How Does GPS Work?

[lordgeoff]

Originally Posted by goboatingnow

without betraying commercial confidences,. I can tell you, you are categorically wrong about Garmin.

The process has been explained to you by several posters. If you continue to ignore the truth and simply peddle your "intuitive" undertasnding, Im sorry, for as Sherlock Holmes said, everyone can " see", few can " observe ". or as some have put it, " theres none so blind as those that will not see "

bye bye

.....just quoting GARMIN. Take it up with them if you are having trouble following their post.

[goboatingnow]

Originally Posted by lordgeoff

.....just quoting GARMIN. Take it up with them if you are having trouble following their post.

You are quoting a garmin marketing style comment , much of what is written by a manufacturer is actually technically wrong.

personally I know for a fact that its wrong. There are dozens of links, some including contributions on technical GPS sites where doppler speed is discussed.

Instantaneous speed ( i.e. as a GPS speedometer display ) does not use successive position computations, I have given you links to papers where that has been proved. its simply not accurate enough , given the probability of position errors and the fact that the principles of position determination , give as a " bye product " doppler speed

[ALAIN97133]

Originally Posted by goboatingnow

we should also thank luminaries like Lorentz, Poincaré, and some others too. many people contributed to special relativity

You're so right ! What surprised me is that a "theory" I considered interesting at most, but "so what?", had implications on our daily life

[lordgeoff]

Originally Posted by goboatingnow

You are quoting a garmin marketing style comment , much of what is written by a manufacturer is actually technically wrong.

personally I know for a fact that its wrong. There are dozens of links, some including contributions on technical GPS sites where doppler speed is discussed.

Instantaneous speed ( i.e. as a GPS speedometer display ) does not use successive position computations, I have given you links to papers where that has been proved. its simply not accurate enough , given the probability of position errors and the fact that the principles of position determination , give as a " bye product " doppler speed

...as I said before I am quoting Garmin information. Discuss with them what you don't agree with. Might I say. I am very comfortable and grateful with what the USA Government, their scientists and technical and development scientists have produced. Not to mention the capital cost of launching, maintaining and replacing an array of satellites for the past 20 years or so.Coupled with companies like Garmin, Raymarine etc who further develop this fabulous gift from the USA and provide all us Yachties with a reliable, accurate, cheap, and safe navigation tool all I can say is THANKS. Probably the only mistake all these genius propeller heads made is they names the system GPS. They should have called it GSS (Global Speed System)....... apparently.
Hope you get the clarifications you are looking for from Garmin.

[transmitterdan]

Originally Posted by lordgeoff

Probably the only mistake all these genius propeller heads made is they names the system GPS. They should have called it GSS (Global Speed System)....... apparently.

If you look closely at the US government documents that describe the features of the US GPS system they clearly state that it is intended to provide position, velocity and time (PVT) information. They specify the performance that can be achieved in each domain of position, velocity and time. Velocity can be determined directly much more accurately than taking two positions and dividing by time. They never talk about nor give an error budget for measuring velocity that way. All the error budgets for velocity are for calculating velocity instantaneously irrespective of computed position or time because GPS provides all three simultaneously.

Read this document for example: http://www.gps.gov/technical/ps/2007...e-standard.pdf

I suspect at least some manufacturers marketing blurbs describe this incorrectly intentionally so they don't have to go through what we are going through right now. And that's unfortunate because some people then think GPS is not good at giving real time accurate velocity. But it is good at doing that precisely because position is not needed to compute velocity.

[Wotname]

OK, now that we have (almost) worked out GPS works, perhaps someone could explain to me why I am strongly drawn back to post #1 of this thread, especially para 2

[Sailor Doug]

I guess anyone who reads this and find it over complicated should stop reading now. I started asking questions when some one said gps was like a policemen's speed gun. It also gets more confusing when a lot of posts included data that just includes a lot of formulas. Sorry I have had to many PHDs who did a good job of remembering but did not full understanding the science. This gets more complicated by the fact you never know the technical competence of the people who reply to you. If I step on your toes sorry.
This gets me back to recreational marine gps. How much of the information is from filtered time broadcast (Kalman filter) and how much is from velocity changes of multiple transmitters?


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[Wotname]

Hey SD, FWIW, my toes have never been stepped by you; others however....

[SURV69]

I wonder what you meant when you asked how does it work?... The magic of GPS is the network of accurately located ground stations, constantly communicating with the satellites and updating the sat's locations.

Your GPS receives the results along with timing factors to ... I think ... a few nanoseconds, and then calculates your position pretty accurately.

