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taxiya

Brigadier
Registered Member
AndrewS said:
Do the satellites switch off over North America, or is that just how they decided to orbit the satellites?
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They don't switch off. More of them are placed in orbits that passes Asia-Pacific than other regions. With the further launches from now to 2020, more MEOs will be placed to orbits that fly over Europe and North America. That is what the goal 2020 meant.

You can see what I am saying by looking at the bottom of the map, Antarctic has better coverage (same as China). Yellow means more Sats.
 
A

Anlsvrthng

Captain
Registered Member
taxiya said:
I don't know the tech details, but using references, 10cm was a design goal of Galileo, also the latest GPS (???). Since the most important factor is the clock, and BeiDou-3's clock is one of the best (if not the best), it is safe to say Beidou-3 is better than GPS or Galileo, at least no worse, I say no worse is only to cover my ass not really I believe so.

I can't comment on Millimeter level because I know nothing about the possible post-processing scheme. Brutally increase the clock is probably not working in today's tech level. Will be interested in reading about the post-processing.

Impact is straight forward as GPS to US. One thing I want to emphasize is the significance of Beidou to China's global ally building. one reason of China's weapon export being small and low tech is because most high-end weapons need GPS guidance, this effectively making Chinese weapon's useless. Now with Beidou, customers are given another choice, pretty much like Chinese investment in Africa gives African's another choice over western countries.
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The accuracy is not about the clock, it is more about frequencies.

The atmosphere can delay the signals, due to weather , so using multiple frequency can increase the accuracy .

I mean different frequencies has different atmospheric propagation paths, so comparing them can give a clue about the "real" receiver-satellite distance.

The post processing that many mention is nothing else just ground stations those transmit the local deviations .

You know the fixed station position, so it can compare the accuracy of the received signal to its position, and it will transmit this data to other receivers , and those can correct they positions as well.
 
T

taxiya

Brigadier
Registered Member
Anlsvrthng said:
The accuracy is not about the clock, it is more about frequencies.

The atmosphere can delay the signals, due to weather , so using multiple frequency can increase the accuracy .

I mean different frequencies has different atmospheric propagation paths, so comparing them can give a clue about the "real" receiver-satellite distance.

The post processing that many mention is nothing else just ground stations those transmit the local deviations .

You know the fixed station position, so it can compare the accuracy of the received signal to its position, and it will transmit this data to other receivers , and those can correct they positions as well.
Click to expand...
It is both. The clock determines the accuracy of intervals of the pulses. The intervals are markers of the time. Receivers measure the time difference between pulses, the measurement is then translated to distance differences of multiple satellites to the receiver which then know its location.

Frequencies are the carrier radio wave. The clock is also the generator of the carrier frequency. Without accurate clock, the carrier frequency will just variate randomly which is useless.

Everything is from the clock.
 
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Anlsvrthng

Captain
Registered Member
taxiya said:
It is both. The clock determines the accuracy of intervals of the pulses. The intervals are markers of the time. Receivers measure the time difference between pulses, the measurement is then translated to distance differences of multiple satellites to the receiver which then know its location.

Frequencies are the carrier radio wave. The clock is also the generator of the carrier frequency. Without accurate clock, the carrier frequency will just variate randomly which is useless.

Everything is from the clock.
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GPS can reach an accuracy of 10cm with ground stations.
Differential Global Positioning Systems (DGPS) are enhancements to the
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(GPS) which provide improved location accuracy, in the range of operations of each system, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations.
As I calculate the error of GPD due to clock is around 10 cm. Everything else is due to atmospheric /receiver/satellite position issues.
 
T

taxiya

Brigadier
Registered Member
Anlsvrthng said:
GPS can reach an accuracy of 10cm with ground stations.
Differential Global Positioning Systems (DGPS) are enhancements to the
Please, Log in or Register to view URLs content!
(GPS) which provide improved location accuracy, in the range of operations of each system, from the 15-meter nominal GPS accuracy to about 10 cm in case of the best implementations.
As I calculate the error of GPD due to clock is around 10 cm. Everything else is due to atmospheric /receiver/satellite position issues.
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What does that has anything to do with "clock is the key"?

Regarding DGPS:
You do know that any frequency carrier or pulse are generated by the clock? Clock determines the stability and accuracy of anything else? It is basic digital signal generator. No matter how you enhance by fancy algorithm, the clock is still where you stand on. You can't make out anything without the thing.

May I ask you what is your professional background? It is very strange to me that you actually argue with the DGPS thing against the critical/degerming factor of clock. If I assume by your calculation that you are educated in some engineering principle, I believe that you did not take courses of "Information theory" in signal processing subject?o_O One of the principle is that your system output accuracy (fidelity) is always lower than your input (the clock).

Regarding "everything else":
Your original statement was "clock vs. frequency"
The accuracy is not about the clock, it is more about frequencies.
Click to expand...
Now you talk about "clock vs. atmospheric /receiver/satellite position".
What do you really what to say? Frequency or something else? Are you trying to move the goal post? I have observed you doing this in other threads and subjects. And I am not going to let that happen here.
 
