You are here: Home page > Gadgets > GPS satellite navigation
Advertisement

Navigating with GPS on an iPhone

GPS satellite navigation

by Chris Woodford. Last updated: November 8, 2014.

Have you ever been lost? Unless you're a polar explorer, a fan of tropical rainforests, or the sort of person who drives across the Sahara desert in a rally car just for fun, chances are that you always have a fairly good idea of where in the world you are. Centuries ago, before the invention of cars, airplanes, and fast ocean ships, Earth must have seemed a huge, dangerous, and scary place. Not any more. With GPS satellite navigation devices (also known as "sat nav"), you can pinpoint your location to within a few feet, wherever you happen to be. Let's take a closer look at GPS and find out how it works!

Photo: Getting lost is a thing of the past thanks to mobile devices like this with built-in GPS receivers and mapping apps.

Using landmarks to find where you are

Drawing of explorer Ferdinand Magellan

Suppose you're in the center of a strange town, where you've arranged to meet a friend. You call them up on your cellphone and try to explain where you are—but how do you do it, exactly? Most people would look around them for landmarks and say something like: "I'm in the square, next to the bank, just across from the statue of George Washington." In other words, we define our location relative to known landmarks. The more landmarks we use, the more precisely we can locate ourselves. If we just say "I'm in the square", that could still be quite vague if the square is a large area. But adding in the extra details about the bank and the statue helps our friend locate us precisely.

Photo: Explorers like Ferdinand Magellan (1480-1521) sailed the globe with great skill and ingenuity, but imagine how much easier their lives would have been with satellite navigation! Public domain engraving courtesy of US Library of Congress.

How satellites work as landmarks in the sky

Ancient navigators, sailing in the open ocean, had no landmarks they could use to locate themselves—so they used the fixed positions of the stars to guide them instead. Modern navigators use a hi-tech version of the same idea called satellite navigation. Instead of looking at lights from the stars, they use radio signals emitted by networks of satellites orbiting around Earth. The satellites are effectively "sky landmarks" that tell you where you are.

NAVSTAR GPS satellite under construction on Earth

GPS and other satellite navigation systems

There are three different satellite navigation systems used around the world, the best known of which is the US Global Positioning System (GPS), which uses 24 satellites named NAVSTAR orbiting 18,000 km (11,000 miles) above Earth. Originally developed by the US military, GPS is now widely used for civilian purposes too; most car-based satellite navigation devices use GPS, for example. In Europe, a rival system called Galileo was launched in 2005 and the Russians have their own system called GLONASS (Global Navigation Satellite System).

Photo: A NAVSTAR satellite pictured during construction on Earth in 1981. You can get an idea how big the satellite is from the engineer pictured some distance beneath it. Picture courtesy of US Department of Defense.

What can we use satellite navigation for?

Satellite navigation systems are incredibly accurate. The NAVSTAR satellites have atomic clocks on board that make their time signals accurate to one second in 300,000 years. That means the military versions of GPS receivers can pinpoint things to within just 5 cm (2 inches). This incredible accuracy makes satellite navigation amazingly useful. It can show ships, aircraft, and cars where they are. It can help farmers to monitor their crop yields. And, combined with an audio system that speaks out directions, it can even help blind people to navigate their way around unfamiliar places.

How GPS works

24 NAVSTAR GPS satellites in orbit around Earth

Photo: An artist's impression of the 24 NAVSTAR satellites in orbit around Earth. Picture courtesy of US Department of Defense.

Satellite navigation systems all work in broadly the same way. There are three parts: the network of satellites, a control station somewhere on Earth that manages the satellites, and the receiving device you carry with you.

Each satellite is constantly beaming out a radio-wave signal toward Earth. The receiver "listens out" for these signals and, if it can pick up signals from three or four different satellites, it can figure out your precise location (including your altitude).

Artwork showing how GPS satnav works

How does that work? The satellites stay in known positions and the signals travel at the speed of light. Each signal includes information about the satellite it came from and a time-stamp that says when it left the satellite. Since the signals are radio waves, they must travel at the speed of light. By noting when each signal arrives, the receiver can figure out how long it took to travel and how far it has come—in other words, how far it is from the sending satellite. With three or four signals, the receiver can figure out exactly where it is on Earth.

Where in the world are you?

  1. If your satellite receiver picks up a signal from the yellow satellite, you must be somewhere on the yellow circle.
  2. If you're also picking up signals from the blue and red satellites, you must be at the black dot where the signals from the three satellites meet.
  3. You need a signal from a minimum of three satellites to fix your position this way (and four satellites if you want to find your altitude as well). Since there are many more GPS satellites, there's more chance you'll be able to locate yourself wherever on Earth you happen to be.

What's the difference between ordinary and military-grade GPS?

Launching a BQM-74 satellite guided drone from the amphibious assault ship USS Essex.

Each GPS satellite constantly beams out two signals made up of long strings of binary data (zeros and ones, like this: 0110111011011....). Although the signals look like random binary "noise" streams, they're generated by a precise mathematical process, so they're described as pseudorandom noise (PRN) codes. The pairs of codes are unique to each satellite so, when a receiver picks them up, it knows exactly which satellite it's listening to.

The two codes each satellite generates are very different. One is called the P (precision) code and the other is known as the C/A (coarse acquisition) code. The P code is an extremely long and precise signal (roughly 6 gigabits—which is about 5–10 times the capacity of the hard-drive in a typical modern PC!) and takes about a week to transmit completely at a rate of 10.23 MHz (Mbps). It was originally designed only for military use and gives absolute, pinpoint accuracy. The C/A code is about 6 million times shorter (1023 bits—roughly two sentences of information) and sent at a lower frequency (1.023 MHz or Mbps) once per millisecond, so it's quicker for receivers to pick up. It's meant for everyday, commercial use and is deliberately much less accurate than military-grade GPS. Military-grade GPS receivers initially try to pick up the C/A code, then try to switch to the P code for greater precision.

Photo: Satellite-guided missiles and drones rely on much more accurate positioning than civilian GPS devices. Photo by Nicholas Messina courtesy of US Navy.

Find out more

On this website

You might like these other articles on our site covering similar topics:

News articles

Simpler books

More advanced books

Sponsored links

Please do NOT copy our articles onto blogs and other websites

Text copyright © Chris Woodford 2007, 2012. All rights reserved. Full copyright notice and terms of use.

Follow us

Rate this page

Please rate or give feedback on this page and I will make a donation to WaterAid.

Share this page

Press CTRL + D to bookmark this page for later or tell your friends about it with:

Cite this page

Woodford, Chris. (2007) Satellite navigation. Retrieved from http://www.explainthatstuff.com/howgpsworks.html. [Accessed (Insert date here)]

More to explore on our website...

Back to top