How does GPS work?

Nowadays, in modern times, a lot of people are in the possession of a GPS device: satellite navigation in vehicles, on your phone or watch or even the more traditional handheld GPS devices. Usually, they work flawless and the very existence and use of GPS-assisted devices has contributed positively to the safety of traffic and for climbers and hikers in the mountains.

But what is GPS and how does it work?

GPS is an abbreviation of Global Positioning System and it works with a system of 24 satellites which orbit our beautiful planet Earth continuously. These satellites emit radio waves, which a GPS device can pick up. These satellites orbit the earth in 12 hours. Due to the positioning of the satellites, there is sufficient reception at any time of the day, at almost any place on earth, to calculate the position.

But how does my GPS-device know where I am?

Contrary to what most people think, the GPS-device calculates where you are. The device is a recipient of the radio waves from the satellites and calculates where you are. There is no reverse signal: the satellites don’t know where you are. Just as much that a radio station doesn’t know that you are listening, although you are receiving the radio waves from the station. Too bad for Hollywood: triangulation only works on devices that emit radio waves themselves, such as telephones.

The GPS-device calculates 3 variables: de position of the satellite, the distance between the satellite and the recipient and the relationshop between the 2 former. At least 3 satellites are needed to triangulate a position:

The position is calculated relative to at least 3 GPS satellites. More satellites? Then the position determination is more accurate.
The position is calculated relative to at least 3 GPS satellites. More satellites? Then the position determination is more accurate.
Before the rise of the smartphone, these devices were quite popular: a handheld GPS device. I loved the thing!
My ancient, solid handheld GPS: Garmin Etrex. This one has seen quite a bit of Europe (and Nepal).

Position of the satellite

The satellite transmits radio waves and with it a specific identification code. Because the conditions on Earth are different everywhere and change from day to day (think of weather and political conditions) there are some ground stations on Earth that compare the position of the satellite with the transmitted position and the actual position. If there are deviations, the ground station calculates a correction factor, which it sends back to the satellite. The satellite sends the correction factor to the receiver.

There are several GPS ground stations. Because these are “fixed”, the system is always calibrated and positions are adjusted.
There are several GPS ground stations. Because these are “fixed”, the system is always calibrated and positions are adjusted.

Distance between the satellite and the receiver

The satellite transmits the time when the signal is sent. Because the speed at which the radio signal travels is known (namely that of light), the GPS receiver can calculate how far the satellite is from its position. It should be noted that the clock of the GPS receiver is less accurate than that of the satellite. Small differences can therefore arise. This is particularly in built-up areas with a lot of disturbance, or in my own experience in the local forest where I regularly go down to -60m. According to the GPS (in my Garmin VivoActive3) at least.

Merging distance

The distance to the satellite itself is of no use. This only becomes valuable data if you know this from several satellites, so that you can, as it were, cross out possibilities (trigonometry). With 2 satellites you already have an idea where you are, namely within the range of both satellites, on a certain line of equal distance between the 2. So you need a third satellite to indicate exactly where you are: in the middle of a triangle. It should be noted that the more satellites there are, the more accurately the position is calculated. To determine the altitude you need at least 4 satellites.

Accuracy

The satellite system is owned by the United States government. It is not in their interest to let everyone on Earth know too accurately where someone is. There is an inaccuracy built in for this, which can be different at every place on Earth (and can vary from moment to moment). This inaccuracy is prompted by the ground stations that send the correction factor to the satellite. Furthermore, the clock in the GPS receiver is less accurate than that of the satellites, so that the uncertainty about the position is even more different. On more expensive devices, the clock is more accurate. Phones are the least accurate, followed by car, boat and traditional GPS. For most applications, this accuracy does not matter much. Count on an accuracy of your GPS of 3-5 meters in every direction (including altitude).

You can easily see this by standing still with your GPS on. Despite the fact that you are sure that you are standing still, the GPS will still put you in different places and you seem to be moving. Incidentally, the more satellites in the image, the more accurate the location. Further south you “see” more satellites than to the north, thanks to the orbit of the satellites.

Manufacturers of GPS devices

Traditionally, it was mainly the manufacturers who were allowed to supply the various armies and build the traditional devices: think of Garmin (world market leader) and Suunto. Later the car navigation manufacturers joined: TomTom, Navteq, Magellan, VDO Dayton and again Garmin, although most systems are now built into the car.

Another category of devices that have gained access to GPS are telephones. Due to the huge number of smartphones sold with GPS, the smartphone is one of the most popular GPS devices today and Apple & Samsung are main GPS-device brands.

The last category is watches. The smartwatches and GPS watches nowadays can do almost the same as your phone. The screen is of course smaller and the battery life is often disappointing. My VivoActive3 charges about every 2 days, although it depends on the activities. There are also sports watches with built-in solar panels that would last up to 60 hours.

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