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Old 23-03-2015, 00:14   #31
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Re: How Does GPS Work?

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Originally Posted by Dockhead View Post
In another thread, there was a serious debate about how GPS works. I'm not sure why there's such a big debate -- it's not a matter of opinion. But in any case -- here's a place to discuss it. Enjoy.

I hope Goboatingnow will come by and straighten us out -- he's a pro in the field and forgot more yesterday, than the rest of us will ever know about how GPS works.
This is the internet, where everyone is entitled to their own facts!
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Old 23-03-2015, 00:47   #32
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by Sailor Doug View Post
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.


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You know what's even more amazing?

Smart phones. Mine can take HD pics, tag them with Lat/Lon using GPS, I can navigate on land or at sea with it using GPS just as one input source, it has accelerometers in it and a compass, it displays time very accurately, diagnose my car's engine via bluetooth, it can help me find the nearest gas station, restaurant, horny woman, motel and in about 15 minutes, it can take a blood sample and tell me if she has HIV or syphilis.

I can also send/receive emails, phone calls and text msgs with it, translate to/from other languages and shop online, order pizza and scan barcodes to shop for the lowest prices then track all of my packages from all over the world to my doorstep. Not to mention read and post on the internet. I think GPS is about 1/10th of what my smartphone does, and they're coming up with new stuff all of the time.


I remember when GPS was a 50lb medium grey unit that required crypto loaded into it periodically and it didn't even have a screen on it! LOL
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Old 23-03-2015, 01:45   #33
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Re: How Does GPS Work?

In all our cheapo GPS, speed is computed from the distance travelled over time.
See the following paper, that used to be online
T.H. Wittea, A.M. Wilson, Accuracy of non-differential GPS for the determination of speed
over ground Journal of Biomechanics 37 (2004) 1891–1898

From the variance given in this paper, it can be calculated that a bit less than 50% of the speed measurements will have an error of plus or minus 0,4 knots at 6 knots

The use of WAAS/EGNOS corrections will help a bit. Roughly plus or minus 0,3 knots.
Time averaging over e.g. 10 s would bring this down to plus or minus 0,1 knots in 50 % of the case, provided the speed is stable. It is not clear to me how such averaging is implemented on Garmin units.
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Old 23-03-2015, 02:42   #34
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Re: How Does GPS Work?

Ungrateful me.

In a moderate blow, partially cloudy with winds from the East, whilst my GPS is working out my position to within 15 feet, my immediate speed over ground, my height above sea level on the waves, my average speed for the last 21 hours and estimated time of arrival, My mind is occupied with 2 things. Knots and Toilet Paper.

I was pretty sure that I packed another 4 rolls, however, they are being elusive in the search. Pointing my handheld GPS in the general direction of the cabin did not help. Did I mention the heads are not working? So now my second preoccupation at the moment is knots...... knots that will hold me connected to the boat as I lean bottie over water. Did I mention I lost the bucket? Hmm.. I thought I had mentioned it but no matter.

I had worked out a place to do the nasty.. Just needed to rig up safety lines so that the squat is comfortable, accurate and above all, secure. I finally decided on 4 lines, it was a little choppy, with an assortment of knots, you cant be too careful you know, and then I positioned myself in the corner. It was then I remembered that I did not conclude my search for toilet paper. Aw well, too late the deed was done.... I sat there perched thinking about the problem when a wave slapped off the stern and shot up the back and covered my nether regions in icy cold water. This was followed by 2 more in quick succession. My problem was solved.

As I hobbled to the cabin in a state of wetness and undress, trying to undo the assortment of knots, my mind reflected that somewhere in Space, miles above, a record of the event had been made. The velocity of discharge from my rectum to the sea had been duly noted both in speed and distance traveled.

And I, the ungrateful wretch, did not give a stuff.
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Old 23-03-2015, 02:50   #35
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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Originally Posted by socaldmax View Post
You know what's even more amazing?

Smart phones. Mine can take HD pics, tag them with Lat/Lon using GPS, I can navigate on land or at sea with it using GPS just as one input source, it has accelerometers in it and a compass, it displays time very accurately, diagnose my car's engine via bluetooth, it can help me find the nearest gas station, restaurant, horny woman, motel and in about 15 minutes, it can take a blood sample and tell me if she has HIV or syphilis.

I can also send/receive emails, phone calls and text msgs with it, translate to/from other languages and shop online, order pizza and scan barcodes to shop for the lowest prices then track all of my packages from all over the world to my doorstep. Not to mention read and post on the internet. I think GPS is about 1/10th of what my smartphone does, and they're coming up with new stuff all of the time.


