How GPS works

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Rayzor

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Re: How GPS works
« Reply #60 on: July 26, 2015, 08:57:08 PM »
But, I love you.

I'm sure he loves you too,  as we all do.   Inability to spell is a genetic fault in EE graduates,  so I take that as confirmation of your bona fides.     So you'll have no trouble verifying that the GPS satellites are in fact in orbit.

As to what that fact means for flat earth theory,   I leave that for others to ponder.

Stop gilding the pickle, you demisexual aromantic homoflexible snowflake.

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Son of Orospu

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Re: How GPS works
« Reply #61 on: July 26, 2015, 09:12:16 PM »
It is EET, not EE.    Also, Android sucks.  So fuck off.  Oh,  banned.  See you in a week.or two. 

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Son of Orospu

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Re: How GPS works
« Reply #62 on: July 26, 2015, 09:21:18 PM »
My handlers have not seen my rule breakage yet.  Maybe I won't get bumm÷d. 

Re: How GPS works
« Reply #63 on: July 27, 2015, 02:27:37 AM »
It is EET, not EE.    Also, Android sucks.  So fuck off.  Oh,  banned.  See you in a week.or two.
Man, you're turning into an embarrassment.

You're like a drunken uncle reeling around at a wedding, calling out people for fights and then falling face down on the dance floor.   I think it's time someone took you outside for some fresh air and a glass of water...
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Yendor

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Re: How GPS works
« Reply #64 on: July 27, 2015, 07:21:53 AM »
The thing that really bothers me about GPS is the poor RF specs. and the internet makes it seem very possible for a system to perform just fine this way. The system works on negative SNR, by the time the signal reaches a GPS receiver the signal is at around -130 to -150dBm. The signal is so low you can't even detect it with a network analyzer. The receiver has to be well designed and should be very expensive just to filter out the interference so it can actually make sense from the very weak signal. With any extra atmospheric interference entering into the system, it wouldn't work. It just seems it is a very marginally designed system. I can't believe any engineer would design a system like this and expect it to work just fine. But it does, that is what turns me off on the whole thing and why I think they are really doing something else and telling us lies. I just can't figure it all out yet, maybe I never will.
"During times of universal deceit, telling the truth becomes a revolutionary act."
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Re: How GPS works
« Reply #65 on: July 27, 2015, 08:43:05 AM »
The thing that really bothers me about GPS is the poor RF specs. and the internet makes it seem very possible for a system to perform just fine this way. The system works on negative SNR, by the time the signal reaches a GPS receiver the signal is at around -130 to -150dBm.

GPS uses spread-spectrum (SS) modulation, so the old rules of thumb regarding SNR don't apply. Correlation, at the receiver, of the pseudorandom (PR) sequence used to spread the bandwidth of the transmitted signal acts as a very powerful filter that can reliably pick the correlated signal from uncorrelated noise.

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The signal is so low you can't even detect it with a network analyzer.

Yep. One of the original purposes for SS was to be able to "hide" the presence of a signal. If it can't be detected, then it can't be intercepted and is harder to jam. The only way to tell the signal is there (and decode it) is to know the PR sequence used to encode it as well as where to look for it.

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The receiver has to be well designed and should be very expensive just to filter out the interference so it can actually make sense from the very weak signal.

The filtering (correlation) is done in a digital signal processor (DSP). Very high performance DSPs are commodities now. If it takes, say, $10 million to design and implement a GPS receiver system and you sell 10 million of them, that's $1 in development cost per receiver. Most modern GPS receivers that are based on chip sets that sell millions and millions of copies, and are themselves almost commodities.

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With any extra atmospheric interference entering into the system, it wouldn't work.

The link margins are good enough that it does work under everyday conditions, despite your uninformed opinion.

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It just seems it is a very marginally designed system.

If you say so.  ::)

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I can't believe any engineer would design a system like this and expect it to work just fine.

Your incredulity is irrelevant. Real RF engineers and DSP programmers don't have insurmountable problems making stuff like this work.

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But it does, that is what turns me off on the whole thing and why I think they are really doing something else and telling us lies. I just can't figure it all out yet, maybe I never will.

If they were lying, why not make up some "more believable" specs? GPS doesn't require you to understand it in order to work.
 
"Everyone is entitled to his own opinion, but not to his own facts." - Daniel Patrick Moynihan

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Yendor

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Re: How GPS works
« Reply #66 on: July 27, 2015, 02:08:29 PM »
Getting back to GPS poor specs. Again I believe if the military was going to develope a navigational system and spend billions on it I would think they would design a system that would be better and not just work on the fringe. GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency. Objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters. The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere. That to me means wide open spaces.

Obviously, mountains and clouds can not be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location. These bandages, if you will, are all ground based.

Assisted GPS (AGPS/A-GPS)

AGPS (Assisted Global Positioning System) is a system that assists conventional GPS when reception of the radio signal from the satellite is poor or non-existent (line of sight is blocked). the A-GPS gains information via a wireless network,on cell towers, to relay the satellite information to the receiver. With this assistance, the GPS doesn't have to calculate the satellite's orbit, which shortens initialization time, and increases battery life.

Differential GPS

To further increase reliability of GPS, DGPS (Differential Global Positioning System) technology was developed. Like the AGPS, the DGPS uses a fixed GPS location (such as a cell tower) to send information to the GPS receiver. DGPS, however, looks at both the satellite and the fixed location adjusts for any difference between the two, (I'm not sure why it needs to look at the satellite information) and then sends that information to the receiver. DGPS is particularly helpful when atmospheric conditions interfere with reception.

