The Physics behind GPS

hello i was wondering the physics that goes into transmitting waves such as: reflection etc.. To achieve a GPS signal, as well as the process of achieving it.

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NoobixCube

Are you asking how gps works?
if so, then basically a large number (25 odd) of satellites orbit the earth and transmit the time of their clock and their current positions. Because the orbital path and the time is known so accurately (through atomic clocks onboard the satellite) four satellies can find the position of the handheld device accurately(4 sat.s needed as their are 4 dimensions to solve for: x,y,z,t). no reflection is needed....

There are so many satellites because, at any one point on the Earth, four have to be visible. To illustrate the concept, a nice GUI is on the wikipedia page

http://en.wikipedia.org/wiki/Global_Positioning_System

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i was more wondering how waves are transferred from source to receiver via satellites. e.g. electromagnetic waves (radio, microwave etc..)

And also could you tell me more about the Atomic Clock on board a GPS. (and in general what is a Atomic Clock)

thank you =]

NoobixCube

L5 (1176.45 MHz): Proposed for use as a civilian safety-of-life (SoL) signal (see GPS modernization). This frequency falls into an internationally protected range for aeronautical navigation, promising little or no interference under all circumstances. The first Block IIF satellite that would provide this signal is set to be launched in 2009

sooo the atomic clock is based on the orbital path of Satellites ??

NoobixCube

basically, one knows that a cesium atom oscillates between to hyperfine levels in its ground state 9 192 631 770 times per second, this is used as the time keeper because it is a stable value.
http://inms-ienm.nrc-cnrc.gc.ca/faq_time_e.html#Q10 [Broken]

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NoobixCube

would wiki be a reliable source?
wiki is always worth a first look when you are not sure on something. When it comes to qouting a reference, never use wiki. Find the references at the bottom of the wiki page you are looking at and check them out for a more legitimate source.

ok thank you very much =] helped me lots...

umm in regards to the cesium clock,,.. would it be relevant information to put in my assignment in context to GPS (Physics based).

could some one tell me the electromagnetic waves involved in transmitting a GPS signal, and how this signal travels from source to receiver then back to source.

NoobixCube

umm in regards to the cesium clock,,.. would it be relevant information to put in my assignment in context to GPS (Physics based).
Very very relevant, it is what GPS is based amount, a very accurate measurement of time

I am having trouble understanding the cesium atomic clock. What exactly does it do? and how is this done?

DaleSwanson

GPS works because there are a number of very accurate clocks in satellites. The satellites broadcast the current time on radio frequencies. That travels out at the speed of light. Since the speed of light is very fast, but not instant it takes some amount of time for the time info to reach the gps receiver (thing in your hand that tells you where you are). The receiver can then figure out how long the signal took to reach it from the satellite. Since the speed of light is known the receiver then knows how far away the satellite is. The location of the satellite above the Earth is also known. Knowing all these things allows the receiver to figure out it must be on a sphere whose center is the satellite.

To think about this, if I told you I was 100 miles from New York City, you would be able to draw a sphere whose surface I must be on. At first you may think a circle, since we are used to dealing with flat surfaces, but remember I could be 100 miles directly above NYC, that's why it's a sphere.

So one satellite gets you a sphere, and if you add another one it adds a different sphere. You must be somewhere where these two spheres touch, since you must be on the surface of both. Going back to cities, if I told you I was also 300 miles from Philadelphia you could draw a second circle. There would be only two points where both circles touched, and I would have to be at one of those. Again really it would be two spheres, as I could be in the air. If you keep adding satellites it keeps limiting the spots you could be. When you have 4 satellites you have enough to know there is only 1 spot you could possibly be.

To review the atomic clock is just a very accurate clock, which is needed because you are measuring tiny fractions of a second. The satellite just broadcast the time, and their location. The receiver only receives, it doesn't send anything back to the satellites.

Andy Resnick

The magazine "Physics Today" had a great feature article on GPS a few years ago- it's worth finding and reading.

in regards with waves being transmitted to the satellite, is that wave being reflected back to the source through the satellite?

and also a GPS device consists of a transmitter and a receiver?

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my assignment is asking me to answer these things through the device of GPS:

1) Describe the energy transfers and transformations that occur in using this communication technology (GPS).

2) Explain wave motion, in terms of energy sources in the communication device and oscillations produced. i.e. Transverse, electromagnetic, mechanical

yeah these questions im kinda having trouble with...

molinaro

There are different kinds of GPS devices.

The basic unit, or original design type, is as described above. It receives a time signal from multiple satellites and calculates it's position. No signal leaves the unit in your hand going back to the satellites.

It's a radio transmitter/receiver setup as far as waves are concerned. Nothing special in that regards. Instead of that signal encoding some music, it's the time, to a very high number of decimal points. That allows an accurate position calculation.

Newer GPS units can have outbound signals for various reasons. However, they are not part of the position display function for the one holding the unit. The unit could for example provide two-way text messaging. It could use either the same cellular networks a phone or pager would be using. Or it could be satellite, but not the time broadcasting satellites, rather the other communications satellites that are in use by some phones and pagers also.

And it's this same kind of communication that can allow the units to update to a web site so a trucking dispatcher, ahem, could watch their current position.

So, to answer the technical questions you should consider how does any satellite broadcast a radio signal. And how does any handheld device receive a radio signal. The difference is only in the encoding of the signal and post processing.

could you tell me the physics behind GPS signals being transmitted and received.

and also what are the energy transfers and transformations that occur in using GPS technology.

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