I remember when GPS was a 50lb medium grey unit that required crypto loaded into it periodically and it didn't even have a screen on it! LOL
wow, I have just a boring one, where can I get one that does all that.
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Old 23-03-2015, 02:58   #36
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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I remember when GPS was a 50lb medium grey unit that required crypto loaded into it periodically and it didn't even have a screen on it! LOL
I once had a psycho girlfriend who stalked me accurately better than any GPS. She knew where I was at any time. Took 10 years, 4 country changes, the FBI and a spell in the nice hospital to break the internal locator beacon. Im convinced GARMIN based their system on her.
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Old 23-03-2015, 03:07   #37
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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I once had a psycho girlfriend who stalked me accurately better than any GPS. She knew where I was at any time. Took 10 years, 4 country changes, the FBI and a spell in the nice hospital to break the internal locator beacon. Im convinced GARMIN based their system on her.
na, I don't want one of them models then.

What's this thread about anyways? I didn't understand the op's question, if there was one. Now I've read the thread I'm still none the wiser.

And when I was a cop I had to parrot some paragraph in court about how a radar gun works and the precise meaning of 'Doppler', made us all sound like experts. I never did understand what we had to say in court..
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Old 23-03-2015, 03:26   #38
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Re: Speed Through GPS Versus Old Fashioned Paddle Log

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No, he used the same Wikipedia, bur he's better at cutting and pasting than you, apparently.

Here is your post. Notice the missing word "use." He wasn't disputing you, he was correcting your quote, which is somehow missing a word. This made me question the accuracy of your quote, since a simple cut n paste would not have dropped a word in the middle of the sentence. It honestly made me think it was edited, that's why I looked it up myself.
Ahh... yes, I see what Chala was referring to so I stand corrected.
A big thanks to both Chala and Socaldmax

The next bit is a bit boring so please feel free to jump forward to the next post

I currently can't cut and paste . My computer died a week back and since then, I have being comming to grips with a base model tablet with and outdated browser. In fact, I can hardly use it properly at all. It driving me to drink which is what I decided to do last night when it froze up in the middle of retyping the wiki quote. When I finally got the +=% tablet working again, I retyped the wiki quote again with the help of a rather nice bottle of red. The results speak for themselves .
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Old 23-03-2015, 04:01   #39
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Re: How Does GPS Work?

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Originally Posted by transmitterdan View Post
.......
The reality is that most GPS brands don't actually make the GPS modules. They are made by a handful of far eastern companies. The companies like Apple and Garmin are mainly software companies and they purchase GPS modules from someone else.
................

Accurate speed is a real challenge for slow moving applications like sailing, jogging, walking, and the like. The fact you can get accurate speed at 0.5 knots updated 10 times a second is truly mind boggling.

Fair winds,
Dan
I have no idea where or how Garmin get their GPS engines these days but let's not forget that the founders of Garmin (Gary Burrell & Min Kao) were avionics design engineers at the forefront of early GPS technology. They had previously worked for BendixKing and Magavox in the GPS development. Right from the early days (around 1991) their product was manufactured in Taiwan but they did the design work.

I suspect but don't know that Garmin still employes serious GPS design engineers.

I also recall their first product (Pronav 100) has a user defined setting to average the speed over various time periods. You had three choices: fast, medium and slow

I sure it is safe to say the unit didn't use doppler techniques then just as I'm sure it's safe to say their high end units today do use doppler techniques.

As to whether these units determine location first and then refine it a few milliseconds later with doppler or vice versa is rather a mute point - IMHO.
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Old 23-03-2015, 04:22   #40
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Re: How Does GPS Work?

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Originally Posted by sailorF54 View Post
In all our cheapo GPS, speed is computed from the distance travelled over time. See the following paper, that used to be online T.H. Wittea, A.M. Wilson, Accuracy of non-differential GPS for the determination of speed over ground Journal of Biomechanics 37 (2004) 1891–1898

From the variance given in this paper, it can be calculated that a bit less than 50% of the speed measurements will have an error of plus or minus 0,4 knots at 6 knots

The use of WAAS/EGNOS corrections will help a bit. Roughly plus or minus 0,3 knots. Time averaging over e.g. 10 s would bring this down to plus or minus 0,1 knots in 50 % of the case, provided the speed is stable. It is not clear to me how such averaging is implemented on Garmin units.
Sorry, but that very paper, from 2004, says:

"It is currently not clear, however, whether the improved positional accuracies of dGPS are mirrored by enhancements in the accuracy of speed determination, since GPS speed determination does not rely solely on
differentiation of position data over time but also depends on Doppler shift of the carrier wave."