WAAS

There are a few things that cause GPS not to be perfectly accurate and interference prone. The charged particles of the ionosphere, and water vapor of the troposphere slow the signal slightly. Multipath, ephemeris errors, and the atomic clocks on the satellites themselves can also contribute to the inaccuracies.

Throughout the continental U.S. are 25 "reference stations." These are GPS receivers that are placed at points that have been very accurately surveyed. These reference stations receive the same GPS signals as the moving receivers. The difference is instead of using the signal's travel time to calculate position, the reference stations use their known position to calculate timing. Because the reference stations know exactly where they are, they can figure out what the travel time of the signals should be. They then compare the calculated times with the actual times. The difference is an "error correction" factor.

The error information is sent to two master stations (one on the U.S. east coast, one on the west coast) which calculate correction algorithms and assess the integrity of the system. A correction message is uplinked to the two WAAS satellites via a ground uplink system.

The two satellites are in geostationary orbits. These satellites then transmit the correction information back down to the GPS user on the GPS frequency. The GPS receiver then decodes this information and applies it to its calculated position to significantly improve the accuracy.

Did you catch that? It uses land based reference stations. Why bother with satellites at all and just use the north star?

I know GPS works out to sea. We all know that. But I also know both military and commercial planes can be equipped with GPS transmitters broadcasting information. Even ships at sea can do it too.

I know my friends like Rayzor and Alpha20mega are going to call me out for all this and explain why i'm so wrong, so be it. But I honestly believe GPS can be done and probably better without satellites involved.








"During times of universal deceit, telling the truth becomes a revolutionary act."
                              George Orwell

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Master_Evar

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Re: How GPS works
« Reply #67 on: July 27, 2015, 02:37:36 PM »
Getting back to GPS poor specs. Again I believe if the military was going to develope a navigational system and spend billions on it I would think they would design a system that would be better and not just work on the fringe. GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency. Objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters. The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere. That to me means wide open spaces.

Obviously, mountains and clouds can not be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location. These bandages, if you will, are all ground based.

Assisted GPS (AGPS/A-GPS)

AGPS (Assisted Global Positioning System) is a system that assists conventional GPS when reception of the radio signal from the satellite is poor or non-existent (line of sight is blocked). the A-GPS gains information via a wireless network,on cell towers, to relay the satellite information to the receiver. With this assistance, the GPS doesn't have to calculate the satellite's orbit, which shortens initialization time, and increases battery life.

Differential GPS

To further increase reliability of GPS, DGPS (Differential Global Positioning System) technology was developed. Like the AGPS, the DGPS uses a fixed GPS location (such as a cell tower) to send information to the GPS receiver. DGPS, however, looks at both the satellite and the fixed location adjusts for any difference between the two, (I'm not sure why it needs to look at the satellite information) and then sends that information to the receiver. DGPS is particularly helpful when atmospheric conditions interfere with reception.

WAAS

There are a few things that cause GPS not to be perfectly accurate and interference prone. The charged particles of the ionosphere, and water vapor of the troposphere slow the signal slightly. Multipath, ephemeris errors, and the atomic clocks on the satellites themselves can also contribute to the inaccuracies.

Throughout the continental U.S. are 25 "reference stations." These are GPS receivers that are placed at points that have been very accurately surveyed. These reference stations receive the same GPS signals as the moving receivers. The difference is instead of using the signal's travel time to calculate position, the reference stations use their known position to calculate timing. Because the reference stations know exactly where they are, they can figure out what the travel time of the signals should be. They then compare the calculated times with the actual times. The difference is an "error correction" factor.

The error information is sent to two master stations (one on the U.S. east coast, one on the west coast) which calculate correction algorithms and assess the integrity of the system. A correction message is uplinked to the two WAAS satellites via a ground uplink system.

The two satellites are in geostationary orbits. These satellites then transmit the correction information back down to the GPS user on the GPS frequency. The GPS receiver then decodes this information and applies it to its calculated position to significantly improve the accuracy.

Did you catch that? It uses land based reference stations. Why bother with satellites at all and just use the north star?

I know GPS works out to sea. We all know that. But I also know both military and commercial planes can be equipped with GPS transmitters broadcasting information. Even ships at sea can do it too.

I know my friends like Rayzor and Alpha20mega are going to call me out for all this and explain why i'm so wrong, so be it. But I honestly believe GPS can be done and probably better without satellites involved.

I think satellites are much better than groundstations, they are not obscured by trees an mountains an one satellite can cover a much larger area than any groundstations that exist. Bouncing signals off layers of the atmosphere only seems like doubling the amount of interference. And you miss the fact that most of these side-technologies still require an actual GPS satellite network, and won't work on the sea where there is no groundstations.
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Yendor

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Re: How GPS works
« Reply #68 on: July 27, 2015, 03:14:18 PM »
Getting back to GPS poor specs. Again I believe if the military was going to develope a navigational system and spend billions on it I would think they would design a system that would be better and not just work on the fringe. GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal. Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency. Objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters. The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere. That to me means wide open spaces.

Obviously, mountains and clouds can not be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location. These bandages, if you will, are all ground based.

