What is GPS ?

Have you ever wondered how your phone can tell your exact location on earth ? its possible due to a technology called GPS and it is our subject today. I hope this gives you a brief information about how our location can be calculated by the GPS technology.

GPS or Global Positioning System is huge network of satellites that orbit earth and they beam down signals to anyone on earth with a GPS reciever. These signals carry a time code and geographical data point that allows the user to pinpoint their exact position, speed and time anywhere on the planet.


GPS was originally intended for military applications, but in the 1980s, the government made the system available for civilian use. GPS works in any weather conditions, anywhere in the world, 24 hours a day and relies on between 24 and 32 satellites that sit in a "constellation" orbiting the planet at medium earth orbit. This is a distance of least 20,000 km above the earth's surface, but no higher than the orbit used by TV, communications and internet satellites, and weather satellites, the so-called geostationary orbit, which is at an altitude of approximately 35,000 km.

How does it work ?

The GPS system is based on concept of time. The satellites carry a very stable Caesium Atomic Clock (i hope you remeber caesium is taken as the standard of time so 1 second is defined as 9192631770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the caesium 133 atom). So the atomic clocks are most accurate and all satellites are synced to each other and are regurlarly maintained here from ground. 

These GPS satellites are constantly transmitting their current time and position. and when you turn on the GPS reciever on your phone it starts recieving them. And then the reciever monitors multiple sattelites and solves some equations to determine the distance of your phone and the satellite, remember that it takes some time for the signal to reach the reciever and that time lag (it just compares the time the signal was sent from the satellite and the time it was recieved by the reciever) multiplied by speed of light gives you the distance! as Cs 133 clocks are very accurate in sensing time differences. Now imagine a circular perimeter around you. This perimeter is the probability of finding you in that area. the radius might be 100km or a few hundred metres. So now your position is not that accurate. Now your phone is connected to another sattelite and it also gives info about you location. and then another satellite is connected. Now the intersection of these circle of a probability function will provide the exact location of you with approx. 3-10 metre radius. The more satellites are there the more accurate your position is. At least minimum 4 satellites are required to give your approx location (three satellites for the x-y-z coordinates and one for clock deviation from satellite).
If you want to know more about GPS read this  http://www.pocketgpsworld.com/howgpsworks.php

What is its use ?

For navigation purposes. Google maps can give you the map as well as direction and GPS tells you your location and both these are a perfect combination for real time navigation. And there are many other uses which you can read here or here.

Why does it take time to lock on GPS ?

The GPS signals are sent at a speed of 50 bytes per second. But that said there are technologies that promises to boost geo-location like the DARPA and Galileo .
And GPS consumes a lot of battery also, as the signals need to be refreshed constantly if you are travelling.

What is the difference between AGPS and GPS ?

In case of an A-GPS device it uses the existing servers for e.g. of the mobile network tower and bases to get the information from the satellites. Since these servers are continually sending and receiving information there is no delay in knowing the exact orbit and time location of the satellites. In other words the time to first fix is a lot faster than a normal GPS. Also these servers have good computation power so they can analyze the fragmentary signals received from the GPS receiver and those received directly from the satellite and thus correct the error. It will then inform the receiver its exact location.

Stands for
Assisted Global Positioning SystemGlobal Positioning System
Source of triangulation information
Radio signals from satellites and assistance servers e.g. mobile network cell sitesRadio signals from GPS satellites
A-GPS devices determine location coordinates faster because they have better connectivity with cell sites than directly with satellites.GPS devices may take several minutes to determine their location because it takes longer to establish connectivity with 4 satellites.
Location determined via A-GPS are slightly less accurate than GPSGPS devices can determine location coordinates to within 1 meter accuracy
It costs money to use A-GPS devices on an ongoing basis because they use mobile network resources.GPS devices communicate directly with satellites for free. There is no cost of operation once the device is paid for.
Mobile phonesCars, planes, ships/boats

What fun can you do with GPS ?

The best fun is Geo-Caching !


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