See also:

Contrary to popular belief, speed is not derived from the distance and time between two position fixes. The GPS satellites are orbiting the Earth at high speeds and there is an observable Doppler effect on their signal frequencies. GPS receivers can therefore measure how fast the satellites appear to be moving towards (or away) from them. Since the GPS receiver already knows a lot about the satellites (position, speed and direction), they can determine their own speed using a mathematical approach called “least squares”. Civilian GPS (horizontal) position fixes are typically accurate to about 15 meters (50 ft) for 95% of the time. However, the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are on average accurate to about 5 meters (16 ft) horizontally. Speeds are typically accurate to within 0.05 m/s (0.1 knots).

Note: Generally speaking, the speed information (Doppler derived) is more accurate than the positional information.

http://www.mikeg.me.uk/guides/GPS_De...Comparison.pdf



Someone made much of the question of why the manufacturers don't talk about Doppler speed measurement. But who is the manufacturer? Neither Garmin, nor Navico, nor Raymarine, produces a single GPS chipset (Furuno produce their own, however). Most of these are made by specialized companies like CSR/SiRF.


Here is what a SiRF engineer says on the matter:

"SiRF's navigation algorithms compute velocity generally using Doppler, but since the Kalman filter is involved, there is a factor from delta position as well. Of course, static navigation can screw things up at slow speeds if it is on. And track smoothing can also change things a bit as it adds in latency on dynamics.

"Doppler measurement is part of the satellite tracking process, done continuously by the hardware and monitored at 100 ms intervals by the software. The velocity solution comes out of the Kalman filter, which uses all the inputs, including primarily the various Doppler readings, but also the previous solution in both position and velocity. We could quantify how much each element contributes only if we knew the uncertainties applied to each -- it is a feature of a Kalman filter that each contributor to a solution is weighted based on the confidence we have in that portion. So if signals are great, Doppler is given heavy weight, but if they are deteriorating, the Kf weights them less and the previous solution gets more weight."

http://www.gpspassion.com/forumsen/t...5&whichpage=27


So the answer is that both position differentiation and carrier Doppler shift are used to determine speed, by applying this very cool algorithm: Kalman filter - Wikipedia, the free encyclopedia. But Doppler shift of the carrier is the most important component of speed measurement.

And this relates to the el-cheapo GPS receivers we all use -- even the ones in our phones, which are also made by companies like SiRF.
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Old 23-03-2015, 06:36   #41
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Re: How Does GPS Work?

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Originally Posted by socaldmax View Post
The problem with your analogy is that a stationary radar gun must be held stationary to accurately measure the velocity of the target, movement by the officer can create a huge error.

A mobile mounted radar gun uses calibrated velocity input from the vehicle to add or subtract from the calculated velocity from Doppler shift.

In the case of a GPS mounted on a boat, it doesn't get a speed input from the boat, so the unit really has no idea what it's velocity is in relation to the velocity of the satellite. Yes, Doppler shift can be measured and used to calculate absolute velocity, but again it has no way of knowing how much is satellite velocity and how much is vessel velocity until after it has calculated position, velocity and time from the data in the signal.
I think by now someone has posted lots of copied text from various sources. Probably this will still not convince some.

The radar gun can see the Doppler from more than just the car. It also sees the Doppler from the stationary objects as well. By looking at the difference it can tell the speed of the car relative to its surroundings. Radar guns do not have to be wired into the police car in any way. Similarly, a GPS receiver can look at the relative speed of more than one satellite. From that it can ascertain its velocity in 3 dimensions. The more satellites the better the accuracy. I can assure you that the GPS algorithms can and do compute velocity very accurately. And it is done without having to wait for two position fixes.
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Old 23-03-2015, 06:44   #42
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Re: How Does GPS Work?

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Originally Posted by Dockhead View Post
So the answer is that both position differentiation and carrier Doppler shift are used to determine speed, by applying this very cool algorithm: Kalman filter - Wikipedia, the free encyclopedia. But Doppler shift of the carrier is the most important component of speed measurement.

And this relates to the el-cheapo GPS receivers we all use -- even the ones in our phones, which are also made by companies like SiRF.
Some were confused as to how GPS can produce velocity data without waiting for 2 or more position updates so it can compute speed by distance divided by time. The answer is that velocity is inherent in the algorithm because it uses Doppler shift. If the algorithm has Doppler shift data then velocity comes from that for free. So the receiver does not wait for two or more position fixes. Velocity can be produced very quickly and in real time as far as humans are concerned. Some specialized receivers only produce velocity and don't bother with the position computation to save cost and/or power.
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Old 23-03-2015, 06:45   #43
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Re: How Does GPS Work?