Assisted GPS (AGPS/A-GPS)

AGPS (Assisted Global Positioning System) is a system that assists conventional GPS when reception of the radio signal from the satellite is poor or non-existent (line of sight is blocked). the A-GPS gains information via a wireless network,on cell towers, to relay the satellite information to the receiver. With this assistance, the GPS doesn't have to calculate the satellite's orbit, which shortens initialization time, and increases battery life.

Differential GPS

To further increase reliability of GPS, DGPS (Differential Global Positioning System) technology was developed. Like the AGPS, the DGPS uses a fixed GPS location (such as a cell tower) to send information to the GPS receiver. DGPS, however, looks at both the satellite and the fixed location adjusts for any difference between the two, (I'm not sure why it needs to look at the satellite information) and then sends that information to the receiver. DGPS is particularly helpful when atmospheric conditions interfere with reception.

WAAS

There are a few things that cause GPS not to be perfectly accurate and interference prone. The charged particles of the ionosphere, and water vapor of the troposphere slow the signal slightly. Multipath, ephemeris errors, and the atomic clocks on the satellites themselves can also contribute to the inaccuracies.

Throughout the continental U.S. are 25 "reference stations." These are GPS receivers that are placed at points that have been very accurately surveyed. These reference stations receive the same GPS signals as the moving receivers. The difference is instead of using the signal's travel time to calculate position, the reference stations use their known position to calculate timing. Because the reference stations know exactly where they are, they can figure out what the travel time of the signals should be. They then compare the calculated times with the actual times. The difference is an "error correction" factor.

The error information is sent to two master stations (one on the U.S. east coast, one on the west coast) which calculate correction algorithms and assess the integrity of the system. A correction message is uplinked to the two WAAS satellites via a ground uplink system.

The two satellites are in geostationary orbits. These satellites then transmit the correction information back down to the GPS user on the GPS frequency. The GPS receiver then decodes this information and applies it to its calculated position to significantly improve the accuracy.

Did you catch that? It uses land based reference stations. Why bother with satellites at all and just use the north star?

I know GPS works out to sea. We all know that. But I also know both military and commercial planes can be equipped with GPS transmitters broadcasting information. Even ships at sea can do it too.

I know my friends like Rayzor and Alpha20mega are going to call me out for all this and explain why i'm so wrong, so be it. But I honestly believe GPS can be done and probably better without satellites involved.

I think satellites are much better than groundstations, they are not obscured by trees an mountains an one satellite can cover a much larger area than any groundstations that exist. Bouncing signals off layers of the atmosphere only seems like doubling the amount of interference. And you miss the fact that most of these side-technologies still require an actual GPS satellite network, and won't work on the sea where there is no groundstations.


If that is what you believe, then tell me why DARPA to re-invent GPS navigation without the use of satellites
By Jamie Lendino on March 27, 2015 at 10:12 am35 Comments
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The Defense Advanced Research Projects Agency has unveiled its plan for the future, and it’s a huge shift, marked by new goals in four main areas. Arguably the biggest change is its plan to reinvent complex military systems and make them more modular, in an effort to ensure “superiority in the air, maritime, ground, space, and cyber domains.” Among the planned developments is a brand new global positioning, navigation, and timing system (GPS) that doesn’t depend on satellites and is resistant to jamming. According to a document (PDF) posted Thursday, DARPA’s new system will be much more advanced than what we have now, and will eventually trickle down to our cars and phones.

First, let’s step back for a moment and look at how current GPS systems work. Many ExtremeTech readers know this already, but the GPS navigation in our cars and phones depends on what was originally a military operation, just like the Internet itself. The U.S. Department of Defense first developed satellite-based GPS, and had an early system called TRANSIT up and running in 1960. By the 1980s, the military had gradually refined its technology to the point where it relied on multiple satellites orbiting the earth.

The system depends on Einstein’s theories of special and general relativity to work, (LIE). It figures out where you are based on signal time stamps from a number of satellites, as well as how far apart those are from each other. Thus enabled, the system triangulates your position on the ground. But the clocks on the satellites advance just a bit faster than ones on the Earth, and moving clocks are slightly slower than ones standing still. Relativity accounts for these differences, which amount to about 38 microseconds per day — a tiny amount, but just enough to misread your position by miles.

Incidentally, consumer technology could use GPS back then as well, but it was rare, being extremely expensive. GPS receivers, once giant backpack-sized behemoths, began to shrink over time as the technology improved. But the real reason you didn’t see much GPS in use in the 1980s and 1990s was because the military purposely scrambled the data civilians could see, rendering it pretty inaccurate, and kept the most accurate data for itself.

DARPA GPS soldier
A U.S. Army infantryman uses a GPS Receiver (DAGR) to document his position during a dismounted patrol in Afghanistan in 2010. (Credit: DARPA)
In the year 2000, president Clinton signed a bill ordering the military to stop scrambling the satellite data for civilians. This move instantly improved the accuracy of existing consumer systems by a factor of 10, and made room for wide civilian adoption of GPS navigation. Today’s network involves several dozen U.S.-based GPS satellites orbiting the earth. The newer Russian GLONASS system augments our own, and the European GNSS (Galileo) system in development soon will do so as well. The network ensures that at least 24 satellites are functioning at any one time, and that at least three are available for any device’s positioning request worldwide.

