Galileo: Europe's Version of GPS Reaches Key Phase 328
another random user sends this quote from the BBC:
"The third and fourth spacecraft in Europe's satellite navigation system have gone into orbit. The pair were launched on a Russian Soyuz rocket from French Guiana. It is an important milestone for the multi-billion-euro project to create a European version of the U.S. Global Positioning System. With four satellites now in orbit — the first and second spacecraft were launched in 2011 — it becomes possible to test Galileo end-to-end. That is because a minimum of four satellites are required in the sky for a smartphone or vehicle to use their signals to calculate a positional fix."
Good to hear (Score:5, Interesting)
There has been far, far too many delays and political fuckery with this. I'm glad to hear it is finally going online.
Satellite navigation is just very important to everything these days (it is the primary nav method for all planes, ships, etc). Having everything rely on GPS, and thus on the budget the US chooses to spend keeping it working, is not a good idea.
This will make things much more reliable since, after an initial hissing match, the US and EU settled down and the systems play nice together and you'll be able to get devices that use both for better accuracy and reliability.
Re:Good to hear (Score:4, Insightful)
Oh, the budget for GPS will pretty much never be cut until the system becomes obsoleted by something newer. The US military relies on GPS. However, the more navigation systems we have, the faster and more reliable fixes can become for civilian use.
Re:Good to hear (Score:5, Informative)
Oh, the budget for GPS will pretty much never be cut until the system becomes obsoleted by something newer. The US military relies on GPS. However, the more navigation systems we have, the faster and more reliable fixes can become for civilian use.
ISTR that due to budget cuts the newer GPS satellites don't operate in polar orbits, giving poor coverage at the poles.
Re:Good to hear (Score:5, Funny)
ISTR that due to budget cuts the newer GPS satellites don't operate in polar orbits, giving poor coverage at the poles.
Poor coverage at the poles is not a problem: If I can see the sky but no GPS satellite, I just have to figure out if there are polar bears or penguins around to know at which pole I am.
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It is simpler than that look for actual land.
The north pole has none, the south pole you can't get to by water.
Re:Good to hear (Score:4, Informative)
You recall incorrectly - the GPS constellation has never had any birds in polar orbit, and has always provided poor coverage at very high latitudes.
Re:Good to hear (Score:5, Interesting)
You recall incorrectly - the GPS constellation has never had any birds in polar orbit, and has always provided poor coverage at very high latitudes.
On a more detailed note, GPS satellites are in inclined orbits at 55 degrees. This means that a receiver at the pole would only ever see a satellite reach a maximum altitude of 55 degrees over the horizon.
While is certainly isn't as great as at lower latitudes, it's more than adequate to provide location information -- it's not like the poles have huge buildings and whatnot that would obstruct the view. I wouldn't really consider that to be "poor" coverage, but your mileage may vary.
The Russian GLONASS system has satellites in inclined orbits at 64.8 degrees as Russia is located at higher latitudes than the continental US. This can get proportionally better coverage at higher latitudes.
Galileo is planned with a 56 degree inclination.
Re:Good to hear (Score:5, Interesting)
In agriculture GPS guidance systems already have the capability of talking to Galileo when it is finished, and Glonass right now. After the military, agriculture is probably the most dependent on positioning technology these days. If GPS guidance goes down (IE our hardware has a problem), we simply cannot drive the machines. They are too wide to drive manually (my sprayer is 120 feet wide-- very difficult to drive that manually at less than 5 feet overlap even with markers) and the inputs too expensive to waste on overlaps. If GPS fails, everyone can switch to Glonass with Glonass correction signals, which should keep us going, but Galileo would offer superior accuracy and also precision. Such a switch, however, is not instantaneous. Would take weeks or months to get the firmwares updated (though the radios already are capable). And if that failed, I guess we can do terrestial positioning signals.
But it's not a matter of if GPS will fail. It's a matter of when. Maybe the US will be able to replace satellites when they die, but if not, it should be very interesting to see what happens.
Satellites are being replaced (Score:3)
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In agriculture GPS guidance systems already have the capability of talking to Galileo when it is finished, and Glonass right now. After the military, agriculture is probably the most dependent on positioning technology these days. If GPS guidance goes down (IE our hardware has a problem), we simply cannot drive the machines. They are too wide to drive manually (my sprayer is 120 feet wide-- very difficult to drive that manually at less than 5 feet overlap even with markers) and the inputs too expensive to waste on overlaps. If GPS fails, everyone can switch to Glonass with Glonass correction signals, which should keep us going, but Galileo would offer superior accuracy and also precision. Such a switch, however, is not instantaneous. Would take weeks or months to get the firmwares updated (though the radios already are capable). And if that failed, I guess we can do terrestial positioning signals.
