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Author Topic: How GPS works  (Read 9613 times)

Offline syhprum

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How GPS works
« on: 29/10/2011 06:14:49 »
Christopher Fry gave a splendid account on TV of how GPS works apparently the "receiving" device transmits to up to four satellites and times the returning pulses to the nearest millisecond and gives the location to within about 30 meters.
This was in reply as to why clocks accurate to one second in thirty billion years are useful.


 

Offline Geezer

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« Reply #1 on: 29/10/2011 16:31:37 »
of how GPS works apparently the "receiving" device transmits to up to four satellites and times the returning pulses to the nearest millisecond

Er, he's talking cobblers then, cos they don't transmit to anything  :D
 

Offline syhprum

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« Reply #2 on: 29/10/2011 19:00:23 »
It is always amusing to listen to actors explain anything technical or to watch them do simple jobs like driving a nail, it is easy to see why they took up acting as there is nowt else they can do. 
 

Offline Geezer

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« Reply #3 on: 29/10/2011 19:11:45 »
I'm not familiar with this nitwit fellow. You wouldn't happen to have a clip that we can see. I imagine it would be pretty amusing!
 

Offline syhprum

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« Reply #4 on: 29/10/2011 20:43:14 »
He is a very popular quiz master on a most amusing program QI broadcast both by the BBC and "Dave", I saw it last night (28/10/11).
If you download BBC I you should be able to see a typical episode but not necessarily the one where GPS is described
« Last Edit: 29/10/2011 20:56:36 by syhprum »
 

Offline CliffordK

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« Reply #5 on: 30/10/2011 04:56:19 »
It is always amusing to listen to actors explain anything technical or to watch them do simple jobs like driving a nail, it is easy to see why they took up acting as there is nowt else they can do. 
I suppose this is a bit off topic, but I was watching a TV show a couple of weeks ago where a family had a "paint party".  Everyone had matching, new T-Shirts.  There wasn't a drop of paint on any of the actors and actresses.  No spots on their spotless clothes.   

Clearly they are more coordinated at painting than me!!!
 

Offline imatfaal

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« Reply #6 on: 31/10/2011 11:25:39 »
Just as a note - the only Christopher Fry I have heard of was a playwright who died in 2005 (the lady's not for burning) - I guess you are referring to Stephen Fry.

Its the difference in the time signal received from the satellites.  in super simplistic terms if you receive a 123455 from both satellite A and B, a 123454 from C etc you are equidistant from A and B, and twice as far from C.  Algorithms allow these differences to be changed into a 3 d position
 

Offline Geezer

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« Reply #7 on: 31/10/2011 15:33:27 »
Yes, I had heard of Christopher Fry the playwright, but he didn't seem to be in a position to be doing much on the telly. Stephen Fry I am aware of. He's usually hilarious, so I doubt that anyone would take him too seriously.
 

Offline syhprum

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« Reply #8 on: 31/10/2011 16:38:23 »
It was of course Stephan I had in mind, the old biological memory chips are getting the worse for wear
 

Offline nicephotog

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« Reply #9 on: 03/11/2011 03:59:25 »
I took a look at "GPS" for both camera EXIF data headers on JPEG and TIFF recently , as much as looking at Mapping system coordinates decimal and deg , min , sec and UTM.
Actually , you need to know a little piece of electronics called "SWR" standing wave ratio
standing wave ratio is the height pitch of the elctromagnetic field produced by frequency / speed of light
e.g. if a rise in output strength to reach peak power of oscillation pitch continues for 1/100th of a second the wave is larger in height than one that reaches peak in 1/10000th of a second , its simply how far the field has travelled in a second by density.
Those satellites a are so far away that time lag can be measured (probably in pico seconds at least - milli seconds is quite useles) and as much the sattelites callibrate and check their internal clocks and synchronise them to stay stable.