Ground-based systems may seem like a good alternative, but there are pitfalls. One design would involve hundreds of thousands of transmitters, which would be ridiculously difficult to build and maintain. Another possibility is differential GPS, which currently sees use in some maritime and nautical settings, but it’s also subject to jamming like satellite GPS. The other is LORAN, a radio-based system that originally saw use in World War II and has seen some renewed interest today thanks to its resistance to jamming, but its accuracy was never very good.

In the paper, DARPA hints at how it’s going to develop a positioning system without the use of satellites: “The need to be able to operate effectively in areas where GPS is inaccessible, unreliable or potentially denied by adversaries has created a demand for alternative precision timing and navigation capabilities. To address this need, DARPA is investing in radically new technologies that have the potential to deliver GPS-quality position, navigation and timing information for military systems, including novel inertial measurement devices that use cold-atom interferometry; chip-scale self-calibrating gyroscopes, accelerometers and clocks; and pulsed-laser-enabled atomic clocks and microwave sources.”


"During times of universal deceit, telling the truth becomes a revolutionary act."
                              George Orwell

Re: How GPS works
« Reply #69 on: July 27, 2015, 03:40:03 PM »
Getting back to GPS poor specs. Again I believe if the military was going to develope a navigational system and spend billions on it I would think they would design a system that would be better and not just work on the fringe.

The system as developed has proved to be quite reliable. It doesn't "just work on the fringe". Why do you think it does? Because you read some S/N spec and jumped to conclusions on a topic you apparently know little about?

Quote
GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal.

If you have fewer satellites (but not less than four), and the ones with good signals are in a limited area in the sky, the computed position will be less accurate. This is well understood. In most cases with as few as four satellites even with unfavorable geometry, your location is still "good enough" for most purposes. 

Quote
Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency.

Even assuming this is correct, and I'm not sure it is, so? 10-meter accuracy is still pretty darn good. Can you cite the source for this information?

Quote
Objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters.

Citation, please.

Quote
The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere.

No argument there.

Quote
That to me means wide open spaces.

To me, it means any location with a clear line of sight to at least four satellites at a time is usually adequate. 360° horizon-to horizon and a dozen satellites may help get somewhat better accuracy, but is not required for basic positioning.

Quote
Obviously, mountains and clouds can not be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location. These bandages, if you will, are all ground based.

WAAS is not ground based. It's satellite-assisted DGPS. See your own description later.

Quote
Assisted GPS (AGPS/A-GPS)

AGPS (Assisted Global Positioning System) is a system that assists conventional GPS when reception of the radio signal from the satellite is poor or non-existent (line of sight is blocked). the A-GPS gains information via a wireless network,on cell towers, to relay the satellite information to the receiver. With this assistance, the GPS doesn't have to calculate the satellite's orbit, which shortens initialization time, and increases battery life.

AGPS is typically used in cell phones to provide location information when a cellular signal is available and GPS is compromised. Apparently it is now mandated for phones sold in the USA to provide a redundant source for location information for emergency services. It does require a cell signal, but, since it uses a different RF wavelength and much stronger signals, it might work when conventional GPS doesn't, like inside buildings. The cost of providing this data is added to your phone rate, or may be considered data use.

Quote
Differential GPS

To further increase reliability of GPS, DGPS (Differential Global Positioning System) technology was developed.

DGPS is used to improve the accuracy and precision of GPS, not reliability.

Quote
Like the AGPS, the DGPS uses a fixed GPS location (such as a cell tower) to send information to the GPS receiver. DGPS, however, looks at both the satellite and the fixed location adjusts for any difference between the two, (I'm not sure why it needs to look at the satellite information) and then sends that information to the receiver. DGPS is particularly helpful when atmospheric conditions interfere with reception.

That's partly right. DGPS uses a receiver at a known location and calculates the error in the pseudorange (essentially, the distance from satellite to receiver) calculated for each GPS satellite it can see based on its known location. Corrections for these errors can be transmitted to other nearby DGPS-enabled receivers and applied to the pseudoranges at the DGPS receiver's location, or saved and applied to the collected data from a suitable receiver in post processing.

Because DGPS only provides a correction to the range to each satellite, the DGPS receiver still, of course, has to acquire the GPS data itself.

Quote
WAAS

There are a few things that cause GPS not to be perfectly accurate and interference prone. The charged particles of the ionosphere, and water vapor of the troposphere slow the signal slightly. Multipath, ephemeris errors, and the atomic clocks on the satellites themselves can also contribute to the inaccuracies.

Throughout the continental U.S. are 25 "reference stations." These are GPS receivers that are placed at points that have been very accurately surveyed. These reference stations receive the same GPS signals as the moving receivers. The difference is instead of using the signal's travel time to calculate position, the reference stations use their known position to calculate timing. Because the reference stations know exactly where they are, they can figure out what the travel time of the signals should be. They then compare the calculated times with the actual times. The difference is an "error correction" factor.

Yep. Basic DGPS so far...

Quote
The error information is sent to two master stations (one on the U.S. east coast, one on the west coast) which calculate correction algorithms and assess the integrity of the system. A correction message is uplinked to the two WAAS satellites via a ground uplink system.

The two satellites are in geostationary orbits. These satellites then transmit the correction information back down to the GPS user on the GPS frequency. The GPS receiver then decodes this information and applies it to its calculated position to significantly improve the accuracy.

