GPS - how does it work?

We use the Global Positioning System (GPS) feature on our smartphone on a daily basis. Every time we use google maps, the weather widget, mobile apps such as Foursquare etc., we are using the GPS feature to help us understand our physical location and its attributes relative to our surroundings. But how exactly does the GPS technology work? How come it is so important and yet we apparently get to use it for free? To learn answers to these questions, given below are some links to relevant webpages. Have a read through and let me know if they whet your curiosity.

http://www.physics.org/article-questions.asp?id=55

http://stackoverflow.com/questions/33637/how-does-gps-in-a-mobile-phone-work-exactly

Recently I came across an article that said that India has plans to set up its own GPS system which piqued my interest. This article by Sandhya Ramesh that appeared in the Wire publication, spells out the details of the Indian GPS. The link to this article is given below. Looking forward eagerly to the launch of this much anticipated desi GPS!

http://thewire.in/2016/01/22/irnss-indias-very-own-gps-19892/