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Sorry, but that very paper, from 2004, says:

"It is currently not clear, however, whether the improved positional accuracies of dGPS are mirrored by enhancements in the accuracy of speed determination, since GPS speed determination does not rely solely on
differentiation of position data over time but also depends on Doppler shift of the carrier wave."

See also:

Contrary to popular belief, speed is not derived from the distance and time between two position fixes. The GPS satellites are orbiting the Earth at high speeds and there is an observable Doppler effect on their signal frequencies. GPS receivers can therefore measure how fast the satellites appear to be moving towards (or away) from them. Since the GPS receiver already knows a lot about the satellites (position, speed and direction), they can determine their own speed using a mathematical approach called “least squares”. Civilian GPS (horizontal) position fixes are typically accurate to about 15 meters (50 ft) for 95% of the time. However, the advancement of technology means that today, civilian GPS fixes under a clear view of the sky are on average accurate to about 5 meters (16 ft) horizontally. Speeds are typically accurate to within 0.05 m/s (0.1 knots).

Note: Generally speaking, the speed information (Doppler derived) is more accurate than the positional information.

http://www.mikeg.me.uk/guides/GPS_De...Comparison.pdf



Someone made much of the question of why the manufacturers don't talk about Doppler speed measurement. But who is the manufacturer? Neither Garmin, nor Navico, nor Raymarine, produces a single GPS chipset (Furuno produce their own, however). Most of these are made by specialized companies like CSR/SiRF.


Here is what a SiRF engineer says on the matter:

"SiRF's navigation algorithms compute velocity generally using Doppler, but since the Kalman filter is involved, there is a factor from delta position as well. Of course, static navigation can screw things up at slow speeds if it is on. And track smoothing can also change things a bit as it adds in latency on dynamics.

"Doppler measurement is part of the satellite tracking process, done continuously by the hardware and monitored at 100 ms intervals by the software. The velocity solution comes out of the Kalman filter, which uses all the inputs, including primarily the various Doppler readings, but also the previous solution in both position and velocity. We could quantify how much each element contributes only if we knew the uncertainties applied to each -- it is a feature of a Kalman filter that each contributor to a solution is weighted based on the confidence we have in that portion. So if signals are great, Doppler is given heavy weight, but if they are deteriorating, the Kf weights them less and the previous solution gets more weight."

GpsPasSion Forums - SiRFdemo tutorial (static navigation)


So the answer is that both position differentiation and carrier Doppler shift are used to determine speed, by applying this very cool algorithm: Kalman filter - Wikipedia, the free encyclopedia. But Doppler shift of the carrier is the most important component of speed measurement.

And this relates to the el-cheapo GPS receivers we all use -- even the ones in our phones, which are also made by companies like SiRF.
Thank you, your point is well taken.
However the doppler speed estimation cannot be vastly more accurate than the position location (otherwise it could be used to improve the position location !)
Therefore it does not matter too much which weighing and time averaging of inputs the filter applies. The accuracy is experimentaly low at slow speed, as the position location jitter is important. My numbers, derived from experimental data, should still be in the right ballpark...
I only assumed that the position jitter is random, i.e. the successive positions are not spatially correlated, and this might be too strong an assumption...The accuracy could be a bit better... However, looking at GPS speed data, one can often see huge (up + / - 1 knot) short-time (a few seconds) drifts re speedo speed.
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Old 23-03-2015, 07:17   #44
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Re: How Does GPS Work?

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Thank you, your point is well taken.
However the doppler speed estimation cannot be vastly more accurate than the position location (otherwise it could be used to improve the position location !)
Therefore it does not matter too much which weighing and time averaging of inputs the filter applies. The accuracy is experimentaly low at slow speed, as the position location jitter is important. My numbers, derived from experimental data, should still be in the right ballpark...
I only assumed that the position jitter is random, i.e. the successive positions are not spatially correlated, and this might be too strong an assumption...The accuracy could be a bit better... However, looking at GPS speed data, one can often see huge (up + / - 1 knot) short-time (a few seconds) drifts re speedo speed.
Inaccuracy of low speed speed measurement is not inherent in the process -- it's a question of computing power of your receiver and quality of the software. The Doppler measurement requires -- as Dave told us -- measurements as fine as 1/3 of a hertz in the carrier signals -- so requires also extremely high precision clock. These capabilities are improving all the time, and more recent "el cheapo" GPS receivers, for example based on SiRF Star III and IV chipsets, are typically accurate to 0.1 knots at all speeds. I do not observe on my system such jitter as you report. I'm actually not sure what chipset my Simrad GS25 receiver uses. The Simrad GS15 used the SiRF Star III chipset. My old Raystar 120 DID give speed jitter like you report -- it had the old SiRF Star I chip which had far less computing power.