A

antiterror13

Brigadier
Anlsvrthng said:
The accuracy is not about the clock, it is more about frequencies.

The atmosphere can delay the signals, due to weather , so using multiple frequency can increase the accuracy .

I mean different frequencies has different atmospheric propagation paths, so comparing them can give a clue about the "real" receiver-satellite distance.

The post processing that many mention is nothing else just ground stations those transmit the local deviations .

You know the fixed station position, so it can compare the accuracy of the received signal to its position, and it will transmit this data to other receivers , and those can correct they positions as well.
Click to expand...

nope, you are WRONG. the GPS is all about the clock, how accurate the clock would determine how accurate the GPS would be. I am not saying clock is the only factor ... many many factors, but clock accuracy is the most important

Thats why China and others trying to develop more and more accurate atomic clock. At the moment, Hydrogen maser is the best atomic clock both Galileo and Beidou 3 have those clocks

Galileo sat has 2 hydrogen and 2 rubidium atomic clocks

Chinese Beidou 3 sats also have rubidium and hydrogen atomic clock
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Quickie

Quickie

Colonel
The theoretical max accuracy of the GPS depends entirely on the accuracy of the atomic clock. Post-processing of the GPS signals in the most optimum environmental conditions will allow the GPS to get as near as possible to this theoretical max accuracy but will not allow the GPS to pass the theoretical limit.
 
A

Anlsvrthng

Captain
Registered Member
taxiya said:
What does that has anything to do with "clock is the key"?

Regarding DGPS:
You do know that any frequency carrier or pulse are generated by the clock? Clock determines the stability and accuracy of anything else? It is basic digital signal generator. No matter how you enhance by fancy algorithm, the clock is still where you stand on. You can't make out anything without the thing.

May I ask you what is your professional background? It is very strange to me that you actually argue with the DGPS thing against the critical/degerming factor of clock. If I assume by your calculation that you are educated in some engineering principle, I believe that you did not take courses of "Information theory" in signal processing subject?o_O One of the principle is that your system output accuracy (fidelity) is always lower than your input (the clock).

Regarding "everything else":
Your original statement was "clock vs. frequency"

Now you talk about "clock vs. atmospheric /receiver/satellite position".
What do you really what to say? Frequency or something else? Are you trying to move the goal post? I have observed you doing this in other threads and subjects. And I am not going to let that happen here.
Click to expand...
It is basic geometry .

- definition of clock accuracy : If we start more than 1 clock at the same time, after T time the standard deviation of clock error will be sT.
ten times better clock by this definition means a clock that has the same deviation after 10T

-definition of GPS accuracy : going through the components there is three main contributor :
a. clock error
b. orbital error - it growing by time unpredictably due to perturbations.
c. atmospheric issues

So, considering that the first step to definite the GPS satellite orbits should be to collect the timing data from satellites and creating the orbits from this ( I presume) .
Then the correction of the errors is an ongoing process, practically the above 3 parameters gives a continuous error and a correction value, and the difference between the two gives the theoretical precision of GPS.

So, on face value the clock can improve the accuracy, because it will decrease one component of the final error but the same effect can be achieved by using laser retro-reflectors, and using them to separate the clock error from the orbital error . Increase of the frequency of independent check could decrease proportionally this error.

So, quick google gives that the GLONASS using laser retro-reflectors for independent distance measurement - the GPS doesn't use it ( I don't have time to check what they use now for error separation - two GPS sat used it during the 90s,).

So, the final accuracy depend on :
number of ground stations
time between independent position measurements.
clock accuracy
 
T

taxiya

Brigadier
Registered Member
Anlsvrthng said:
It is basic geometry .

- definition of clock accuracy : If we start more than 1 clock at the same time, after T time the standard deviation of clock error will be sT.
ten times better clock by this definition means a clock that has the same deviation after 10T

-definition of GPS accuracy : going through the components there is three main contributor :
a. clock error
b. orbital error - it growing by time unpredictably due to perturbations.
c. atmospheric issues

So, considering that the first step to definite the GPS satellite orbits should be to collect the timing data from satellites and creating the orbits from this ( I presume) .
Then the correction of the errors is an ongoing process, practically the above 3 parameters gives a continuous error and a correction value, and the difference between the two gives the theoretical precision of GPS.

So, on face value the clock can improve the accuracy, because it will decrease one component of the final error but the same effect can be achieved by using laser retro-reflectors, and using them to separate the clock error from the orbital error . Increase of the frequency of independent check could decrease proportionally this error.

So, quick google gives that the GLONASS using laser retro-reflectors for independent distance measurement - the GPS doesn't use it ( I don't have time to check what they use now for error separation - two GPS sat used it during the 90s,).

So, the final accuracy depend on :
number of ground stations
time between independent position measurements.
clock accuracy
Click to expand...
Whatever. Ignored @Anlsvrthng.
 
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