So instead of relaying the DGPS corrections directly to nearby receivers, it relays the corrections to geostationary satellites that can provide them to WAAS-equipped receivers over a Wide Area (thus, Wide-Area Augmentation System).

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Did you catch that? It uses land based reference stations.

Sure. It's quite clever, actually. It uses land-based reference stations to make small corrections to the signals received by mobile receivers, improving their accuracy from a couple dozen meters to typically less than ten.

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Why bother with satellites at all and just use the north star?

Several reasons. Can you see the North Star in daylight or when it's cloudy? How about when it's screened by trees, buildings, or other obstructions even if the rest of the sky is clear? How can you tell your longitude from the North Star? How accurately can you tell your latitude from the North Star? How convenient would it be to gather the measurements?

Quote
I know GPS works out to sea. We all know that. But I also know both military and commercial planes can be equipped with GPS transmitters broadcasting information. Even ships at sea can do it too.

Maybe they could be, but are they? How are these ships and planes going to know their own locations accurately? Can you provide a reference that demonstrates that this has ever been done?

Quote
I know my friends like Rayzor and Alpha20mega are going to call me out for all this and explain why i'm so wrong, so be it.

We aim to please. We also don't like blatantly wrong information to stand unchallenged.

Quote
But I honestly believe GPS can be done and probably better without satellites involved.

Do you have anything other than idle speculation to back up that belief?

I see you posted again with something that may address this question after I started this response. I haven't read that post in detail, but did note this paragraph:

Ground-based systems may seem like a good alternative, but there are pitfalls. One design would involve hundreds of thousands of transmitters, which would be ridiculously difficult to build and maintain. Another possibility is differential GPS, which currently sees use in some maritime and nautical settings, but it’s also subject to jamming like satellite GPS. The other is LORAN, a radio-based system that originally saw use in World War II and has seen some renewed interest today thanks to its resistance to jamming, but its accuracy was never very good.

Other systems are certainly possible, but are they practical?
"Everyone is entitled to his own opinion, but not to his own facts." - Daniel Patrick Moynihan

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Yendor

  • 1676
Re: How GPS works
« Reply #70 on: July 27, 2015, 04:17:06 PM »
Getting back to GPS poor specs. Again I believe if the military was going to develope a navigational system and spend billions on it I would think they would design a system that would be better and not just work on the fringe.

The system as developed has proved to be quite reliable. It doesn't "just work on the fringe". Why do you think it does? Because you read some S/N spec and jumped to conclusions on a topic you apparently know little about?

Quote
GPS accuracy is affected by a number of factors, including satellite positions, noise in the radio signal, atmospheric conditions, and natural barriers to the signal.

If you have fewer satellites (but not less than four), and the ones with good signals are in a limited area in the sky, the computed position will be less accurate. This is well understood. In most cases with as few as four satellites even with unfavorable geometry, your location is still "good enough" for most purposes. 

Quote
Noise can create an error between 1 to 10 meters and results from static or interference from something near the receiver or something on the same frequency.

Even assuming this is correct, and I'm not sure it is, so? 10-meter accuracy is still pretty darn good. Can you cite the source for this information?

Quote
Objects such a mountains or buildings between the satellite and the receiver can also produce error, sometimes up to 30 meters.

Citation, please.

Quote
The most accurate determination of position occurs when the satellite and receiver have a clear view of each other and no other objects interfere.

No argument there.

Quote
That to me means wide open spaces.

To me, it means any location with a clear line of sight to at least four satellites at a time is usually adequate. 360° horizon-to horizon and a dozen satellites may help get somewhat better accuracy, but is not required for basic positioning.

Quote
Obviously, mountains and clouds can not be controlled or moved, nor can interference and blockage from buildings always be prevented. These factors then, will affect GPS accuracy. To overcome or get around these factors, other technology, AGPS, DGPS, and WAAS, has been developed to aid in determining an accurate location. These bandages, if you will, are all ground based.

WAAS is not ground based. It's satellite-assisted DGPS. See your own description later.

Quote
Assisted GPS (AGPS/A-GPS)

AGPS (Assisted Global Positioning System) is a system that assists conventional GPS when reception of the radio signal from the satellite is poor or non-existent (line of sight is blocked). the A-GPS gains information via a wireless network,on cell towers, to relay the satellite information to the receiver. With this assistance, the GPS doesn't have to calculate the satellite's orbit, which shortens initialization time, and increases battery life.

AGPS is typically used in cell phones to provide location information when a cellular signal is available and GPS is compromised. Apparently it is now mandated for phones sold in the USA to provide a redundant source for location information for emergency services. It does require a cell signal, but, since it uses a different RF wavelength and much stronger signals, it might work when conventional GPS doesn't, like inside buildings. The cost of providing this data is added to your phone rate, or may be considered data use.

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Differential GPS

To further increase reliability of GPS, DGPS (Differential Global Positioning System) technology was developed.

DGPS is used to improve the accuracy and precision of GPS, not reliability.

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Like the AGPS, the DGPS uses a fixed GPS location (such as a cell tower) to send information to the GPS receiver. DGPS, however, looks at both the satellite and the fixed location adjusts for any difference between the two, (I'm not sure why it needs to look at the satellite information) and then sends that information to the receiver. DGPS is particularly helpful when atmospheric conditions interfere with reception.

