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Old 29-03-2015, 03:27   #166
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Re: How Does GPS Work?

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Originally Posted by Viking Sailor View Post
The following is an example of how a GPS receiver could provided Doppler SOG. There are other possible implementations.

The time it takes to begin providing SOG depends mostly on the acquisition mode of the receiver.

GPS Doppler SOG can be provided once the receiver has acquired at least one satellite with a geometry that includes some minimum Doppler shift due to vessel motion. When the receiver has carrier frequency and code lock, determined the approximate satellite and vessel positions it can remove various Doppler effects from the carrier frequency until all that is left is the uncorrected Doppler shift due to the motion of the vessel. The vessel Doppler shift can then be corrected based on geometry of the receiver and the satellite. As more satellites are acquired this processes can be repeated and applied to a filter to improve the SOG accuracy.
I don't understand how that could work - it seems to me too few operands to solve your velocity - even if you know exact (not approximate) positions. A given Doppler shift could give various velocities, depending on relative motion. Unless you derive that from position differentiation? But you can't do that, either, with a single satellite.

What you say is beyond my feeble understanding of this - please explain.

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Old 29-03-2015, 06:43   #167
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Re: How Does GPS Work?

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OK, now we have some FITK (folk in the know) particating in the thread I wish to ask a perhaps unimportant question but one that has been bugging me for awhile .

Back in the early days (1990ish) of GPS receivers with single channel receivers or at the best, only a small number of channels, the aviation receivers were provided with an altitude input with the intent of creating a "pseudo satellite" that could be added to the number of "real" satellites for the resolution of position. In essence, if this pseudo satellite was considered to be directly overhead, all one needed was one's altitude in order for the pseudo satellite to be considered "real". Remember way back then, the number of satellites were somewhat less than today.

So I got to thinking (maybe a bad habit ) that in the maritime world, one's altitude is always at sea level so for all intents and purposes, one could use this pseudo satellite principle as a built in design feature of the GPS receiver.

Thus my question is: was this feature actually used in maritime GPS receivers in the early days and if so, does it still apply today now we have multi channel receivers and a full complement of satellites?

Yes, that pseudolite is at the center of the earth (mathmatically ECEF X,Y,Z are are 0, no ephemeris required) and treated as having zero motion also, but with a range to the user = to earth radius (ellipsoid model). This is the arrangement for 2-D mode.
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Old 30-03-2015, 05:22   #168
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Re: How Does GPS Work?

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Originally Posted by Ericson38 View Post
Yes, that pseudolite is at the center of the earth (mathmatically ECEF X,Y,Z are are 0, no ephemeris required) and treated as having zero motion also, but with a range to the user = to earth radius (ellipsoid model). This is the arrangement for 2-D mode.
Thanks
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Old 30-03-2015, 05:48   #169
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Re: How Does GPS Work?

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I don't understand how that could work - it seems to me too few operands to solve your velocity - even if you know exact (not approximate) positions. A given Doppler shift could give various velocities, depending on relative motion. Unless you derive that from position differentiation? But you can't do that, either, with a single satellite.

What you say is beyond my feeble understanding of this - please explain.

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I posted this link a week or so ago.

Garmin | What is GPS?

I found it was ample information to understand GPS and how the satellite and GPS receiver establish a position on the globe. Particularly the section below copied courtesy of the GARMIN link.

"GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. (Doppler) The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more."


I thinks some of the posters are possibly planning to build a satellite/GPS Network of their own and need more information........
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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 30-03-2015, 06:27   #170
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Re: How Does GPS Work?

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Originally Posted by lordgeoff View Post
I posted this link a week or so ago.

Garmin | What is GPS?

I found it was ample information to understand GPS and how the satellite and GPS receiver establish a position on the globe. Particularly the section below copied courtesy of the GARMIN link.

"GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. (Doppler) The time difference tells the GPS receiver how far away the satellite is. Now, with distance measurements from a few more satellites, the receiver can determine the user's position and display it on the unit's electronic map.

A GPS receiver must be locked on to the signal of at least three satellites to calculate a 2D position (latitude and longitude) and track movement. With four or more satellites in view, the receiver can determine the user's 3D position (latitude, longitude and altitude). Once the user's position has been determined, the GPS unit can calculate other information, such as speed, bearing, track, trip distance, distance to destination, sunrise and sunset time and more."


I thinks some of the posters are possibly planning to build a satellite/GPS Network of their own and need more information........
You inserted the word "Doppler", where it does not belong. That part of the Garmin marketing blurb is talking only about the measurement of distance, and no Doppler shift is involved.

There is no reason for anyone to have a deep understanding of how GPS works, if they're not interested in it. All of us, including even actual rocket scientists, use lots of technology every day which we don't understand.

But if you don't understand it, and are not willing to learn about it, you shouldn't make bold statements about how it works!

And if you ARE interested in how it works, you need to go further than superficial marketing-oriented materials.
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Old 30-03-2015, 07:37   #171
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Re: How Does GPS Work?

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Originally Posted by lordgeoff View Post
I posted this link a week or so ago.

Garmin | What is GPS?

I found it was ample information to understand GPS and how the satellite and GPS receiver establish a position on the globe. Particularly the section below copied courtesy of the GARMIN link.

"GPS satellites circle the earth twice a day in a very precise orbit and transmit signal information to earth. GPS receivers take this information and use triangulation to calculate the user's exact location. Essentially, the GPS receiver compares the time a signal was transmitted by a satellite with the time it was received. (Doppler)
That is not Doppler. Doppler is not a time difference. It is a frequency difference. A frequency difference can be measured at a single moment in time whereas a time difference requires some time to elapse between events. They are two different concepts.
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Old 10-04-2015, 09:32   #172
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Re: How Does GPS Work?

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I do not observe on my system such jitter as you report.
Here is some.
Definitely no sign in the pictures of great Doppler accuracy.

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Originally Posted by Dockhead View Post
your speed log is simply not sensitive enough to detect 0.1 knot speed variations.
How can you be so sure?
Mine does detect 0.1 knot water flow variations and it accurately display zero knot in the present location.

In short as observed:
GPS log accuracy can be out by more than 10 nautical miles per 24 hours
GPS speed accuracy + 0.3 to – 0.2 knot
And I definitely use STW for fine tunning.
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Old 10-04-2015, 10:04   #173
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Re: How Does GPS Work?

Gps is subject to fractal errors, the slower you go the rougher the water and the higher your antenna the bigger the errors. When measuring distance travelled these errors accumulate. I am not trying to but gps down it is great but a few thing some sailors may want it is not so good.



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