How Does GPS Work?

[goboatingnow]

Originally Posted by leftbrainstuff

Except for the contribution of current effects.

Sent from my SM-N900T using Cruisers Sailing Forum mobile app

what so you mean here

[El Pinguino]

Originally Posted by goboatingnow

for the technical minded heres a good paper on the subject http://www.gps-results.com/GPS_Speed.pdf

I really think, unless you have significant technical understanding, debates on such subjects need to be left to people that do. This isn't being elitist , is simply a fact of life.

Dave

An excellent link which in the first two paras and the conclusion tells us all we simple folks need to know. Thank you for posting that .

The chap who wrote that has that rare skill of being able to deal with a complex subject in a simple form.
Ping

[socaldmax]

Originally Posted by boatman61

Well.. there's a button marked on/off..
Press and Bingo.. it works..

Still too complicated for my ex-GF. Hand her the GPS and somehow the batteries fall out.

[gbgreen59]

@goboatingnow: Nice writeup.

[a64pilot]

I can't believe it, six pages and none of you know how GPS works?

Simple it's PFM and if it's not PFM, then it's certainly Smoke and Mirrors

[gbgreen59]

Oh, you can be sure that receiver design is exactly as Erickson38 and I have described. Interferometry is a very general term.

[Canibul]

"Any sufficiently advanced technology is indistinguishable from magic". A.C. Clarke

[goboatingnow]

Originally Posted by a64pilot

I can't believe it, six pages and none of you know how GPS works?

Simple it's PFM and if it's not PFM, then it's certainly Smoke and Mirrors

funny that , I was sitting beside a Boeing Wing design engineer, I asked him tho explain the latest theory of flight, PFM he said

seems to in use everywhere . somewhat like the magic smoke that has to remain inside all electronics to make them work

[goboatingnow]

Originally Posted by gbgreen59

Oh, you can be sure that receiver design is exactly as Erickson38 and I have described. Interferometry is a very general term.

Yes of course, but its the basis of how range determination works in GPS ( and other systems )

[gbgreen59]

Originally Posted by goboatingnow

Yes of course, but its the basis of how range determination works in GPS ( and other systems )

Definitely makes it more comprehensible. It makes it much more understandable by the masses. Very good job in your writeup.

[Canibul]

Originally Posted by goboatingnow

Hi , I have signed Non Disclosure agreements and therefoer I cannot go into what specific manufactures do. But let me say the two or three posts from a couple of people here have expressed what happens.

Rather like the recent thread on power in a heating element, very few people have the time, knowledge or inclination to understand how GPS works at a first principles level. Hence they make intitutiuve claims or reference articles they either didn't read or understand ( as Dockhead has debunked)


Without descending into the technical issues of pseudo random code correlation , let me state what happens in as non tech as I can make it

(a) ALL gps receiver detect position by detecting the range of the satellite, this is done by inferometry , i.e. phase measurement, By default speed ( and using more satellites , velocity, is determined. instantaneous speed is a function ( note function) of phase detection and relies on the Doppler shift principles.

(b) Instantaneous speed ( I am particular about the term) is not a function of position determination, but as has been pointed out , unless you have a position lock ( in effect a clock correlation ) you havent for a speed. So its more correct to say that both are a function of a lock, but speed is not in itself a function of position determination ( more later)

(c) Because errors in these measurements are distributed randomly or pseudo randomly, simple averaging cannot be sued to remove " noise " or extraneous results. In simplistic terms , various forms of estimation, are used to in effect determine what errors are outliers. These filters typically use both speed and position to essentially attempt to curve fit the data and remove the outliers.

(d) Instantaneous speed is more accurate then simply using successive positions, simply because the distribution of position errors is such that it would result in a poorer accuracy then using the doppler effect. This is clearly shown in the technical paper referenced previously and in other technical papers.

Remember GPS error positions are statistical probabilities not fixed. Garmin for example uses the term Estimated Position Error circle, but is often mistakenly assumed by people that all positions lie within the circle , this is not the case.


Now, what a manufacturers determines what you see on the screen of a GPS, varies all over the place. Garmin in early models ( upto about 12xl) often showed speed computed from position. However in most cases today instantaneous speed is primarily derived from Doppler calculations


Equally what is stored and output on the NMEA interface can also be different , Garmin for example never historically stored speed in the track log, until fairly recently.

for the technical minded heres a good paper on the subject http://www.gps-results.com/GPS_Speed.pdf


Note GPS based inferometry is capable of determining heading , including absolute heading while stationary, however its needs two physically seperate antenna and significant processing power. I have on such device sitting on my desk as we speak.