[Wotname]

Originally Posted by SURV69

I wonder what you meant when you asked how does it work?... The magic of GPS is the network of accurately located ground stations, constantly communicating with the satellites and updating the sat's locations.

Your GPS receives the results along with timing factors to ... I think ... a few nanoseconds, and then calculates your position pretty accurately.

The whole thread was a spin off from another thread discussing STW and GPS derived SOG.
A part that thread got bogged down with how GPS determines speed so the mods ( bless their cotton socks) hived off this aspect and made it the subject of a new thread. The title is somewhat arbitrary.

The vast majority (and the handful of actual experts) agree that GPS uses doppler effect as part of it's technique to determine speed. A few (1?) apparently still insist that GPS speed is purely a function of distance travelled divided by time.

Feel free to have an input; informed or otherwise, the rest of us have!

[El Pinguino]

Originally Posted by goboatingnow

Agreed, you may continue to use a device invented by people , far far smarter then you.

The fact that someone is an expert in their chosen field does not make them smarter than anyone else.... many of whom will have their own area of expertise.

The truly 'smart' person is the one who can share a complex idea with laymen in an understandable form in one thousand words or less.......

[Sailor Doug]

Originally Posted by Wotname

The whole thread was a spin off from another thread discussing STW and GPS derived SOG.
A part that thread got bogged down with how GPS determines speed so the mods ( bless their cotton socks) hived off this aspect and made it the subject of a new thread. The title is somewhat arbitrary.

The vast majority (and the handful of actual experts) agree that GPS uses doppler effect as part of it's technique to determine speed. A few (1?) apparently still insist that GPS speed is purely a function of distance travelled divided by time.

Feel free to have an input; informed or otherwise, the rest of us have!

Sent from my iPad using Cruisers Sailing Forum

[Sailor Doug]

Doppler effect is part of how gps establishes relative speed I can agree on and have recently learned this. I think the definition of relative speed has been under stated. I replied to this thread when it was comparing gps to a paddle wheel. There are still some things that describe acceleration a lot better than gps. ( I am having fun keep the safety boots on)


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[transmitterdan]

Originally Posted by SURV69

I wonder what you meant when you asked how does it work?... The magic of GPS is the network of accurately located ground stations, constantly communicating with the satellites and updating the sat's locations.

Your GPS receives the results along with timing factors to ... I think ... a few nanoseconds, and then calculates your position pretty accurately.

You are right. A little more detail is that GPS satellites transmit data (just a bunch of numbers) that the receiver stores in something called an "almanac". The numbers are sent only so often. These constants along with the measured Doppler frequency shift and time duration between certain code pulses can be used by the receiver to compute position, velocity or time since the GPS epoch. The ground stations verify the accuracy of the same calculations constantly. If the results begin to drift beyond the specified accuracy then the ground stations compute and upload new numbers. When the new data is received by your receiver then the accuracy of calculations is improved.

This is one reason GPS receivers that have been turned off for a long time or have been moved thousands of kilometers from their last location require more time to start working after powering on. They have to wait until the up to date almanac is received so the computations will be right.

[colemj]

Special relativity (the E=mc2 one) is only part of the correction. SR concerns itself with relative velocities, in which time dilation slows down a clock moving in a relatively faster frame of reference. So the SR equations are used to adjust the "slower" clock on the "faster" satellite.

But keep in mind that a boat and a satellite are accelerating relative to each other because each is falling in a differently shaped region of the warped space-time that forms the gravity field.

In the slower acceleration field at the greater height, the clock in the satellite is running faster, not slower, relative to the boat. The general relativity equations must be used to adjust for this. What comes out is the differential result of the combined equations, although in practice, the general relativity equations encompass the special relativity ones.

As for Lorentz, Poicare and others, they certainly worked out almost all of the math and stood right on the precipice of special relativity, but lacked the imagination and courage to take that step because it went against everything they thought was true in physics and the working of the universe. Poincare to his death would not accept relativity, even after all its predictions and testing were proven as fact. Einstein did not have these imagination or courage problems and is credited deservedly for special relativity.

As for general relativity, Einstein had the greatest single thought/insight a human has ever had. He alone did the exquisitely difficult work and came up with a complete theory that is fully contained. It is the only example that I know of in science where a fully complete principle was done by a single human and no further refinements or additions have been necessary. No other persons deserve credit for general relativity, and none ever came even close to having that greatest human thought ever.

BTW, Einstein also discovered quantum mechanics, proved the existence of atoms, and explained the mystery of Brownium motion that had baffled scientists for decades. The first two are also important to GPS because without quantum mechanics, we wouldn't have the clocks to adjust, and without atoms, we wouldn't have anything to put the clocks in (OK, that last one was a stretch!).

Mark