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The Physics Behind the GPS

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Alex Siu

on 7 June 2013

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Transcript of The Physics Behind the GPS

photo credit Nasa / Goddard Space Flight Center / Reto Stöckli The Physics Behind
the GPS What is a GPS? GPS stands for Global Positioning System. The system consists of 24 satellites all orbiting the Earth for a period of 12 hours. They are constantly monitored by the US government's Department of Defence. Created by the American scientist Robert L. Easton, his invention would soon revolutionize the future of navigation. The Impact The GPS was originally created for the use of navigation
for the US government's militia but were made universal in 1983. The invention of then lead to new ideas like Google Street view which enables you to travel around the world at the comfort of your living room.
Pros
You have access to basically the whole world in terms of where your destination is located
It is able to locate and pinpoint exact locations
It is also capable of telling you the altitude/elevation
Cons
Most GPS devices are expensive
Requires a strong signal to be able to work well
It can break easily if not cared for properly How Does it Work? Trilateration How trilateration works depends on the distance from the receiver and three known locations of the satellites. A GPS is able to do that due to the analysis of high frequency waves, and low-power radio signals. Radio waves are considered electromagnetic energy which travels at 3.0 x 10^8 m/s (speed of light).

Trilateration is quickly calculated with a series of imaginary 3D compasses. Your location is where 3 spheres of radius is given by the distance to each of the 3 satellites overlap. If the GPS is able to locate a 4th satellite then the measurements can be double-checked. Where all the spheres intersect is where the GPS receiver is located and pinpointed. From there, the GPS then gives you the directions to the location you want to get to. Distance Measured The satellites are constantly streaming a radio signal that includes the position and the time of the GPS receiver. What the receiver does is decode the data and is subtracted from its own time sent from the satellite.

This can be calculated through the equation:
186,000 miles/hr = x / calculated time

During the time the receiver picks up the signal from the different satellites, the waves take microseconds to travel from the satellite to the receiver, the delay from the satellite is then converted into distance and the location of the GPS receiver. An atomic clock is a clock that measures the most accurate time on Earth. The signals that are sent from the satellites all correspond to a random sequence that the receiver must know, this sequence is called the pseudo-random code. Since the GPS receivers must repeat the code inside the device the atomic clock is very important to keep the two in synch. The receiver is then able to compare the incoming signal and the internal signal to calculate the time needed to get to your destination. Questions to Ponder Some important questions to think about

How does this GPS work?

How will the possible future developments of the GPS impact how the GPS is used in the modern era?

What are the pros and cons of the GPS? The GPS that we own and cherish don't transmit signals but they "receive" the signal from the satellite. Each of the 24 operational satellites transmits data to be able to find location and the current time. There are 3 factors to how a GPS can display your location: trilateration, distance measured and atomic clocks. The Theory of Relativity General Relativity: states that time will go slower in under the gravitational pull of the Earth (9.8m/s^2) than in space where gravity does not exist. So time in space is more precise and accurate than the time on Earth

Special Relativity: states that because since the satellites' clocks are ticking corresponding to the time on Earth, the times will appear slower The Numbers Power of transmitter: 50 W (signal from satellite)
Satellites orbit around the Earth at a height of 11,500 miles at a speed of 3.6 km/s (3600 m/s)
Due to the signal being an electromagnetic wave, gravity (9.8 m/s^2) will be acting upon the signal and apply Einstein's general relativity theory.

In the video below, it summarizes how GPS and relativity are correlated in a simple and creative video! Future Uses and Applications From a device that revolutionized the world of navigation, there are many new future aspirations to be looking towards from the development of the GPS. Ideas like expanding the limited space for a working signal, improved signal from anywhere in the world, planning of new communities. The development of the GPS can also benefit certain fields like aviation, agriculture, and disaster relief. The GPS is not possible without the concepts of physics derived from Einstein's theory of relativity and the discovery of satellites. Atomic Clock By Alex Siu Works Cited http://airandspace.si.edu/gps/work.html

http://www.trimble.com/gps_tutorial/dgps-how.aspx

https://illumin.usc.edu/70/the-evolution-of-gps/

http://www.thespacereview.com/article/626/1

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

http://www.pocketgpsworld.com/howgpsworks.php

http://www.mio.com/technology-trilateration.htm

http://tech2.in.com/features/all/what-is-agps-how-does-it-work/115142
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