constellations—we know where they are supposed to be at any
given time.
The ground stations use radar to make sure they are actually where we think they are.
A receiver, like you might find in your phone or your parent's car, is constantly listening for a
signal from these satellites. The receiver figures out how far away they are from some of them.
Once the receiver calculates its distance from four or more satellites, it knows exactly where you
are. Presto! From miles up in space your location on the ground can be determined with
incredible precision! They can usually determine where you are within a few yards of your actual
location. More high-tech receivers, though, can figure out where you are within a few inches!
The ancient sailors of history would be flabbergasted by the speed and ease of pinpointing your
location today.
GPS in everyday life
There are a whole lot of important things GPS is used for—but perhaps nothing is more
important than finding the quickest slice of pizza! Check out our fun Space Place in a Snap
animation “GPS and the Quest for Pizza” to learn more about how GPS works.
By NASA science
Satellite Navigation - GPS - How It Works
Satellite Navigation is based on a global network of satellites that transmit radio signals from medium
earth orbit. Users of Satellite Navigation are most familiar with the 31 Global Positioning System (GPS)
satellites developed and operated by the United States. Three other constellations also provide similar
services. Collectively, these constellations and their augmentations are called Global Navigation Satellite
Systems (GNSS). The other constellations are GLONASS developed and operated by the Russian
Federation, Galileo developed and operated by the European Union, and BeiDou, developed and operated
by China. All providers have offered free use of their respective systems to the international community.
All providers have developed International Civil Aviation Organization (ICAO) Standards and
Recommended Practices to support the use of these constellations for aviation.
The basic GPS service provides users with approximately 7.0 meter accuracy, 95% of the time, anywhere
on or near the surface of the earth. To accomplish this, each of the 31 satellites emits signals that enable
receivers through a combination of signals from at least four satellites, to determine their location and
time. GPS satellites carry atomic clocks that provide extremely accurate time. The time information is
placed in the codes broadcast by the satellite so that a receiver can continuously determine the time the
signal was broadcast. The signal contains data that a receiver uses to compute the locations of the
satellites and to make other adjustments needed for accurate positioning. The receiver uses the time
difference between the time of signal reception and the broadcast time to compute the distance, or range,
from the receiver to the satellite. The receiver must account for propagation delays or decreases in the
signal's speed caused by the ionosphere and the troposphere. With information about the ranges of three
satellites and the location of the satellite when the signal was sent, the receiver can compute its
three-dimensional position. An atomic clock synchronized to GPS is required to compute ranges from
these three signals. However, by measuring a fourth satellite, the receiver avoids the need for an atomic
clock. Thus, the receiver uses four satellites to compute latitude, longitude, altitude, and time.
References:
1. How does GPS work? | NASA Space Place – NASA Science for Kids. (n.d.).
https://spaceplace.nasa.gov/gps/en/#:~:text=GPS%20is%20a%20system%20of,figure%2
0out%20where%20you%20are.
2. Satellite navigation - GPS - How it works. (n.d.). Federal Aviation Administration.
https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/nav
services/gnss/gps/howitworks
