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Youe can not help men permanently by doing for them what they could and should do for themselves. ..."Abraham Lincoln(1809-1865, US President 1861-1865"
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Author Topic: How GPS works  (Read 26931 times)
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Fu-Kwun Hwang
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on: April 23, 2007, 08:23:25 pm »

The Global Positioning System (GPS) is actually a constellation of 27 Earth-orbiting satellites (24 in operation and three extras in case one fails). The U.S. military developed and implemented this satellite network as a military navigation system. Each of these solar-powered satellites circles the globe at about 12,000 miles (19,300 km), making two complete rotations every day. The orbits are arranged so that at any time, anywhere on Earth, there are at least four satellites "visible" in the sky.
Each GPS satellite has atomic clocks on board. Each GPS satellite transmits data that indicates its location and the current time. All GPS satellites synchronize operations so that these repeating signals are transmitted at the same instant. The distance to the GPS satellites can be determined by estimating the amount of time it takes for their signals to reach the receiver.


The GPS receiver is located at sphere surfaces centered at each satellite.
So the location is at the intersection for those three sphere surface.

The intersection for two sphere surface is a circle.
And the intersection for the above circle and the other sphere surface can be at two intersect points. However, only one of the point is on the earth surface (the other one will be on the outer space, not on the earth surface). You unique location can be determined.




This following applet shows how the GPS works. It needs at least three satellites.
You can drag those three blue dots to change the location for satellite.
Check "satellite 1/2" two display intersection for two satellite (yellow circle).
Then un-check "satellite 1/2" to find the intersection between circle and the third sphere surface (the red dot).
I hope this applet can help you understand how satellite works.
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Press the Alt key and the left mouse button to drag the applet off the browser and onto the desktop. This work is licensed under a Creative Commons Attribution 2.5 Taiwan License
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http://www.youtube.com/watch?v=_8P3DrKFjYc



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dalewilley
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Reply #1 on: April 09, 2009, 01:42:37 am »

Can you please verify that the GPS applet is working.  I repeatedly get runtime errors

thanks
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ivgelder
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Reply #2 on: November 20, 2009, 12:04:30 am » posted from:Antwerp,Antwerpen,Belgium

Thank your for this applet and for sharing it under a cc license. I used it for an educational project.

I have 1 remark on the applet: the simulation would be clearer if the user would be able to rotate the earth. In that case, the intersection of the spheres would become more understandable.
« Last Edit: November 20, 2009, 12:13:57 am by ivgelder » Logged
Fu-Kwun Hwang
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Reply #3 on: November 20, 2009, 09:28:16 am » posted from:Taipei,T\'ai-pei,Taiwan

Thank you for your comment:
Code:
User would be able to rotate the earth

You can change the view point with mouse drag or do you meaning something else?
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ivgelder
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Reply #4 on: November 25, 2009, 03:56:21 am » posted from:Antwerp,Antwerpen,Belgium

Thank you for your comment:
Code:
User would be able to rotate the earth

You can change the view point with mouse drag or do you meaning something else?

Oh I'm sorry. My computer was too slow to run the applet. Therefor I didn't notice you can change the view point. I tried it on a faster computer and now it works very well.
I really like the applet. Thank you for your work.
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