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Old 25-03-2015, 19:02   #121
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Re: How Does GPS Work?

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Originally Posted by lordgeoff View Post
...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.
Yup. I'm thankful also, The good 'ol US of A.

Now, when is the super low cost, super high speed, global Internet service coming? I need that in the middle of the Pacific! Sometimes the Atlantic too.

Jets come, pass overhead, and speed away. I could care less about the doppler effect.
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Old 26-03-2015, 00:57   #122
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Re: How Does GPS Work?

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Originally Posted by Wotname View Post
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!
ahh, no thanks, I've understood little of this thread.

All I know about GPS is it saves me doing a hell of a lot of drawing on paper charts and other stuff like that.
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Old 26-03-2015, 03:10   #123
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Re: How Does GPS Work?

OK, here's the pseudo-OP checking back in!

The first post in this thread was arbitrarily selected by Dockhead from that preceding stew where the question of the working of GPS came up. I really didn't have the intention of starting this thread, but I have found it quite informative, so thanks for all the far better informed folks who have attempted to explain an incredibly complicated and sophisticated system to mere mortal yotties.

I hope that by now the Doppler-deniers have come to grips with the facts and are willing to admit that their conceptions of velocity measurement were wrong. If not, well, I reckon there is no hope, and they will just have to carry on using this marvelous tool without an accurate idea of its inner workings. Probably works for them...

And for all those who reminded us of the giants of physics upon whose shoulders this system stands... thanks for those humbling reminiscences!

Jim, still using Ann's computer, lying in Eden awaiting the passage of a nasty front before continuing northward. Incidentally, this marks our 26th crossing of Bass Straits... and it still commands our complete respect!
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Old 26-03-2015, 03:12   #124
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Re: How Does GPS Work?

Who needs GPS! Print this out and keep it within arms length for best accuracy:
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Old 26-03-2015, 04:52   #125
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Re: How Does GPS Work?

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Originally Posted by noelex 77 View Post
Who needs GPS! Print this out and keep it within arms length for best accuracy:
only took 7 pages
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Old 26-03-2015, 05:10   #126
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Re: How Does GPS Work?

To see a technical tutorial on how GPS works go to:

https://buildingmytrimaran.shutterfl...itioningsystem
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Old 26-03-2015, 07:52   #127
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Re: How Does GPS Work?

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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.
transmitterdan. Again. I'm not the one to talk to. Don't be shy. Send your questions to GARMIN...or Raymarine or whatever.
Cheers buddy. OMG I just noticed my GARMIN calculated my position 83 times it took to reply to your post sorry now 123 time.
...........I'm tied up to the dock BTW
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how long has this been going on and why wasn't I told about it earlier.....
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Old 26-03-2015, 11:29   #128
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Re: How Does GPS Work?

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Agreed, you may continue to use a device invented by people , far far smarter then you.
Exactly like when you use a computer.
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Old 26-03-2015, 11:59   #129
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Re: How Does GPS Work?

To be objective, let's say a person doesn't know anything about GPS and does a Google search. Half of the sources you find indicate that Doppler shift is used for velocity calculations, half of them, including mfr's own websites, do not. I see no good reason for them to lie about something like that, but the "experts" insist that their version is right, and that Garmin is lying, with no explanation why.

Meanwhile, the "top expert" who has forgotten more than all of us will ever know, says it uses "inferometry" which I guess, means that he forgot the correct spelling of interferometry. Not a big confidence booster, considering that I rarely ever use that word (since I'm not an expert on GPS) but I have no problem spelling it.

So it's pretty easy for me to see why there is plenty of confusion about how GPS calculates velocity with all of the disinformation spread out there. How do you determine who to believe? The mfr or the guy who can't spell interferometry correctly?

Taking it to a different issue: who publishes the output specs for solar panels? The mfrs do. If you can't trust GPS mfrs, can you trust solar panel mfrs? How do you tell which mfrs are lying and which aren't?
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Old 26-03-2015, 14:02   #130
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Re: How Does GPS Work?

I don't think anyone believes that Garmin or any other company is telling lies. A lie is some untruth knowingly told with an intent to deceive. No such knowing intent exists as far as I can tell. So let's be clear that there is no issue with trusting GPS manufacturers as far as this thread is concerned.

The simple answer is that for 99.9% of GPS users this question of velocity doesn't matter. And for probably 90% of boaters it doesn't matter either. But for a few it might matter because they wrongly draw a conclusion that velocity updates are "slower" for GPS than simple paddlewheels or other speed measuring devices that work in real time. The fact is that GPS velocity works in real time as was the intent of the system designers. The answer to how that's possible is complex and simply too much for marketing material intended for the mass market to explain. Even here people don't believe it's possible to measure speed without knowing position. So the marketing folks punt as best they can and no one complains about it which is ok.
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Old 26-03-2015, 15:00   #131
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Re: How Does GPS Work?

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Originally Posted by transmitterdan View Post
I don't think anyone believes that Garmin or any other company is telling lies. A lie is some untruth knowingly told with an intent to deceive. No such knowing intent exists as far as I can tell. So let's be clear that there is no issue with trusting GPS manufacturers as far as this thread is concerned.

The simple answer is that for 99.9% of GPS users this question of velocity doesn't matter. And for probably 90% of boaters it doesn't matter either. But for a few it might matter because they wrongly draw a conclusion that velocity updates are "slower" for GPS than simple paddlewheels or other speed measuring devices that work in real time. The fact is that GPS velocity works in real time as was the intent of the system designers. The answer to how that's possible is complex and simply too much for marketing material intended for the mass market to explain. Even here people don't believe it's possible to measure speed without knowing position. So the marketing folks punt as best they can and no one complains about it which is ok.
It really doesn't matter how fast the GPS chipset measures/produces speed/velocity calculations. What matters is how fast the data hits the wire coming out of the unit. That has absolutely nothing to do with how it's measured/calculated within the chipset. So, yes, some can draw the correct conclusion by stating a paddlewheel is faster. A gps outputting dampened SOG at 1hz could certainly be slower to update than a paddlewheel dumping STW on the wire in realtime. One second is discernible to a human.
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Old 26-03-2015, 15:27   #132
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Re: How Does GPS Work?

If you are looking at the GPS display there is no wait for the wire. And even at 4800 baud the update rate is faster than once per second.
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Old 26-03-2015, 15:35   #133
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Re: How Does GPS Work?

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If you are looking at the GPS display there is no wait for the wire. And even at 4800 baud the update rate is faster than once per second.
hmm...watch the NMEA wire, SOG comes out of the unit at 1hz...how is that faster the once a second? I'll bet any display uses the same output.
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Old 26-03-2015, 15:47   #134
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Re: How Does GPS Work?

Dan you give yourself a disservice saying any processor works in real time. Even the brain has a problem doing that. I believe flies are best because they are hard wired to miss our flyswatters. Gps has to process data to give velocity away from our last fix.


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Old 26-03-2015, 16:02   #135
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Re: How Does GPS Work?

I know the GPS units that truckers use to display speed on the highway update much faster than once per second. I am sure there are integrated GPS displays that update the display faster than once per second.

Doug, the velocity is not the speed away from the last fix. It is a continuously updated value. There is no fundamental reason it cannot be updated 10 times or more per second. In fact, there are GPS units that can do that (e.g. Auto Meter 5289, Auto Meter Universal GPS Speedometer Interface | Auto Meter). Here's one that updates 100 times per second with 5mS latency (https://www.onosokki.co.jp/English/h...8100_8200.html) but watch out, they talk about Doppler shift so don't get mad at them.
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