But it's not a matter of if GPS will fail. It's a matter of when. Maybe the US will be able to replace satellites when they die, but if not, it should be very interesting to see what happens.
If 5' of overlap in unacceptable, then Glonass will not help. The military signal provides 10 meter resolution, and the civilian signal provides one quarter of that (20 meter resolution - exercise to the reader why that is one-quarter resolution). In fact, I'm surprised that you can get consistent 5 foot accuracy with GPS. You might have more overlap than you think.
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It's pretty common to use a localization signal in agricultural radio navigation to provide much greater accuracy than is available over the air.
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Satellite positioning doesn't work that way. The absolute error might be 10m, but it will be fairly consistent over tens of square kilometers and several hours. For reducing overlap it's fine.
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They even drove 10 miles through the snow! Uphill! Both ways!
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You seem to have drifted into a different discussion. He was listing out as fact that many machines require GPS. You may think that's stupid, and you may correctly point out that different machines could be made to accomplish the same thing without GPS, but you aren't arguing against his point at all that it is required.
I don't actually know how true this is so I'm not taking his side per se -- I grew up in a rural area but not on a farm and haven't lived there in almost a decade, but honestly this is th
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...Having everything rely on GPS, and thus on the budget the US chooses to spend keeping it working, is not a good idea.
Everything does not rely on GPS... the iPhone, for example has had support for GPS as well GLONASS since the iPhone 4S (http://www.apple.com/iphone/specs.html). The iPhone seamlessly switches between the two satellite constellations (as well as Wi-Fi and GSM triangulation) so the consumer never really knows which system is being used to provide location services). So clearly the world is not solely dependent on the budget the US chooses to spend to keep GPS working; it is dependent upon the combined budgets
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Right... Because the US almost exclusively using GPS-guided munitions, being in the process of converting the entirety of civil aviation over to GPS, and more, is a clear indication they're going to just let those GPS satellites fall out of the sky ANY TIME NOW.
If other countries want to waste their money, be my guest, but let's be honest here... Other countries are putting up their own GPS
Re:Good to hear (Score:5, Funny)
Its simply not reliable enough to use when peoples lives are at stack.
Just allocate people on the heap, and we're all safe?
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Just allocate people on the heap, and we're all safe?
But then you have to make sure to eliminate them yourself, and you must have a reference counter to prevent eliminating people anyone still needs. Just look at the over-population we are facing due to unused people running around, still allocating resources. Maybe we should move all suspects to 72930S 1100016E for some time to make sure...
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push human
push human
pop human
cooperation between systems? (Score:2)
So now there's GPS, GLONASS, and Galileo. Is there going to be cooperation between the different sets of satellites, or will a given device only talk to its own set of satellites? It sucks, for example, when I'm hiking and can't get a GPS fix because I'm in a canyon with a view of only part of the sky. Ditto when all the visible satellites are near the horizon, so the vertical position's accuracy goes to hell, like a couple of weeks ago when I was at 7000' and it told me I was at 14000'. If we had a large n
Re:cooperation between systems? (Score:4, Informative)
Yes [novatel.com]
As we add satellites, even from different systems, the accuracy will get better. It is very cool.
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a minimum of four satellites are required (Score:5, Informative)
because a minimum of four satellites are required in the sky for a smartphone or vehicle to use their signals to calculate a positional fix.
Lets be more accurate here. A minimum of 4 satellites are required to be in the sky that can be observed at the same time from the same point on earth. Hopefully these satellites are relatively close together, because otherwise they might never all be visible at the same time. And if they are, since they are in low earth orbit they will pass by relatively quickly and only be briefly useable during each orbit. So, if the orbits are close this may allow a little bit of testing, but the "system" is still too satellite poor to be of any real use for navigation (at least unless you combine the signals with info from other U.S. or Russian satellites).
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because a minimum of four satellites are required in the sky for a smartphone or vehicle to use their signals to calculate a positional fix.
Lets be more accurate here. A minimum of 4 satellites are required to be in the sky that can be observed at the same time from the same point on earth. Hopefully these satellites are relatively close together, because otherwise they might never all be visible at the same time. And if they are, since they are in low earth orbit they will pass by relatively quickly and only be briefly useable during each orbit. So, if the orbits are close this may allow a little bit of testing, but the "system" is still too satellite poor to be of any real use for navigation (at least unless you combine the signals with info from other U.S. or Russian satellites).
If you'd quoted the previous sentence: it becomes possible to test Galileo end-to-end, it'd be clear that they meant that it is possible to test the system, no one said it's a usable navigation system.