That's partly right. DGPS uses a receiver at a known location and calculates the error in the pseudorange (essentially, the distance from satellite to receiver) calculated for each GPS satellite it can see based on its known location. Corrections for these errors can be transmitted to other nearby DGPS-enabled receivers and applied to the pseudoranges at the DGPS receiver's location, or saved and applied to the collected data from a suitable receiver in post processing.

Because DGPS only provides a correction to the range to each satellite, the DGPS receiver still, of course, has to acquire the GPS data itself.

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WAAS

There are a few things that cause GPS not to be perfectly accurate and interference prone. The charged particles of the ionosphere, and water vapor of the troposphere slow the signal slightly. Multipath, ephemeris errors, and the atomic clocks on the satellites themselves can also contribute to the inaccuracies.

Throughout the continental U.S. are 25 "reference stations." These are GPS receivers that are placed at points that have been very accurately surveyed. These reference stations receive the same GPS signals as the moving receivers. The difference is instead of using the signal's travel time to calculate position, the reference stations use their known position to calculate timing. Because the reference stations know exactly where they are, they can figure out what the travel time of the signals should be. They then compare the calculated times with the actual times. The difference is an "error correction" factor.

Yep. Basic DGPS so far...

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The error information is sent to two master stations (one on the U.S. east coast, one on the west coast) which calculate correction algorithms and assess the integrity of the system. A correction message is uplinked to the two WAAS satellites via a ground uplink system.

The two satellites are in geostationary orbits. These satellites then transmit the correction information back down to the GPS user on the GPS frequency. The GPS receiver then decodes this information and applies it to its calculated position to significantly improve the accuracy.

So instead of relaying the DGPS corrections directly to nearby receivers, it relays the corrections to geostationary satellites that can provide them to WAAS-equipped receivers over a Wide Area (thus, Wide-Area Augmentation System).

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Did you catch that? It uses land based reference stations.

Sure. It's quite clever, actually. It uses land-based reference stations to make small corrections to the signals received by mobile receivers, improving their accuracy from a couple dozen meters to typically less than ten.

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Why bother with satellites at all and just use the north star?

Several reasons. Can you see the North Star in daylight or when it's cloudy? How about when it's screened by trees, buildings, or other obstructions even if the rest of the sky is clear? How can you tell your longitude from the North Star? How accurately can you tell your latitude from the North Star? How convenient would it be to gather the measurements?

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I know GPS works out to sea. We all know that. But I also know both military and commercial planes can be equipped with GPS transmitters broadcasting information. Even ships at sea can do it too.

Maybe they could be, but are they? How are these ships and planes going to know their own locations accurately? Can you provide a reference that demonstrates that this has ever been done?

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I know my friends like Rayzor and Alpha20mega are going to call me out for all this and explain why i'm so wrong, so be it.

We aim to please. We also don't like blatantly wrong information to stand unchallenged.

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But I honestly believe GPS can be done and probably better without satellites involved.

Do you have anything other than idle speculation to back up that belief?

I see you posted again with something that may address this question after I started this response. I haven't read that post in detail, but did note this paragraph:

Ground-based systems may seem like a good alternative, but there are pitfalls. One design would involve hundreds of thousands of transmitters, which would be ridiculously difficult to build and maintain. Another possibility is differential GPS, which currently sees use in some maritime and nautical settings, but it’s also subject to jamming like satellite GPS. The other is LORAN, a radio-based system that originally saw use in World War II and has seen some renewed interest today thanks to its resistance to jamming, but its accuracy was never very good.

Other systems are certainly possible, but are they practical?


Below is the link i got most information from.

http://www.maps-gps-info.com/gps-accuracy.html


"During times of universal deceit, telling the truth becomes a revolutionary act."
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Rayzor

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Re: How GPS works
« Reply #71 on: July 27, 2015, 05:40:53 PM »
Ok,  I'll try once more,  see if it sinks in this time. 

GPS transmitters can be ANYWHERE,  they can be up a tree in your backyard,  they can be on top of a nearby hilltop.   They can be in high flying aeroplanes,   you can put the transmitters anywhere you like,  the system will still work just fine,  you DON'T HAVE TO HAVE THE TRANSMITTERS ON ORBITING SATELLITES.   

Have I made myself clear,  because I'm getting sick of repeating it over and over. 

What you ABSOLUTELY MUST HAVE, NO EXCEPTIONS,  NO VARIATIONS, NO OTHER POSSIBILITES.   is the REAL ACTUAL  location of the transmitter, and the time code sequence being sent.   That tells the receiver where the transmitter is and how far away from that location the receiver is.   With that data from multiple transmitters the receiver can calculate it's own position. 

That's how GPS works.     ( read the thread title )

So,  where are the transmissions coming from on my little GPS receiver, that I can plug a serial port into.    The signals I am receiving are all over 10,000 miles away and scattered all over the sky, some directly overhead and some close to the horizon,  all with reasonable SNR.

If you want to check for yourself you need a GPS receiver and a serial connection,  to see the NMEA sequences,  look for the $GPSRV sequence, that tells you the azimuth and elevation of all the satellites the receiver can see as well as the SNR. 

Getting the pseudo range data requires a GPS receiver that will allow access to the raw binary data stream,  and a bit of software. 

34357446.85408 104694102.10708 25567381.585 9 25567371.841 9 25567379.659 7  76.000 84.000

The highlighted numbers in the above stream are the distance from the receiver to the transmitter in meters,   so that transmitter is 25,567.381585 km away or 15,979 miles away from the receiver.   

