The last time you used your phone to Google Maps pinpoints your exact location on a map, have you ever wondered how GPS works so accurately?
The Global Positioning System (GPS) system was launched in 1973 by the United States Department of Defense (known as NAVSTAR). In 1993, 24 GPS satellites were in orbit, transmitting orbital and positional data that the military could use for navigation and other military purposes. At the time of writing, there are 28.
In the 1980s, data transmitted by GPS satellites was opened up to the public, opening up a whole market for the wide range of GPS navigation devices we have today.
At the time of writing this article, Russia, China, Europe and India all have their own active GPS systems. Japan is developing its own GPS system that should be operational by 2023.
How does GPS work?
While the satellite technology on which GPS is based is very advanced, the way the system works is impressively simple.
Each individual GPS navigation system consists of three components.
- Satellites: GPS satellites orbit the Earth and transmit their current time and orbit position to all GPS receivers on their side of the planet.
- Command center: The command center transmits orbital data, time corrections, and the orbital position of other satellites to the orbiting satellites.
- GPS receivers: A GPS receiver on Earth receives orbital times from as many satellites in range and calculates its position on Earth based on the positions of at least four GPS satellites.
GPS receivers use a mathematical principle known as triangulation to calculate their own location.
How does GPS triangulation work?
From any point on the planet, if you’re holding a GPS receiver (like the one in your phone), a GPS receiver will receive timestamps from the synchronized clocks on each of the GPS satellites overhead.
Using the differences in timestamps and the constant speed of light at which radio waves travel, the GPS receiver can determine the distance between where you are and each satellite.
This gives the GPS receiver the radius of the spheres with the satellites in the center and your location on the edge of the sphere.
Since each satellite travels in a predictable orbit around the Earth, the receiver can use a stored almanac of the current known position of all GPS satellites to determine approximately where these satellites are currently above the Earth.
With the known position of each satellite and the measured distance between those satellites and your position, your GPS reception can calculate your approximate location by determining where the intersection of those three spheres converge on the Earth’s surface.
The receiver then shows you that location on a map.
Three satellites provide a rough location, and GPS receivers need a fourth signal from another GPS satellite to determine your current elevation on the Earth’s surface using another mathematical principle known as trilateration.
How your phone’s GPS sensor works
Most modern smartphones today are equipped with a GPS receiver chip. This chip can receive the radio signals from GPS satellites.
Your phone’s clock is not an atomic clock, so time is not synchronized with the atomic clocks of the orbiting satellites. However, this doesn’t matter when it comes to calculating location using signals from those satellites.
This is because your phone’s GPS receiver focuses on the data it receives from the satellites and on a database of known satellite locations above the Earth. Since all satellites have an atomic clock, the current time on each satellite is exactly the same at all times.
Due to the distance from the satellite and the fact that the radio signals travel at the speed of light, the differences between each received timestamp reveal the distance between your phone and each of the satellites.
Here’s how this GPS process works:
- All four satellites will transmit the exact same timestamp to your phone at 5:12:14 PM.
- Your phone will receive that timestamp at 5:12:15 from satellite 1.
- It receives the timestamp at 5:12:16 from satellite 2.
- Finally, at 5:12:17, it receives the timestamp from satellite 3.
This tells your GPS receiver that it took 1 second for the radio signal to reach it from satellite 1, 2 seconds from satellite 2 and 3 seconds from satellite 3.
The speed of light is a known constant of 299,792,458 meters per second.
Using simple math, the receiver can calculate that the distance is about 300 thousand meters from satellite 1, 600 thousand meters from satellite 2 and 900 thousand meters from satellite 3.
Using a lookup table from a GPS satellite database, your phone’s GPS receiver knows the approximate current location above the Earth of all three satellites, which gives the latitude and longitude coordinates of all three.
With that information, your phone can calculate: your own latitude and longitude coordinates on earth.
Using your known coordinates, your GPS receiver can use the distance between itself and a fourth satellite to determine your altitude above the Earth.
What is an Assisted Global Positioning System?
Before smartphones started integrating GPS circuits, people mostly used portable GPS receivers that ran on AA batteries. Or they installed GPS units in cars, which were connected to the phone’s battery.
This was because radio communications require more power. The limitation of this is that you often had to wait several minutes for your GPS receiver to “lock” enough GPS satellites to calculate your position.
Smartphone manufacturers have circumvented this battery limitation by combining their existing technology of cellular triangulation. Long before phones were GPS enabled, they could use signals from cell towers to triangulate your position, using the same sort of triangulation timestamp and range technology as GPS satellites.
Unfortunately, this navigation calculation is much less accurate because cell phone towers are at ground level. So your smartphone’s GPS software first uses cell signal triangulation to determine your approximate position and then updates that position once the GPS satellite data is ready.
This allows modern smartphones to reserve battery power by only using GPS data when those location updates are required. This is why you often see your location on Google Maps it occasionally jumps to a new location when more accurate data is available.