How Does GPS Work?

[Sailor Doug]

Dan I can't argue the math only a little about the logic. Since the speed we are discussing is a very low relative speed between two moving objects. I believe this means it is a relative velocity either to or from something on earth I have been or heading too or from. Some days the size of waves are distorting any readings.
I am learning a lot but I must push the logic.


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[DotDun]

The Sirfstar IV updates at 1hz
http://dlnmh9ip6v2uc.cloudfront.net/...S/GP-2106b.pdf

SkyTrak default is 1hz, capable of up to 40z
http://www.skytraq.com.tw/products/Venus816_PB_v1.pdf

Broadcom (used in a lot of cell phones) is up to 2hz
https://www.broadcom.com/products/GP...utions/BCM4750

[44'cruisingcat]

Originally Posted by transmitterdan

But the only thing I hope folks take away from this is that GPS receivers can calculate velocity very fast and very accurately. So fast and so accurately in fact that many trucks and high end cars use GPS speed instead of more traditional cable speedometers.

Fair winds,
Dan

Really? Which cars? This seems like a very bad idea to me. What happens in underground carparks or tunnels ?

[44'cruisingcat]

A digital display that updates much more than every second would be pretty much unreadable anyway.

[DotDun]

The move away from NMEA0183 to NMEA2000 has enabled GPS makers to increase the update rate from the standard 1hz to 5hz. The Maretron unit which is N2k only updates position at 5hz, speed at 4hz.

[Rustic Charm]

Very True

[lordgeoff]

Originally Posted by socaldmax

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?

The best and totally accurate way to verify the claims of solar panel manufacturers and suppliers??

.....
......
......
.......
Buy one of their panels......
There are a lot of variables. It is not a simple answer. I was very surprised at how the output degrades in very high temperatures (40C and above). Good supplementary source of energy but expensive and of course very intermittent energy production.

[socaldmax]

Originally Posted by transmitterdan

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.

But why is Garmin's explanation worded the way it is?

Doppler shift is not hard for anyone to understand. Why bother to obfuscate it? I just don't see any reason for them to give a different explanation. There's always a way to simplify it yet still remain technically correct.

The only reason people in this thread questioned it was because of all of the online sources that give a different explanation.

[Dockhead]

Originally Posted by socaldmax

But why is Garmin's explanation worded the way it is?

Doppler shift is not hard for anyone to understand. Why bother to obfuscate it? I just don't see any reason for them to give a different explanation. There's always a way to simplify it yet still remain technically correct.

The only reason people in this thread questioned it was because of all of the online sources that give a different explanation.

A couple of thoughts on this:

1. Garmin does not manufacture GPS modules. These are made by SiRF, a different company.

2. Garmin's marketing blurb is written by non-technical people for non-technical people. It will naturally be simplified, technically vague, and may be erroneous on some points. This should not surprise anyone.

3. Non-technical people will naturally be attracted to marketing blurb, which will seem clearer and easier to read, than real technical information. But they should not be deceived that marketing blurb is intended to give serious answers to serious technical questions -- it's not intended for that. If you want a real technical explanation of something like how speed measurement works, you will have to read real technical literature.

4. In any case, I did not read the Garmin blurb cited by several above, to say that speed is measured by position differentiation. It just said that "first a position is fixed, then . . . ". You shouldn't jump to conclusions that this means that a series of positions is used for speed measurement -- it doesn't say that. It just skips over the details about how that is done.

5. Someone above claimed that "about half" of the literature you can find says that GPS measures speed by position differentiation. This is false. All of the serious technical literature that gets into how speed is measured, talk about Doppler shift of the carrier. Simply because that is how it, in fact, is done, although that is also a big simplification. For a really good explanation of how it's done in more detail - how this data together with other data is put into a Kalman filter -- see the explanation by the SiRF engineer -- engineer from the company which makes the modules used in Garmin GPSs -- cited above. That is, if you don't believe our own Dave, who is a real expert and professional in the field. Why anyone would, instead of listening to this, insist on his own half-understanding of marketing blurb -- is beyond my understanding.

[goboatingnow]

Originally Posted by socaldmax

But why is Garmin's explanation worded the way it is?