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Wikipedia seems to indicate that the Galileo orbits are medium-earth orbits
Not to be confused with Middle Earth Orbits, which are where we place the spy satellites used to monitor Sauron.
Re:a minimum of four satellites are required (Score:4, Funny)
Wikipedia seems to indicate that the Galileo orbits are medium-earth orbits
Not to be confused with Middle Earth Orbits, which are where we place the spy satellites used to monitor Sauron.
I guess you mistake the for Middle Earth Orbs used for the same purposes.
The ones made by the Elves of Valinor in the Uttermost West, and marketed under the Palantir [wikipedia.org] brand, were considered the best... until it was discovered that not only they tracked the location of its users but also subtly altered the information, in a way targeted for each particular user (very much like the targeted ads today):
1. Saruman looked through the Orthanc stone, and saw what he thought was an unassailable strength in Mordor, helping to corrupt him
2. Sauron, looking the other way with voyeuristic intent thought, thought Pippin had the One Ring
3. when Denethor used the stone, it convinced him there was no hope for Minas Tirith, driving him to suicide
Needless to say, once all of the above has been exposed, their customers lost confidence, so the production of new devices was interrupted.
It took quite a long time until the brand was resurrected and today it seems to enjoy commercial viability again.
A word of caution for the would-be consumers of their services: rumours have it that the new management team can not resist co-contracting alongside trolls [wikipedia.org] if they were given an opportunity.
Do you really need 4-5? (Score:2)
I wonder, since you can pretty much figure out what city you are in through ordinary radio and wifi beacons, not to mention the help you could get from having a clock and a sun locator, couldn't you really use GPS on the road with just two or three satellites?
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All the equipment you listed is either more expensive, less precise, or not global; and triangulation in 3d space does not really work that way. Using just 2 satellites, you would get a circular area of possible positions, of which 2 would intercept the surface of the Earth and most likely the other would be far from Europe, but you lose altitude measurement and consumer products using the technology will not be reliable outside of Europe. An argument can be made that 3 is sufficient and most GPS devices wo
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You also need to solve for time. Quartz oscillators aren't precise enough for a fix to within even kilometers, let alone meters.
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You also need to solve for time. Quartz oscillators aren't precise enough for a fix to within even kilometers, let alone meters.
I hadn't considered that we could use the 4th signal this way... that's really quite brilliant.
Survey quality GNSS systems (Score:2)
Re:...Why? (Score:5, Informative)
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Selective availability can be turned on by geographic region, in order to degrade the civilian signal. This allows you to only degrade the middle-east, while still maintaining precision position determination for the rest of the world. If the US wants to deny position determination, they would just jam Galileo. I supposed the EU could always send the US a sternly worded letter if that were to occur.
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Yeah, except that jamming Galileo would provoke a vastly different response from the EU than the US turning some switches off on their own positioning system. You know, this isn't a computer game where the only thing that counts is what you can *technically* do.
Sternly worded letter? Fights happen in economy today, not on the battlefield. US citizens like to think they're safe just because no country is stupid and suicidal enough to drop bombs on them.
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Re:...Why? (Score:4, Informative)
Why is Europe spending billions to create their own GPS constellation when the US government already went through the hassle and expense? The GPS system is free and open to use by anyone with a GPS receiver. This strikes me as nothing but a political move, as if to say "We're independent and don't need America to provide anything for us". This is a completely redundant and pointless project by the EU.
Sigh,
It is a measure of trust. No one, trusts that the US will not screw with GPS if it would give them a military or economic advantage. Sure they say right now that they won't, but who knows what will happen in 5, 10 or 15 years in the future. And trust me, the value of an accurate navigation/timing system makes it well worth the efforts the Europeans, the Russians and the Chinese are making to field their own versions.
Why can't we all be friends?
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"Why can't we all be friends?"
Because some of those "friends" will eat your lunch.
Don't be paranoid... but don't be a fool, either.
--
Peace through superior firepower.
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I bet the US is already working on finding a way to scramble Galileo and other GPS satellite signals.
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Amazing, isn't it? As long as the European Parliament is the US's pet dog we will never be a superpower.
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But we can jam the US's system now without any damage to Galileo, so it's a win/win situation!
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It has nothing to do with trust. There is nothing the U.S. or any country could do to gain the trust of other countries with a crucial technology like GPS. If Galileo had been developed first, the U.S., Russians, and Chinese wouldn't want to be reliant upon it either,
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I think destroying the satellites would be the equivalent of a declaration of war. I'm not sure the U.S. would want to declare war on its allies.
Of course should the U.S. and Europe no longer be allies at the time that happens, then if the U.S. kills the Galileo satellites, I guess Europe's answer would be to kill the GPS satellites. Again, not exactly what the U.S. wants.