I really think, unless you have significant technical understanding, debates on such subjects need to be left to people that do. This isn't being elitist , is simply a fact of life.

Dave

You're referring to Kalman filtering techniques, correct?

Re: determining heading using only two stationary antenna, how do you resolve which side of the baseline you are on? how accurate is the heading derived from two systems with a +/1 x (n) meters positional accuracy? and how complicated the installation? I was involved some years back with various navy trials exploring various ways to get platfform attitude using three GPS receivers, and the filtering alone prevented us from using it for real time platform attitude. We needed to know the surface ship's attitude at the time an acoustic signal arrived at a hydrophone. The GPS systems artificially removed the 'jitter' and came up with a CEP they liked. The predictive filters in use at that time couldn't keep up with real rapid changes.

Would you agree that the GPS manufacturers sacrifice absolute accuracy in the interest of showing the customers a nice smooth track and position? Many GPS users I run across couldn't tell you the difference between accuracy, resolution, and repeatability. And in some cases, they obviously don't know the difference between course and heading.

[Dockhead]

Originally Posted by goboatingnow

Hi , I have signed Non Disclosure agreements and therefoer I cannot go into what specific manufactures do. But let me say the two or three posts from a couple of people here have expressed what happens.

Rather like the recent thread on power in a heating element, very few people have the time, knowledge or inclination to understand how GPS works at a first principles level. Hence they make intitutiuve claims or reference articles they either didn't read or understand ( as Dockhead has debunked)


Without descending into the technical issues of pseudo random code correlation , let me state what happens in as non tech as I can make it

(a) ALL gps receiver detect position by detecting the range of the satellite, this is done by inferometry , i.e. phase measurement, By default speed ( and using more satellites , velocity, is determined. instantaneous speed is a function ( note function) of phase detection and relies on the Doppler shift principles.

(b) Instantaneous speed ( I am particular about the term) is not a function of position determination, but as has been pointed out , unless you have a position lock ( in effect a clock correlation ) you havent for a speed. So its more correct to say that both are a function of a lock, but speed is not in itself a function of position determination ( more later)

(c) Because errors in these measurements are distributed randomly or pseudo randomly, simple averaging cannot be sued to remove " noise " or extraneous results. In simplistic terms , various forms of estimation, are used to in effect determine what errors are outliers. These filters typically use both speed and position to essentially attempt to curve fit the data and remove the outliers.

(d) Instantaneous speed is more accurate then simply using successive positions, simply because the distribution of position errors is such that it would result in a poorer accuracy then using the doppler effect. This is clearly shown in the technical paper referenced previously and in other technical papers.

Remember GPS error positions are statistical probabilities not fixed. Garmin for example uses the term Estimated Position Error circle, but is often mistakenly assumed by people that all positions lie within the circle , this is not the case.


Now, what a manufacturers determines what you see on the screen of a GPS, varies all over the place. Garmin in early models ( upto about 12xl) often showed speed computed from position. However in most cases today instantaneous speed is primarily derived from Doppler calculations


Equally what is stored and output on the NMEA interface can also be different , Garmin for example never historically stored speed in the track log, until fairly recently.

for the technical minded heres a good paper on the subject http://www.gps-results.com/GPS_Speed.pdf


Note GPS based inferometry is capable of determining heading , including absolute heading while stationary, however its needs two physically seperate antenna and significant processing power. I have on such device sitting on my desk as we speak.


I really think, unless you have significant technical understanding, debates on such subjects need to be left to people that do. This isn't being elitist , is simply a fact of life.

Dave

I was wondering when you would come along and straighten us out. What a breath of fresh air, when someone who actually understands the subject inside and out, saves the blind from further leading the blind. Thanks to you and GBGreen and others who have shed actual light on the subject, to dissipate some of the smoke.

[goboatingnow]

Originally Posted by Canibul

You're referring to Kalman filtering techniques, correct?

No , Heading derived from interferometry using two physical seperate antennas . commonly called RTK and result in a compass that is superior to a magnetic one.

google is your friend here.

Re: determining heading using only two stationary antenna, how do you resolve which side of the baseline you are on? how accurate is the heading derived from two systems with a +/1 x (n) meters positional accuracy? and how complicated the installation

?

the solution provides for resolution of the baseline. Too complex to discuss here, but there are many sources, google " satellite compass"
accuracy can be down to fractions of a degree, I quoted some specific figures based on Hemispheres range of OEM compass solutions, reporting at about 10-20 Hz.

Installation is simple , no more difficult then ordinary single antenna GPS installs, many ships are now using GPS compass as a backup or replacement to Gyros.

