There are circular paths around the earth, and they are called orbits. In GPS system, there are six orbits in different planes across the earth. (see figure). In each orbit, six satellites are going around the earth. Totally, 30 satellites revolve around the earth. At least four satellites will be visible to you wherever you are on this planet.
The satellites continuously send signals. Your GPS phone receive them and calculates the time of travel. Then, using the formula,
satellite's distance = speed of light(signal) * time of travel.
the phone calculates the distance of the satellite.
On receiving signal from one satellite, the GPS in your phone construct sphere of which the satellite is the center. The 'distance' is the radius of the sphere. You are somewhere on the sphere.This information is not enough to find you. If you get another reading from another satellite, you get a second sphere. Both spheres intersect and intersecting area form a circle. (see figure). Now you are somewhere on this circle. Your location info. greatly improved. If you get a third reading from a satellite, three sphere intersects and form a smaller circle and the accuracy of your location further improves. Four satellites give enough accuracy. The GPS software in the phone calculates all this. That is why, we are able to use the google map efficiently. Readings from 10 satellites give location accuracy close to an inch.
Foot note: Satellites send signals with their time stamp.
Einstein theory of relativity is taken into account here.
a. Clocks in the satellite run slowly (milliseconds).
b. clocks in the space are affected.
Hence, due corrections are made by GPS software.
Nobody can hide with working smart phone.
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