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What is GPS?

By Hannah Walsh

Find out about what GPS is, and how the big navigation brands use it in your sat nav or sat nav app.

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The Global Positioning System, more commonly known as GPS, has a wide variety of uses, but its basic job in everyday life is to tell gadgets, such as smartphones or sat navs, and consequently the owner, their location.

GPS ensures that map apps on your phone can show you were you are, that fitness trackers can tell you how far you've walked, and that internet connected devices can serve you content that is most relevant to where you are.

If you use a sat nav, sat nav app, a smartphone or fitness tracker, then the chances are you’re already using GPS on a daily basis. If you get on a plane or boat, then GPS will certainly be used to plot the course to your destination.

Read our expert recommendations of the best sat navs, best smartphones and best fitness trackers, based on independent Which? tests.

How does GPS work?

Originally designed for military use in the 1960s, GPS is a navigation system made up of a series of orbiting satellites. GPS satellites circle the Earth twice a day at an altitude of approximately 20,000km. Each orbiting satellite transmits a signal carrying a time code and geographical data point, which is picked up by a GPS receiver in your device. A signal from a fourth satellite is also received, but is only needed to confirm the results from the first three satellites.

Why use GPS?

GPS is used to track a device's location and, by extension, devices using GPS can also work out your speed, altitude and how far you’ve travelled. This makes it useful for informing speed alerts on your sat nav when you start driving a bit too fast.

Do I need wi-fi, 3G or 4G to use GPS?

No. GPS works separately to wi-fi, 3G or 4G networks so doesn’t rely on these to work. The signals transmitted by GPS satellites are carried by radio waves and travel by line of sight.

What are the benefits of GPS?

It works in any weather conditions, and can be used almost everywhere with no subscription fees or setup charges.

Which devices use GPS?

GPS is used by many of the devices we use every day, such as wearable technology with built-in GPS, sat navs and sat nav apps, dashboard cameras, smartphones, and even some cars.

Apple iPads and iPhones, the Apple Watch Series 2, Android smartphones, Samsung Gear S3 – these are just some of the devices that use GPS.

What stops GPS working?

Your GPS receiver must be able to connect to three GPS satellites in order to carry out a trilateration process, which is the calculation of your position on Earth to the nearest few meters based on your distance from each of the satellites. GPS signals travel by line of sight, meaning they will pass through clouds, glass and plastic, but may struggle in areas that are particularly built up or mountainous. Most modern receivers are sensitive enough to pick up GPS signals when you’re inside.

How accurate is GPS?

GPS generally has a good level of accuracy to within a few meters, but it isn’t always perfect.

A particularly frustrating issue when using your sat nav or sat nav app is GPS dropout – when you drive through an area and lose your GPS signal. This can be because of interference between the GPS receiver in your device and the three or four orbiting satellites, such as atmospheric changes. It will be the most accurate when you’re outside in a clear area – being inside a building or in an area with a lot of high-rise buildings can impact the quality of your GPS data.

We’ve tested all the latest sat nav and sat nav apps using a GPS simulator to replicate driving a set route, including minor and major roads, in a variety of areas. Using a simulator allows us to replicate exactly the same circumstances for every test sample, so we can answer key questions, such as how good the visual and audio instructions are, and whether the device experiences frequent GPS dropout.

Some fitness trackers and smartwatches, and most fitness watches, have built-in GPS, and we check the accuracy of distance tracking using GPS data for these devices. The majority of wearables are good at distance tracking and can accurately calculate your distance travelled using GPS, although we have found a smartwatch that understated it by nearly 50%.

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