I was involved some years back with various navy trials exploring various ways to get platfform attitude using three GPS receivers, and the filtering alone prevented us from using it for real time platform attitude. We needed to know the surface ship's attitude at the time an acoustic signal arrived at a hydrophone. The GPS systems artificially removed the 'jitter' and came up with a CEP they liked. The predictive filters in use at that time couldn't keep up with real rapid changes

Interesting, Altitude can also be determined by GPS interferometry , so I suspect that issue has been solved given the greater computing power available today.

Would you agree that the GPS manufacturers sacrifice absolute accuracy in the interest of showing the customers a nice smooth track and position? Many GPS users I run across couldn't tell you the difference between accuracy, resolution, and repeatability. And in some cases, they obviously don't know the difference between course and heading.

No , not as you put it. GPS companies, or moire correctly the small number of chip makers behind the scenes, make increasingly used more and more sophisticated configurations to improve the accuracy of GPS, within the cost constraints of the given marketplace. The " smoothness" you see today is a function of better error filtering, using predictive estimation filtering, improved clock correlation techniques, and other " tricks".


As for users, yeah sure, they have no idea whats really going on. LOL :big grin:

[Wotname]

Originally Posted by goboatingnow

Hi , I have signed Non Disclosure agreements and therefoer I cannot go into what specific manufactures do. But let me say the two or three posts from a couple of people here have expressed what happens.

Rather like the recent thread on power in a heating element, very few people have the time, knowledge or inclination to understand how GPS works at a first principles level. Hence they make intitutiuve claims or reference articles they either didn't read or understand ( as Dockhead has debunked)


Without descending into the technical issues of pseudo random code correlation , let me state what happens in as non tech as I can make it

(a) ALL gps receiver detect position by detecting the range of the satellite, this is done by inferometry , i.e. phase measurement, By default speed ( and using more satellites , velocity, is determined. instantaneous speed is a function ( note function) of phase detection and relies on the Doppler shift principles.

(b) Instantaneous speed ( I am particular about the term) is not a function of position determination, but as has been pointed out , unless you have a position lock ( in effect a clock correlation ) you havent for a speed. So its more correct to say that both are a function of a lock, but speed is not in itself a function of position determination ( more later)

(c) Because errors in these measurements are distributed randomly or pseudo randomly, simple averaging cannot be sued to remove " noise " or extraneous results. In simplistic terms , various forms of estimation, are used to in effect determine what errors are outliers.







These filters typically use both speed and position to essentially attempt to curve fit the data and remove the outliers.

(d) Instantaneous speed is more accurate then simply using successive positions, simply because the distribution of position errors is such that it would result in a poorer accuracy then using the doppler effect. This is clearly shown in the technical paper referenced previously and in other technical papers.

Remember GPS error positions are statistical probabilities not fixed. Garmin for example uses the term Estimated Position Error circle, but is often mistakenly assumed by people that all positions lie within the circle , this is not the case.

Now, what a manufacturers determines what you see on the screen of a GPS, varies all over the place. Garmin in early models ( upto about 12xl) often showed speed computed from position. However in most cases today instantaneous speed is primarily derived from Doppler calculations


Equally what is stored and output on the NMEA interface can also be different , Garmin for example never historically stored speed in the track log, until fairly recently.

for the technical minded heres a good paper on the subject http://www.gps-results.com/GPS_Speed.pdf

Note GPS based inferometry is capable of determining heading , including absolute heading while stationary, however its needs two physically seperate antenna and significant processing power. I have on such device sitting on my desk as we speak.


I really think, unless you have significant technical understanding, debates on such subjects need to be left to people that do. This isn't being elitist , is simply a fact of life.

Dave

Thanks for the excellent write up about GPS Dave

However I strongly disagree with your last paragraph. This suggestion does not promote any advancement of knowledge to those of us who may not have significant technical understanding. It could be seen as "just dumming down" even though I dont think that is your motive.

I reckon many of us posting here or just reading do not have "significant technical understanding" otherwise the issue would not have raised. You and several others have provided us with a now greater understanding than we had last week. For some you have confirmed what we thought we knew and for others presumably you have corrected some fallacies.

FWIW, it is easier (for me anyway) to understand this subject when it is written by mariner (ie you and others on CF) then trying to google a reliable source that isn't written by say a maths professor for an advanced scientific engineering design group or whatever.

[Wotname]

Originally Posted by socaldmax

Still too complicated for my ex-GF. Hand her the GPS and somehow the batteries fall out.

Try handing her a battery operated device she does understand