When was the last time you used your phone to get Google Maps to pinpoint your exact location on a map, have you ever stopped and wondered how GPS works so accurately?
The Global Positioning System (GPS) was actually launched by the US Department of Defense in 1973 (known as NAVSTAR). By 1993, there were 24 GPS satellites orbiting the Earth, transmitting orbital and positional data that the military could use for navigation and other military purposes. Today, at the time of this writing, there are 28 of them.
In the 1980s, data from GPS satellites was opened to the public, opening up a whole market for the wide variety of GPS navigation devices we have today.
At the time of this writing, Russia, China, Europe, and India have their own active GPS systems. Japan is developing its own GPS system, which is scheduled to be operational in 2023.
How does GPS work?
Although the satellite technology on which GPS is based is very advanced, the system is impressively simple.
Each individual GPS navigation system has three components.
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- Satellites: GPS satellites revolve around the Earth and broadcast their current time and orbital 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 up to the satellites in orbit.
- GPS receivers. A GPS receiver on Earth receives orbital time data from so many satellites within range and calculates its own 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 location.
How GPS triangulation works
From anywhere in the world, if you hold a GPS receiver (for example, in your phone), it receives time stamps from a synchronized clock on each of the GPS satellites overhead.
Using the difference in time stamps and the constant speed of light at which the radio waves travel, the GPS receiver can determine the distance between your location and each satellite.
Thus, the GPS receiver knows the radius of the spheres with satellites in the center and your position at the edge of the sphere.
Since each satellite is moving in a predictable orbit over the Earth, the receiver can use the stored almanac of the current known position of all GPS satellites to roughly determine where those 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 receiver can calculate your approximate position by locating the intersection of these three spheres on the Earth’s surface. P>
The receiver then displays this location on a map.
Three satellites provide an approximate location, and GPS receivers require a fourth signal from another GPS satellite in order to determine your current altitude on the earth’s surface using another mathematical principle known as trilateration.
How does the GPS sensor on your phone work
Most modern smartphones today are equipped with a GPS receiver chip. This chip can receive radio signals from GPS satellites.
Your phone’s clock is not an atomic clock, so its time is not synchronized with the atomic clock of satellites in orbit. However, this does not matter when it comes to calculating the position from the signals of these satellites.
This is because your phone’s GPS receiver focuses on the data it receives from the satellites and a database of known satellite locations on Earth. Since all satellites have atomic clocks, the current time on each satellite is the same at any time.
However, due to the distance from the satellite and the fact that radio signals travel at the speed of light, the difference between the time stamps received indicates the distance between your phone and each of the satellites.
Here’s how this GPS process works:
- All four satellites send the same exact time stamp to your phone at 17:12:14.
- Your phone receives this time stamp at 5 o’clock. : 12: 15 from satellite 1.
- It gets the time stamp at 5:12:16 from satellite 2.
- Finally at 5:12:17 it gets the time stamp from the satellite 3.
This tells your GPS receiver that it took the radio signal 1 second to reach it from satellite 1, 2 seconds from satellite 2 and 3 seconds from satellite 3
The known constant speed of light is 299,792,458 meters per second.
Using simple math, the receiver can calculate that it is approximately 300,000 meters from satellite 1, 600,000 meters from satellite 2, and 900,000 meters from satellite 3.
Using the lookup table from the GPS satellite database, your phone’s GPS receiver knows the approximate current location of all three satellites above the Earth, which provides the longitude and latitude of all three satellites.
With this information, your phone can calculate your own longitude and latitude coordinates on Earth.
Using your known coordinates, your GPS receiver can use the distance measured between it and the fourth satellite to determine how high you are above the Earth.
What is Assisted GPS?
Before smartphones started integrating GPS circuits, people used to use portable GPS receivers that run on AA batteries. Or they installed GPS devices in cars that would run on the phone’s battery.
This is because radio communication requires more power. The limitation of this is that you often had to wait several minutes for your GPS receiver to “acquire†enough GPS satellites to calculate your location.
Smartphone manufacturers have bypassed this battery limitation by integrating existing cellular triangulation technology. Long before phones had GPS enabled, they could use signals from cell towers to triangulate your location using the same triangulation technology used for time and distance stamps as for GPS satellites.
Unfortunately, since the cell towers are at ground level, this navigation calculation is much less accurate. Thus, your smartphone’s GPS software will first use cell triangulation to determine your approximate location, and then update that position when GPS satellite data is ready.
This allows modern smartphones to reserve battery power using GPS data only when these location updates are required. This is why you can often see your location on Google Maps, sometimes navigating to a new location when more accurate data is available.
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