It's easy to see how if you remember the time lag it took talking to the astronauts, when humans landed on the moon in the nineteen sixties.
You can put sattelites much closer to earth than the moon and have a sensible measurable lag for radio waves.

-Yes the hand set does transmit, because the "request signal" is time lag translated between more than one sattelite(sent to a set of sattelites in different positions so each has the time of signal arrival and there own known position to compare and triangulate in a pyrimid shape down to the point of request signal origin).
« Last Edit: 03/11/2011 04:07:11 by nicephotog »
 

Offline Geezer

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« Reply #10 on: 03/11/2011 04:56:55 »
-Yes the hand set does transmit,

Those chappies must be really clever to figure out a way to shrink a whopping great transmitter like that into a little GPS unit. Quite a feat for something that costs less than $100

(They are receive only.)

http://en.wikipedia.org/wiki/Global_Positioning_System#Basic_concept_of_GPS
« Last Edit: 03/11/2011 05:29:46 by Geezer »
 

Offline nicephotog

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« Reply #11 on: 03/11/2011 05:23:42 »
Then you are saying there is no sattelite/sattelites used in GPS positioning, the short code pulse has to come from that position physically.
Or are you implying something similar to the time it took an overseas phone call to the other side of the earth that took around 10 - 30 seconds between each point because of lag in receiving the audio, which happens to be security taping it and listening to it then hitting a button between each exchange(of speech) to allow the taping to be played onto the transmitter to be sent.

Then where the hell is the position the person is! , i'll give you a clue, when hunting pigs and foxes(professional vermin extermination : Australia , by law shoot on site) using dogs there are GPS tracking collars, and they have an aerial on the collar.
https://buy.garmin.com/shop/shop.do?pID=89564&ra=true
http://www.barkcontrol.com.au/shop/bark-collars/01?gclid=CJ3mv7bcmawCFaMF4god_CA8Pg
http://www.barkcontrol.com.au/shop/dog-tracking-collars/05
« Last Edit: 03/11/2011 05:33:14 by nicephotog »
 

Offline Geezer

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« Reply #12 on: 03/11/2011 05:32:14 »
Sorry! I just edited my post to include this link. GPS receivers only receive.

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

Offline nicephotog

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« Reply #13 on: 03/11/2011 05:38:03 »
Your splitting hairs about the synchronisation of the reciever request signal to the sattelite, in any case it does.
So it calculates the reception of each sattelites time pulse set inverse to transmitting it.
..."implying something similar to the time it took an overseas phone call"...
which is just as sneaky.
« Last Edit: 03/11/2011 05:53:34 by nicephotog »
 

Offline Geezer

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« Reply #14 on: 03/11/2011 06:27:24 »
It's not all that sneaky. It's just triangulation (or maybe quadrangulation).

The receiver knows where it is because it knows how far it was from the sats when they were at known locations.
 

Offline syhprum

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« Reply #15 on: 03/11/2011 06:28:34 »
I assumed that all correspondents were well aware how the GPS system works my post was about the garbled account an actor gave
 

Offline nicephotog

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« Reply #16 on: 03/11/2011 08:01:40 »
Sneakier again!!! thats a bit like saying "because of mathematical prediction of where the sattelites would be" you could then remove them and just press the button from the time of day.
Economically thats viable, the technology is in the handset so the users pay, while all the company needs to pay is for a sattelite that is a second hand spark transmitter, e.g. hauled up from the Titanic.
« Last Edit: 03/11/2011 08:04:53 by nicephotog »
 

Offline Bored chemist

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« Reply #17 on: 03/11/2011 20:27:34 »
This
"Actually , you need to know a little piece of electronics called "SWR" standing wave ratio
standing wave ratio is the height pitch of the elctromagnetic field produced by frequency / speed of light
e.g. if a rise in output strength to reach peak power of oscillation pitch continues for 1/100th of a second the wave is larger in height than one that reaches peak in 1/10000th of a second , its simply how far the field has travelled in a second by density."
is mainly wrong.

