by Many of us use GPS almost every day – getting directions with mapping apps on our phones, tracking our meal deliveries, logging our running routes – but have you ever stopped to wonder, how GPS works?
Almost all smartphones use GPS technology, so let’s find out what that really means and why having a GPS receiver and a clear view of the sky means you’ll never get lost again.
What is GPS?
When people talk about “GPS”, they usually mean a GPS receiver, but the Global Positioning System (GPS) is a constellation of many satellites that orbit the Earth.gps.gov/systems/gps/space/”>31 in operational orbit and four classified as “reserve” or “unhealthy”).
The US military first developed and implemented this satellite network as a military navigation system, and then opened it up to everyone else.
Each of the 3,000- to 4,000-pound (1,361- to 1,814-kg) solar-powered satellites circles the globe at an altitude of about 12,427 miles (20,000 km), making two complete rotations each day. The orbits are arranged so that four satellites are “visible” in the sky at any time, anywhere on Earth.
A GPS receiver uses these satellites to calculate the exact location of the person operating the device.
How Does GPS Work?
GPS receivers work by finding four or more of these satellites, determining the distance to each one, and using this information to determine their own position.
This operation is based on a simple mathematical principle called trilateration. Trilateration in three-dimensional space can be a bit tricky, so we’ll start with an explanation of simple two-dimensional trilateration.
Trilation 2D
Imagine that you are somewhere in the United States and you are completely lost; for whatever reason, you have no clue where you are. You find a friendly local and asks, “Where am I?” It says, “You are 625 miles from Boise, Idaho.”
This is nice, hard stuff, but not particularly useful on its own. You could be anywhere on a circle around Boise with a radius of 625 miles, like this:
You ask another person where you are, and she says, “You are 690 miles from Minneapolis, Minnesota.” Now you are getting somewhere.
If you combine this information with Boise’s information, you have two circles that intersect. You now know that if you are 625 miles from Boise and 690 miles from Minneapolis, you must be at one of these two intersections:
If a third person tells you that you are 615 miles from Tucson, Arizona, you can eliminate one of the possibilities, because the third circle only intersects one of these points. You now know exactly where you are: Denver, Colorado.
This process is called 2D trilateration because all the intersection points are located on a two-dimensional plane. When we start removing height/height – hello, third dimension – 3D trilatation comes into play.
Trilation 3D
In essence, three-dimensional trilateration is not much different from two-dimensional trilateration, but it is a little more difficult to imagine. Imagine the rays from the previous examples going in all directions. So instead of a series of circles, you get a series of spheres.
If you know that you are 10 miles from satellite A in the sky, you could be anywhere on the surface of a giant imaginary sphere with a radius of 10 miles. If you also know that you are 15 miles from satellite B, you can overlap the first sphere with another larger sphere.
The spheres intersect in a perfect circle. If you know the distance to the third satellite, you will find the third sphere, which intersects this circle at two points.
The Earth itself can act as a fourth “satellite” or sphere; only one of the two possible points will be on the surface of the planet, so you can eliminate the one in space. However, receivers typically look at four or more satellites to improve accuracy and provide precise altitude information.
How GPS Devices Calculate Your Location
To function properly, a GPS device needs to know two things:
-
Location of at least three satellites above you
-
The distance between you and each of those satellites
GPS receivers do both of these things by analyzing high-frequency, low-power radio signals from Earth-orbiting satellites. Better units have multiple receivers, so they can pick up signals from several satellites at the same time.
Radio waves are electromagnetic energy, which means they travel at the speed of light (about 186,000 miles per second, or 300,000 km per second in a vacuum). The receiver can determine how far the GPS signal has traveled by the time it took for the signal to arrive.
GPS Math: Using Timing to Calculate Distance
At this point, you could confidently say to someone that you want to convince them that GPS works by trial and error. But you should be prepared for the follow-up question: How does the GPS device know the distance between those GPS satellites? As it turns out, it is a matter of time.
At a certain time (say midnight), the satellite starts transmitting a long digital pattern called a random pseudo code. The receiver also starts running the same digital pattern exactly at midnight. When the satellite signal reaches the receiver, the transmission of the pattern will be slightly behind the receiver’s playing of the pattern.
The length of the delay is equal to the travel time of the signal. The receiver increases this time by the speed of light to determine how far the signal has traveled. Assuming the signal travels in a straight line, this is the distance between the receiver and the satellite.
Maintaining Synchronicity
One major caveat is that the measurement only works if the GPS unit and the satellite have clocks that can be synchronized down to the nanosecond. This level of accuracy is only possible with atomic clocks, but those cost between $50,000 and $100,000 a pop.
Our tax dollars already pay for GPS satellites, but what about GPS receivers? Even Apple iPhones would be hard to sell at that price point.
The Global Positioning System has a smart and efficient solution to this problem: Each satellite has an expensive atomic clock, but the receiver itself uses a regular quartz clock, which it constantly resets.
In a nutshell, the receiver looks at incoming signals from four or more satellites and measures its own inaccuracy. By constantly resetting and rechecking its time against the GPS signals coming from the satellites, a low-end smartphone gets the accuracy of an atomic clock “for free.”
Differential GPS
GPS works well, but inaccuracies come up. For one thing, this method assumes that the radio signals will make their way through the atmosphere at a constant speed (the speed of light).
But satellite signals deal with interference all the time. The earth’s atmosphere slows the signals down, and large objects like skyscrapers can also affect their path.
Differential GPS (DGPS) helps correct these errors. The basic idea is to measure GPS inaccuracy at a stationary receiver station with a known position. Since the station’s DGPS hardware already knows its own position, it can easily calculate the inaccuracy of its receiver.
The station then broadcasts a radio signal to all DGPS-equipped receivers in the area, providing signal correction information for that area. In general, access to this correction information makes DGPS receivers much more accurate than conventional receivers.
Original article: How does GPS work?
Copyright © 2024 HowStuffWorks, a division of InfoSpace Holdings, LLC, a System1 Company