Now do you understand?

 
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Re: How GPS works
« Reply #72 on: July 27, 2015, 08:51:08 PM »
Below is the link i got most information from.

http://www.maps-gps-info.com/gps-accuracy.html
Thanks, I see where you got those numbers, but they don't provide any citations for their source nor background describing how they were determined.

Here's the type of analysis[nb]With further references.[/nb] that I'm more familiar with: https://en.wikipedia.org/wiki/Error_analysis_for_the_Global_Positioning_System. They don't have an entry in their error table saying anything like "noise = 10m" or "blocked by building or mountain = 30m", and I don't see how this would happen; if the signal from a given satellite is too noisy, it will simply be ignored, which reduces the number of satellites used for the fix, which can (but doesn't necessarily) degrade the quality of the location some.
 
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Yendor

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Re: How GPS works
« Reply #73 on: July 28, 2015, 09:54:26 AM »
Ok,  I'll try once more,  see if it sinks in this time. 

GPS transmitters can be ANYWHERE,  they can be up a tree in your backyard,  they can be on top of a nearby hilltop.   They can be in high flying aeroplanes,   you can put the transmitters anywhere you like,  the system will still work just fine,  you DON'T HAVE TO HAVE THE TRANSMITTERS ON ORBITING SATELLITES.   

Have I made myself clear,  because I'm getting sick of repeating it over and over. 

What you ABSOLUTELY MUST HAVE, NO EXCEPTIONS,  NO VARIATIONS, NO OTHER POSSIBILITES.   is the REAL ACTUAL  location of the transmitter, and the time code sequence being sent.   That tells the receiver where the transmitter is and how far away from that location the receiver is.   With that data from multiple transmitters the receiver can calculate it's own position. 

That's how GPS works.     ( read the thread title )

So,  where are the transmissions coming from on my little GPS receiver, that I can plug a serial port into.    The signals I am receiving are all over 10,000 miles away and scattered all over the sky, some directly overhead and some close to the horizon,  all with reasonable SNR.

If you want to check for yourself you need a GPS receiver and a serial connection,  to see the NMEA sequences,  look for the $GPSRV sequence, that tells you the azimuth and elevation of all the satellites the receiver can see as well as the SNR. 

Getting the pseudo range data requires a GPS receiver that will allow access to the raw binary data stream,  and a bit of software. 

34357446.85408 104694102.10708 25567381.585 9 25567371.841 9 25567379.659 7  76.000 84.000

The highlighted numbers in the above stream are the distance from the receiver to the transmitter in meters,   so that transmitter is 25,567.381585 km away or 15,979 miles away from the receiver.   

Now do you understand?

 

I'm sorry Rayzor, but I for one have never heard you say, "GPS transmitters can be ANYWHERE,  they can be up a tree in your backyard,  they can be on top of a nearby hilltop.   They can be in high flying aeroplanes,   you can put the transmitters anywhere you like,  the system will still work just fine,  you DON'T HAVE TO HAVE THE TRANSMITTERS ON ORBITING SATELLITES". I'm not disputing you, just I've never heard you say it.

The NMEA data is information the processor shows the user after the RF section. The NMEA data can be anything  the programmer wants you to know. One thing i'm curious about, maybe you know, I can't find it anywhere. If you have numerous transmitters at different locations all transmitting on the same frequency, how does the receiver discriminate one transmitter from the other. The receiver front end is still RF and the digital information is extracted later on. One  way I can think of is if the receiver is going by signal strength. If that is the case, then the receiver can figure out the direction the strongest signal is coming from and ignore the other weaker signals. This strong signal then can send it's location, altitude and time. Rayzor, do you have any information about this? Thanks.
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Re: How GPS works
« Reply #74 on: July 28, 2015, 10:58:17 AM »
One thing i'm curious about, maybe you know, I can't find it anywhere. If you have numerous transmitters at different locations all transmitting on the same frequency, how does the receiver discriminate one transmitter from the other. The receiver front end is still RF and the digital information is extracted later on.

Great question!

The discrimination is achieved by the correlation used to "de-spread" the received spread-spectrum (SS) signal. Each satellite uses a different pseudorandom (PR) sequence when creating the SS signal transmitted, and the correlator finds only the signal that has been spread by the particular PR sequence it's searching for.

There is a lot of information available on line about SS using PR or "pseudonoise" encoding.

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One  way I can think of is if the receiver is going by signal strength. If that is the case, then the receiver can figure out the direction the strongest signal is coming from and ignore the other weaker signals. This strong signal then can send it's location, altitude and time.

This is not correct. The receiver has no knowledge of the direction the signal came from until after its own location has been determined. Even if it did know the direction to the satellite with the strongest signal, it still doesn't have enough information to determine its own location.

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Rayzor, do you have any information about this? Thanks.

Not Rayzor, but I hope this helps.
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Yendor

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Re: How GPS works
« Reply #75 on: July 28, 2015, 12:16:03 PM »
One thing i'm curious about, maybe you know, I can't find it anywhere. If you have numerous transmitters at different locations all transmitting on the same frequency, how does the receiver discriminate one transmitter from the other. The receiver front end is still RF and the digital information is extracted later on.

Great question!

The discrimination is achieved by the correlation used to "de-spread" the received spread-spectrum (SS) signal. Each satellite uses a different pseudorandom (PR) sequence when creating the SS signal transmitted, and the correlator finds only the signal that has been spread by the particular PR sequence it's searching for.