Doppler shift is not hard for anyone to understand. Why bother to obfuscate it? I just don't see any reason for them to give a different explanation. There's always a way to simplify it yet still remain technically correct.

The only reason people in this thread questioned it was because of all of the online sources that give a different explanation.

heres a copy of a post on Sci.... Gps , after a long winded debate and the usual; few "deniers"

Garmin engineering has replied to my inquiry as to which method
(PVT or distance/time) they use to compute speed. Their answer
is as follows:

Joe,

We do a full PVT computation on all of our products.
Position/time yields poor results for velocity calculation, so we
rely more on Doppler measurements when calculating velocity.

Garmin Engineering

 

[transmitterdan]

Originally Posted by socaldmax

Doppler shift is not hard for anyone to understand. Why bother to obfuscate it? I just don't see any reason for them to give a different explanation. There's always a way to simplify it yet still remain technically correct.

The only reason people in this thread questioned it was because of all of the online sources that give a different explanation.

I guess for me Doppler shift isn't totally easy to understand. It seems not that many people can believe and fewer still understand how the exact same radio wave can be at several different frequencies at the same time. The difference being the relative velocity of the several observers and/or the type of matter the wave travels through. The fact that a $20 piece of electronics can accurately measure relativistic phenomena is amazing. As a first year physics student it was quite mind boggling and still is to me.

[socaldmax]

Originally Posted by goboatingnow

heres a copy of a post on Sci.... Gps , after a long winded debate and the usual; few "deniers"

See? That wasn't so hard, was it?

So why couldn't they have simply posted that on their website, instead of the gobbledygook that they have up there.

That would have made an excellent follow up question.

[Dockhead]

Originally Posted by transmitterdan

I guess for me Doppler shift isn't totally easy to understand. It seems not that many people can believe and fewer still understand how the exact same radio wave can be at several different frequencies at the same time. The difference being the relative velocity of the several observers and/or the type of matter the wave travels through. The fact that a $20 piece of electronics can accurately measure relativistic phenomena is amazing. As a first year physics student it was quite mind boggling and still is to me.

OK, look, I am not a technical person -- my background (at least, degrees) are in law, music, and philosophy. So this is my perhaps simplistic non technical understanding. But the idea of the Doppler shift seems fairly straightforward to me. I first got acquainted with the concept in childhood when reading about red shift as used in astronomy. We know the characteristic spectrum of light of different elements, which are kind of like a fingerprint. When you do spectral analysis of stars, you recognize these fingerprints, just shifted down towards red -- like music played in a different key. So you can compare the frequencies (colors) the light should be according to the element you're looking at and calculate the precise degree of shift redwards. From that, you can calculate the exact speed at which the star is moving away from you. That's how we figured out that the universe is expanding. And from the constant according to which the universe is expanding, we can figure out the distance of the star from us.

GPS speed measurement works on the same principle. The GPS receiver knows the carrier frequency of every satellite. It compares the received frequency to the frequency it is supposed to be, and can measure the difference (apparently to 1/3 herz -- astonishing how they do that). Now someone asked a good question -- that way we might well get our speed in relation to that satellite, but how do we get from there to our speed in relation to earth? I'll leave that to Dave to explain more thoroughly and correctly, but remember, we are doing this with several different satellites at the same time, and we know their positions. So we can use the same process of trilateration (like triangulation, except we use the sides, not the angles) we use to find position, to find speed in relation to earth, by comparing our relative speeds with several satellites whose positions are known.

Dave will correct me here if I'm wrong, I hope.

That really doesn't seem all that hard to understand, to me.

[Wotname]

Originally Posted by Dockhead

...... So we can use the same process of trilateration (like triangulation, except we use the sides, not the angles) we use to find position, to find speed in relation to earth, by comparing our relative speeds with several satellites whose positions are known.

Dave will correct me here if I'm wrong, I hope.

That really doesn't seem all that hard to understand, to me.

And presumably this is spherical trilateration ( curved sides)???
Not trying to be smartypants, just wanting to get a more complete understanding; again maybe Dave or others who know can correct if required.

[Sailor Doug]

Are all gps chip sets equal? Other than marine friendly interface does my phone perform as well as my garmin?


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