Where does Google Map’s blue dot come from?
Suppose we’re on city 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. This article will try to explain, in simple words, what is the 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.
GPS is a positioning system which works thanks to a network of more than 30 satellites orbiting the Earth at altitudes of approximately 20.000km, which corresponds to that of Medium Earth Obit (MEO).
As the “G” letter states, 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)
As well as the two regional systems:
- QZSS (Japan)
- IRNSS (India)
GPS is the first GNSS, and the terms GPS and GNSS are often used interchangeably. It started its operations in the early 1990s in charge of the Department of Defense of the United States. It was firstly designed for military purposes, but with time, 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 gathering with four friends (let’s call them Jimmy, Sandra, Patrick and Helen) and we unknown our location. However, we have the great idea to call them and ask them for theirs. They reply us with their address and the distance to us. Now with this data we can open a map, plot them into it and by knowing their respective distances to us, we will end up finding a place where all distances cross into a common point, then that’s our location.
This analogy, however, exemplifies the geometrical concept of trilateration. For GPS, the location is also obtained thanks to the information that satellites send us, but the process is slightly different and explained below. Notice that our friends did not share their coordinates to us, instead they provided their current addresses. We will come back to this in coming paragraphs.
Which data does the smartphone need and how does it get it?
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, Parick and Helen, we do not call the satellites, instead, they send information continuously, broadcasting it. Satellites do not send their position, they send parameters in the navigation message that allow the smartphone to compute it. This is analogous to our friends sending us their current address (i.e. street name and number) instead of their exact coordinates.
Also, in the trilateraion example, our friends sent us their distances, but satellites don’t do this, actually our smartphone is also the responsible for calculating this by processing the broadcast satellite signals. So, in short, the GPS chip:
- Receives the satellite signals.
- Process them and estimates the distance to each satellite.
- Reads (or decodes, to be more precise) the navigation message.
- Calculates each satellite position.
- Computes its own position by with trilateration.
Now that we have briefly explained how GPS works, what 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 satellite navigation topics!