The thingy in my car which tells me where I am is receiver and not a transmitter.
Not only is it simply too small to transmit a signal that would reach a satellite with enough power to carry a signal, nor even that if it did so its batteries would last about a minute,
You need to look at the other end.
There are millions of GPS systems. There's no way the satellites could respond to all of them at once.
Read the wiki article and/or think it through.
 

Offline nicephotog

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« Reply #18 on: 04/11/2011 00:54:18 »
Quote
it simply too small to transmit a signal that would reach a satellite with enough power to carry a signal, nor even that if it did so its batteries would last about a minute,

Its not a mobile phone or a sattelite phone, so its power sustainability requirements would have been "sweet FA" for both usage in send and sustainability, second, the signals would only need to get above the weather and would need to be around only 10 watts for a fraction of a moment, after the weather , the signal can keep going effectively and be heard on the other side of the universe for human studies.

*Apart from being preposturous, i've thought of something even more silly to think of, with a mobile phone battery system around the size of an ipod and a filtered spark transmitter(no huge adjustment circuits just good generation property for the filters for an intensely short period required), about 50 watts could be generated and would probably always burn through a cyclone.

Quote
There are millions of GPS systems. There's no way the satellites could respond to all of them at once.
Certainly no good for a user pay system, until its selling the handsets.
« Last Edit: 04/11/2011 01:05:27 by nicephotog »
 

Offline Bored chemist

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« Reply #19 on: 04/11/2011 07:55:32 »
What does a "User pays" system have to do with the price of fish?
It's designed for the military and they are not normally going to have a few spare coins with them to put into the GPS  receiver.

This bit" the signal can keep going effectively and be heard on the other side of the universe for human studies." is nonsense too. The signal would soon get lost among the noise.

While this "Its not a mobile phone or a sattelite phone, so its power sustainability requirements would have been "sweet FA" for both usage in send and sustainability, second, the signals would only need to get above the weather and would need to be around only 10 watts for a fraction of a moment, after the weather "
even if it were true, ignores the fact that my GPS updates its position every few seconds or so.

Nicephotog
READ THIS
http://electronics.howstuffworks.com/gadgets/travel/gps3.htm
 

Offline nicephotog

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« Reply #20 on: 16/11/2011 13:54:29 »
Quote
...ignores the fact that my GPS updates its position every few seconds or so.

"every few seconds or so" , mobile phones tx/rx in duplex constantly , why anyone would worry about " 'a few' tiny pips 'a second' " at that type of frequency compared to blazing the side of their head with an oven at point blank.
 

Offline Bored chemist

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« Reply #21 on: 16/11/2011 19:20:10 »
Quote
...ignores the fact that my GPS updates its position every few seconds or so.

"every few seconds or so" , mobile phones tx/rx in duplex constantly , why anyone would worry about " 'a few' tiny pips 'a second' " at that type of frequency compared to blazing the side of their head with an oven at point blank.

What did you mean by that?
 

Offline Mazurka

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« Reply #22 on: 17/11/2011 13:06:48 »
As an aside, some people may be interested to hear that the MoD are trialing ground based GPS specific jamming systems using a 10 - 15w transmitter which has an expected range in of a few 10's of km...
 

Offline syhprum

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« Reply #23 on: 19/11/2011 22:36:12 »
I would not want a GPS jammer but I would really like a device to move my apparent location to maybe some nearby military establishment.
Then when the GPS guided missile comes to destroy me it hits something less important.
We did something similar during the war with the guidance system the Luftwaffe were using.
 

Offline Geezer

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« Reply #24 on: 19/11/2011 23:05:03 »
All you need to do is get enough of the sats to fib about their positions, and that should pretty much take care of it, although it might be possible to fool a receiver with some "synthetic" satellites.

I wonder if that's why certain other countries are creating alternative GPS systems?
 

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« Reply #24 on: 19/11/2011 23:05:03 »

 

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