How Does GPS Work?

[DeepFrz]

Course (navigation) - Wikipedia, the free encyclopedia

Before the thread gets back on track I would like to put in a plug for a free eBook, Navigation For The Rest Of Us.

[Dockhead]

Originally Posted by Rustic Charm

You have a way with words Dockhead! God help students if you ever attempt to teach in a class room.

LOL!

I've been doing it on a regular basis for a few decades. So far no students have committed suicide They say I'm strict, though

[Wotname]

Originally Posted by DeepFrz

Course (navigation) - Wikipedia, the free encyclopedia

Before the thread gets back on track I would like to put in a plug for a free eBook, Navigation For The Rest Of Us.

Not sure about the rest of that wiki page but they got it wrong about the bit about the Course Deviation Indicator. Anyhow probably not relevant to thread

[gbgreen59]

Originally Posted by Dockhead

Rather than guessing or supposing what "heading" means, according to what "sounds correct" to you, I suggest you look it up. It has a precise definition, which is really important for navigation.

Thanks for direction. Very good comment.

[DeepFrz]

Wotname, yes, the wording of the article isn't all that great. I guess someone could always edit it...(hint, hint). The definitions and diagrams though are clear and it was for that reason I posted it. That bit about the course deviation indicator lost me but the link takes you to a picture and a better description.

It has been such a long time since I flew I had to remind myself what they were talking about.

[StuM]

Originally Posted by DeepFrz

Wotname, yes, the wording of the article isn't all that great. I guess someone could always edit it...(hint, hint). The definitions and diagrams though are clear and it was for that reason I posted it. That bit about the course deviation indicator lost me but the link takes you to a picture and a better description.

It has been such a long time since I flew I had to remind myself what they were talking about.

At sea level, it's more commonly know as Cross Track Error (XTE)

[Wotname]

Originally Posted by StuM

At sea level, it's more commonly know as Cross Track Error (XTE)

Well yes but only if the said CDI was coupled to a GPS, if in the original configuration with the CDI coupled to a VOR or LOC transmiter, then the deviation shown on the indicator is actually a bearing deviation, not a displacement deviation.

Of course at sea level, most are using GPS not V/L so your statement is mostly accurate

Anyhow back to regular programming - if such a thing ever existed on this thread

[El Pinguino]

I am but a simple sailor man... however...

Transit Satnav was 100% doppler but that is neither here nor there...

For a GPS to work out your speed via doppler it first needs to know which direction you are moving in.....frinstance

A simple example. A GSP sat is zooming along on a N/S course somewhere to the west of you...
No probs thus far...
So you start moving west... and the doppler shift associated with the sat changes...
Easy peasy so far... if you are moving 'due west' and it knows that it can compute your speed quite easily.. but lets say you are moving SW or NW... the frequency shift will give you quite a different speed...
So to give accurate speed it must know your direction of travel... to know that it must know where you were and where you are now....

Just a thought...

Anyone up for a chat about the derivation of the Haversine formula and its place in traditional navigation??

[Wotname]

Originally Posted by El Pinguino

I am but a simple sailor man... however...

Anyone up for a chat about the derivation of the Haversine formula and its place in traditional navigation??

Yep, of course but I gotta say it will be pretty boring to most. You will be doing all the talk and I will be going "un huh, really, is that so, gosh, well I never" and so on and so forth.

I sotta recall the expression and maybe it was something about helping get a LOP but just tell on what page of my GPS does one find the Haversine stuff?

[transmitterdan]

Originally Posted by El Pinguino

For a GPS to work out your speed via doppler it first needs to know which direction you are moving in.....frinstance

snip

Obviously velocity cannot be determined from a single satellite Doppler measurement. However, there are many satellites in the US constellation (24 minimum) and for most of the world there are at least 6 visible at all times. So the answer is that the receiver uses Doppler shift of multiple satellites. There will be only one correct velocity and position solution that simultaneously matches the Doppler shift and ranging data from multiple satellites.

[gbgreen59]

Originally Posted by transmitterdan

Obviously velocity cannot be determined from a single satellite Doppler measurement. However, there are many satellites in the US constellation (24 minimum) and for most of the world there are at least 6 visible at all times. So the answer is that the receiver uses Doppler shift of multiple satellites. There will be only one correct velocity and position solution that simultaneously matches the Doppler shift and ranging data from multiple satellites.

What he said....

[boatman61]

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

[leftbrainstuff]

Originally Posted by Sailor Doug

From Doppler they know if a satellite is moving away or towards them that allows calculated correction of satellite position only.
Gps is quite amazing, at lest three satellites rotating very very high, very fast elliptical orbit. Spinning semi sphere and a boat siting on it in a fluctuating fluid. Device on the boat (gps) is calculating relative direction, speed and position on this rotating sphere. Only a very weak common message is given to all devices trying to solve this problem. It is amazing how accurate and quickly you processor does this. Did I also mention from memory the gps needs to know orbit of satellites, surface of earth at boats position and correct it for surface undulation.

If you give up your knot log you might as well give up your watch and compass cause gps does all three of them better.


Sent from my iPad using Cruisers Sailing Forum

Except for the contribution of current effects.

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

[gbgreen59]

Originally Posted by El Pinguino

Just a thought...

Anyone up for a chat about the derivation of the Haversine formula and its place in traditional navigation??

Or how about discussing WGS-84 ellipsoidal model instead for a model of the Earth's shape. Or more generally, what is the latest greatest ellipsoidal model? Normally, GPS receivers use the WGS-84 model of the ellipsoid and not a sphere.

[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