Also don't forget that speed itself might really be varying in tenths of a knot. Your GPS may have different damping settings from your speed log, or your speed log is simply not sensitive enough to detect 0.1 knot speed variations.
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Old 23-03-2015, 11:39   #45
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Re: How Does GPS Work?

I didn't read all of the posts in this thread. I'm just responding to the title. I really don't have any credentials built in this community, so I understand why people may not accept what I say as creditable information. Anyway, I'll dive in:

The first thing to look at is the error budget for determining GPS accuracy. There are statistically defined errors attributed to the equipment controlling the GPS satellites, the satellites themselves, and the receivers (bought by consumers). The major contributors to position error are satellite clock and ephemeris, troposphere, ionosphere, receiver oscillator and receiver measurements. For velocity, the major contributors to error are receiver clock and receiver measurement error. As a first order, position accuracy comes out to be around 20 meters in a 1-sigma (67%) sense, and velocity is around 0.1 m/s in a 1-sigma sense. The errors will vary depending on how a manufacturer defines the various error components. In addition, the geometry of the satellites play a direct roll in the accuracy output from a GPS receiver. This geometry is call Dilution of Precision (DOP). DOP is broken down into Geometric DOP (GDOP), Horizontal DOP (HDOP), Vertical DOP (VDOP) and Time DOP (TDOP). DOP is a measure of how many satellites are used to calculate position/velocity and how the satellites are distributed around the receiver.

As an aside, time is computed very accurately by GPS receivers too. However, it is not usually important to consumers to know time down to 10's of nanoseconds, so this isn't normally made available to the consumer.

The next thing to look at is the methods for determining position and velocity. The first thing to note is that there are two tracking loops running in a GPS receiver. One loop tracks the carrier frequency (1575.42 MHz) and the other tracks the pseudorandom (PN) code. PN code is a sequence of 1's and 0's that repeats every 1024 chips. Also there is a 50Hz data sequence riding on the PN code.

The purpose of the carrier loop is to track the carrier signal (sine wave modulated by the PN code). The carrier loop produces a value called "delta-range" (probably why people think velocity is based on range measurements). Delta range is a measure of the change in carrier wavelength over the tracking interval (many time 50Hz). Delta range is really a velocity measurement that is changed into frequency error by combining it with the speed of light and the carrier wavelength. So delta range and frequency error of the loop are essentially the same value converted to each other by constants. The significant thing to note is, the carrier loop produces measurements of frequency error from the fundamental transmit frequency of 1575.42 MHz.

The code loop tracks the phase of the PN code. The code loop cannot determine frequency error due to code phase measurements being to noisy. So the "delta range" measurement from the carrier loop is used to help track the code, since delta range is really a measure of frequency error in the tracking loops. The significant thing to note is, the code loop produces measurements of range to the satellite called pseudorange.

Now there are two sources of measurements. One source is used to compute velocity and the other position. These two types of measurements are frequency offset from the fundamental, and range to the satellite. Both of the measurement types have the prefix "pseudo" attached, because each of these measurements have error in them as part of the measurement. Hence, pseudorange has satellite errors, atmospheric error, local oscillator error still in them. So at a very high level, the receiver removes as much of these errors as it can and then produces position from a group of pseudorange measurements and velocity from a group of frequency offset measurements.

Measurements from a minimum of four satellites are needed to produce a 3-dimensional solution (fewer if there is a good estimate of time and/or height). For position, a set of four range measurements produce 3-D position and a time offset in the local oscillator. For velocity, a set of four frequency measurements produce 3-D velocity and frequency offset for the local oscillator. As state earlier, the DOP has a direct impact on accuracy. Lower DOP produces a more accurate solution. Receivers fielded today track all satellites that are in view of the receiver. So the accuracy is better than the early receivers that could only produce measurements from four satellites.

This is how a GPS receiver works in a generic sense. Much innovation has occurred in producing more accurate measurements, faster satellite acquisitions, receiver sensitivity, and multipath mitigation...other areas too. But, I covered the basic operation that holds for all receivers. With these concepts understood, a universal understanding of GPS operation is had.
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