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GPS – behind the scenes

GPS – Where does Google Map’s blue point come from?

…deciphering the mysterious origin of that little blue point in Google Maps…

Suppose we’re on city town looking for a particular place, we all almost instantaneously open Google Maps app, a blue point shows somewhere in the map indicating our location and we are then ready to continue our way to our destination. Well, this article will try to explain what’s the process before that blue point appears in our map.

What is GPS?

GPS stands for Global Positioning System. We could end this first answer here since the acronym says it all. However, there are still some interesting facts.

As you could imagine, GPS is a positioning system which works thanks to a network of more than 30 satellites orbiting the Earth at an altitude of approximately 20.000km, which correspond to an altitude of a Medium Earth Obit (MEO). As the “G” letter announces, it provides global coverage, making of GPS one of the four Global Navigation Satellite Systems, or GNSS:

  • GPS (USA)
  • Galileo (Europe)
  • GLONASS (Russia)
  • BeiDou (China)

It is important to also know the regional systems such as:

  • QZSS (Japan)
  • IRNSS (India)

GPS is the first GNSS (the terms GPS and GNSS are often used interchangeably), which came up in the early 1990s in charge of the Department of Defence of the United States. It was firstly designed for military purposes, but with time its civil usage has grown up until becoming the most used service so far.

But coming back to the blue point…

Where does it come from?

Our smartphone, more precisely a device called GPS chip (or GNSS chip) in our smartphone, performs algorithms to calculate our position which is expressed in latitude and longitude and displayed in Google Maps.

How does it calculate the position and which data does that algorithm process?

The algorithm needs to know the position of at least four satellites and the respective distance to them. Then, by knowing this information the computation is based in a principle called trilateration. To exemplify, this is analogous to the following: suppose we are walking with three friends (let’s call them Jimmy, Sandra and Hellen) and we unknow our location, but we have the great idea to call them and ask them for theirs. They reply not only with their address but also with the distance to us. Now with this data we can open a map, plot them into it and by knowing their respective distance to us we will end up matching a place where all distances cross into a single point, then that’s our location.

This example, however, exemplifies the geometrical concept of trilateration. For GPS, the location is also obtianed thanks to the information that the satellite send us, but the process is slightly different and explained below.

Which data does the smartphone need and how does it get it?

The satellites send information towards the Earth, the smartphone receives it and calculates the distance to the satellites and their position thanks to the information carried in the satellite signals, known as navigation messages. Coming back to the analogy with our friends Jimmy, Sandra and Hellen, we do not call the satellites, instead, they send information continuously. In fact to be more precise, satellites don’t send their position, they send parameters that allow the smartphone to compute their position, just like if our friends send us the street name and number instead of their exact coordinates. Also, in the trilateraion example, our friends sent us the distance to us, but satellites don´t do this, actually our smartphone is also the responsible for calculating this by means of the signals that satellites are continuously broadcasting.

Now that we know how GPS works, how is the process before that mysterious blue point appears and what information the smartphone needs in order to make that processing, did you know that a lot of GPS applications go far beyond positioning? For example, GNSS is widely used in meteorology, have a look in the previous blog post GPS & Meteorology. Furthermore, GNSS can be used not only on Earth, but also in Space, check out the article GPS works in Space? Yes!

Stay tuned for more articles related in Space navigation topics!

Thank you for reading!


REFERENCES

NASA. How Does GPS Work? Retrieved from: https://spaceplace.nasa.gov/gps/en/
GPS.GOV. The Global Positioning System. Retrieved from: https://www.gps.gov/systems/gps/
James Webb Space Telescope. Retrieved from: https://www.jwst.nasa.gov/
The Planetary Society. How to Search for Exoplanets. Retrieved from https://www.planetary.org/explore/space-topics/exoplanets/how-to-search-for-exoplanets.html