You are on your way to a friend's wedding in a new city and suddenly you realized his direction to the wedding venue was not very clear and now you needed to find it. You dialed his number and it was switched off (well, he may probably be doing the last minute plan for the wedding happening in an about an hour). Then you remembered you have a GPS capability on your smartphone and you input the venue and follow the direction prompt to the wedding venue. Bravo. Technology to the rescue. But what is  GPS and how does it work?
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<center>https://res.cloudinary.com/hpiynhbhq/image/upload/v1512837835/exzreelqizq3ikcvcw4n.png)</center>
<sup>Image Source: pixabay</sup>
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GPS stands for Global Positioning System. It is a space-based navigation system that allows anyone on earth with an unobstructed view of at least 4 GPS satellites to be able to get location and time information. 

It is a system that was originally developed for US military intelligence but now is available on most mobile phone, car navigation system or just about any device with a GPS receiver built into it.
<div class="pull-right"><center><img src="https://steemitimages.com/DQmUPo8svhT3bq4ArqHuihJMX3cMUYLK5FjUX7fgpDZfBux/200w_d.gif" /><br/><em><a href="https://giphy.com/gifs/2011-A2cOueekVG1jy">Source: Giphy</a></em></center></div>

GPS is a constellation system of thirty (30) Earth-orbiting satellites.
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#### How it works
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For a complete global coverage, twenty-four (24) satellites are needed. The other six (6) serves as spares.

The orbits are positioned around the Earth (at over 20,200km) in a manner that there is always four of the satellites visible to people with each orbiting the earth every 11 hours 58 minutes.



![orbits.gif](https://steemitimages.com/DQmPw555PhejJYChBXJsjLEgCS3X4TCPHWA1b3z885o1gHF/orbits.gif)
<sup>[Source](https://www.colorado.edu/geography/gcraft/notes/gps/gif/orbits.gif): GPS Satellites in Orbit</sup>

The four satellites are needed in order for the GPS receiver to determine its own location in the 3-dimensional coordinates.

This location of itself is done via a process known as **trilateration**.
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<center>![gps_system_how_it_works.png](https://steemitimages.com/DQmRotPKb5JKvVU4UF6hWzv6geSussoFB2LPjQVvFoub8sW/gps_system_how_it_works.png)</center>
<sup>[Source](http://www.geeksforgeeks.org/how-gps-works/) </sup>

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Trilateration is the mathematical method of measuring distances which is sometimes confused with triangulation (measuring angles).

Each GPS satellites broadcasts messages to earth containing highly synchronized time provided by an atomic clock and satellite position at these broadcast time.

The distance between the GPS receiver and satellite is calculated by checking the difference in time the signal was sent from satellite to the time it was received by the GPS receiver

This is done by multiplying the time difference with speed of light, this yields distance between satellite and receiver.



If you know your distance from satellite 1, you could be anywhere on a huge imaginary circle (100km in any direction within the circle in the example). 
<center>https://image.slidesharecdn.com/gps-130331091353-phpapp01/95/gps-17-638.jpg?cb=1364721288</center>
<sup>[Source](https://image.slidesharecdn.com/gps-130331091353-phpapp01/95/gps-17-638.jpg?cb=1364721288)</sup>

Then if you know your distance from Satelite 2, you can overlap the first circle intersecting at two points, it had narrowed your location but still not exact. If a third satellite distance from you is gotten, the intersection of these three points would be where you are.

https://image.slidesharecdn.com/ppt-localtrackingandgps-101016101959-phpapp01/95/ppt-local-tracking-and-gps-17-728.jpg?cb=1287224659
<sup>[Source](https://image.slidesharecdn.com/ppt-localtrackingandgps-101016101959-phpapp01/95/ppt-local-tracking-and-gps-17-728.jpg?cb=1287224659)</sup>

Of course, this illustration is in two-dimensional space but the same concept applies in three-dimensional space where instead of circles we have spheres. From here, you could see only three satellites were needed (theoretically) to get a location but to improve accuracy four are needed.




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#### Uses of GPS
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1. Location: To determine position of aircraft and satellites

2.  Tracking: remote monitoring of the environment, tracking and monitoring object/people.

3. Mapping: the world maps are created with aid of the GPS

4. Timing: with the inboard atomic clock precise timing is sent to the world.

5. Navigation: Getting from one location to the other.

Ref: [1](http://www.physics.org/article-questions.asp?id=55),[2](https://spaceplace.nasa.gov/gps/en/),[3](https://www.faa.gov/about/office_org/headquarters_offices/ato/service_units/techops/navservices/gnss/faq/gps/),[4](http://www.geeksforgeeks.org/how-gps-works/).