There is a lot of information available on line about SS using PR or "pseudonoise" encoding.

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One  way I can think of is if the receiver is going by signal strength. If that is the case, then the receiver can figure out the direction the strongest signal is coming from and ignore the other weaker signals. This strong signal then can send it's location, altitude and time.

This is not correct. The receiver has no knowledge of the direction the signal came from until after its own location has been determined. Even if it did know the direction to the satellite with the strongest signal, it still doesn't have enough information to determine its own location.

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Rayzor, do you have any information about this? Thanks.

Not Rayzor, but I hope this helps.

Thank you Alpha20mega. I'm familiar with spread spectrum, in fact i've designed frequency hopping switched filter banks to go along with SS receivers. I can see the discrimination after the signal is demodulated I just don't see how it can be demodulated until the RF source is  established. To me it would be the same as any two sources transmitting a modulated signal at the same time and on the same frequency. The Rx could not distinguish between the two unless one had enough power to over ride the other. Then the Rx can demodulate the signal. Doesn't cell phones work this way?
"During times of universal deceit, telling the truth becomes a revolutionary act."
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Rayzor

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Re: How GPS works
« Reply #76 on: July 28, 2015, 06:04:34 PM »
Thank you Alpha20mega. I'm familiar with spread spectrum, in fact i've designed frequency hopping switched filter banks to go along with SS receivers. I can see the discrimination after the signal is demodulated I just don't see how it can be demodulated until the RF source is  established. To me it would be the same as any two sources transmitting a modulated signal at the same time and on the same frequency. The Rx could not distinguish between the two unless one had enough power to over ride the other. Then the Rx can demodulate the signal. Doesn't cell phones work this way?

That's exactly right,   but they are never on the same frequency at the same time,  that's how spread spectrum works.    Different pseudo random sequences.   Each satellite has a unique PRN number  ( pseudo random number) that identifies which satellite it is.    If I look at the  constellation display for gpsd,   it looks like this,  ( this is not my location )



On the left is the satellite number PRN number,  then elevation,  azimuth,  signal to noise ratio,  and whether the satellite is used in the position calculation.

If you don't want to use the NMEA data,  ( or if you think it's faked )  then you need a gps receiver that gives access to the raw binary data,  some people do that to get cm level accuracy using RTK
http://navspark.mybigcommerce.com/ns-raw-carrier-phase-raw-measurement-output-gps-receiver/
« Last Edit: July 28, 2015, 06:07:06 PM by Rayzor »
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Yendor

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Re: How GPS works
« Reply #77 on: July 29, 2015, 08:51:09 AM »
Thank you Alpha20mega and Rayzor, you both have been very helpful.
"During times of universal deceit, telling the truth becomes a revolutionary act."
                              George Orwell

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Pezevenk

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Re: How GPS works
« Reply #78 on: July 31, 2015, 01:17:58 PM »
I have a BS in Elecronics Engendering Technology, much of which had to do whith communications.  I also have an AAS in Industrial Electricity/Electronics, which had a lesser degee of communications, but it was still there.  I was also trained as an RTO and FRO the USMC.  I think I know a thing or two about communications. 

Ok, your turn, expert communicator.

If by "BS" you mean bullshit, then ok. Although I have no idea what giving a gender to electronics has to do with anything.

Oh wait. I thought universities brain washed you and everything they taught you is a lie.

By the way, there is no way to make GPS data bounce off of the ionosphere. It's a known myth.
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Pezevenk

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Re: How GPS works
« Reply #79 on: July 31, 2015, 01:29:51 PM »
Thank you Alpha20mega and Rayzor, you both have been very helpful.

Yendor, you seem like a nice guy who simply wants to learn. So, I have to ask you, why do you think Einstein's special and general relativity are lies? Because if you want to, I can derive at least special relativity for you. First of all, do you believe that the speed of light is finite and the same for every observer, moving or not?
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markjo

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Re: How GPS works
« Reply #80 on: July 31, 2015, 03:34:56 PM »
Yeah, like it is simply impossible for a ground based transmitter to send you the same signal as a flying trashcan transmitter.   ::)
That would be correct.  Ground based transmitters do not exhibit Doppler shift like satellite based transmitters.
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Pezevenk

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Re: How GPS works
« Reply #81 on: August 01, 2015, 05:39:06 AM »
Yeah, like it is simply impossible for a ground based transmitter to send you the same signal as a flying trashcan transmitter.   ::)
That would be correct.  Ground based transmitters do not exhibit Doppler shift like satellite based transmitters.

If I could like this comment, I would.
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Art

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Re: How GPS works
« Reply #82 on: August 03, 2015, 10:42:31 PM »
Yeah it is kinda impossible at GHz frequencies to transmit across the Earth.
Zero signal propagation at those frequencies means many more very powerful transmitters than phone towers.. everywhere on Earth including complete coverage of the World’s oceans.
where from Space, the signal is only impeded by our atmosphere.
Also from Earth, signal strength would be the giveaway, increasing or decreasing greatly with distance from the transmitter.

But most convincingly, just one flying trashcan has a massive footprint compared to anything we could put on Earth
operating at the same frequency. If the same footprint could be achieved, it would certainly kill anyone close to it.
« Last Edit: August 03, 2015, 10:46:00 PM by Art »
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