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Is there any practical way of shooting down satellites in medium earth orbit? GPS satellites orbit around 20,000km -- that's a long way for any anti-satellite missile.
Re:...Why? (Score:5, Insightful)
Why is Europe spending billions to create their own GPS constellation when the US government already went through the hassle and expense? The GPS system is free and open to use by anyone with a GPS receiver. This strikes me as nothing but a political move, as if to say "We're independent and don't need America to provide anything for us". This is a completely redundant and pointless project by the EU.
Even as an American I can see the value in having a completely separate system for satellite navigation. Even ignoring the ability of the USA to reduce the accuracy (or completely shut off) the system, the system is still a potential single point of failure subject to software problems or a rogue agent controlling the ground stations. Much better to have a completely separate redundant system with no common elements.
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There are also navigation systems that use geostationary satellites, e.g. Egnos and Inmarsat.
EGNOS augments other satellite navigation systems (GPS, GLONASS, Galileo) by providing ionospheric correction data and other relevant information. It is not a full-fledged navigation system in its own right -- it cannot be used for navigation on its own.
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Redundancy improves reliability. It's the same reason why you should keep backups, even though it's a redundant copy of the data.
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"Galileo is much more precise than GPS. And that's just for starters."
No, it isn't. It's just that unlike GPS, the precise part is open to the general public.
Also, I think OP is incorrect. The only reason for a 4th lock using GPS is to get the precision you would normally get with 3, if you were military. If the new system is open and precise, 4 sats should probably not have to be visible.
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No, it isn't. It's just that unlike GPS, the precise part is open to the general public.
Correct. The L5 intermediate-precision Galileo signal will be freely available to the public. However, this signal is not as precise as the GPS encrypted "precise acquisition" (aka military) signal.
The freely available L1 signal has essentially an identical format as the GPS "coarse acquisition" signal, and is therefore expected to offer broadly similar performance.
The advantage of having 2 distinct frequencies available to the public is that it is possible to correct for atmospheric dispersion. Within the
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"The 4th sat is for elevation. 3 to fix you in 2D. 4 to fix you in 3D."
No. That's with GPS, and that's after the fixes for precision that are necessary because the "high precision" part of U.S. GPS is restricted to military.
With an open system, it should require no more than 3 visible sats to fix your position in 3D.
Somebody tried to argue this point with me a couple of weeks ago, and it's simply false. You can get a rock-solid 2D position with only 2 observation points. 3 (and even just the first 2, if they are transmitting the appropriate data) can establish elevation.
Re:...Why? (Score:5, Interesting)
You need at least one other sat to fix you in time. The receiver doesn't have an atomic clock and so needs an extra point of data for the extra unknown.
Simple logic might win an argument, but that doesn't make you correct.
Re:...Why? (Score:5, Informative)
Nevertheless, I am still correct. The 4th sat is only for precision.
No it's not.
It is not necessary for location.
Yes it is.
Quoting the Navy [nps.edu]: "Therefore there are 4 unknowns at each timeline where a solution is computed, 3 for position and 1 for time. This is why the minimum number of satellites for a solution is 4."
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The air force on the other hand
So some of the military only need 3.
For the civilian stuff the more that can be seen the better to cope with the randomising that the military don't have to deal with. I think that also applies to everyone at high lattitudes, but since I don't know how high these things are and in what orbit I don't know how much of an effect getting the signal through a bit more atmosphere has.
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"No, you're not correct. The 3D information that you'd get without the fourth satellite would be so inaccurate as to be meaningless. You need the fourth satellite for a 3D fix that puts you in the right city, let alone the right street. That's qualitatively different from 'only for precision'."
Then why do the military only require 3 satellite locks? Eh? Answer me that, you fucking genius.
They have accurate clocks?
I don't know, maybe I'm not a "fucking genius" enough for that to be the right answer.
scientific reasoning? (Score:3)
Just quoting someone else without knowing what you are talking about doesn't make you right. Come up with scientific reasoning using trigonometry and publicly available specs for GPS to prove your point, or keep your mouth shut, please.
Triangulation (without altitude) works with three points if you have one unknown transmitter and three known receivers. This is the other way around. Using just two receivers would give you two possible locations, without altitude. The other way around, in a real life situati
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"That's not unique to the military signal. It's just a matter of how much money you want to throw at the problem."
The U.S. military GPS system was designed to work properly with only 3 visible satellites.
Put that in your pipe and smoke it, or do whatever the hell you want with it. I didn't invent the system. But I do know just a little about it.
The US military system is, these days, no different to the civilian system. The system was descrambled in the 90s because it became apparent that it was just too expensive to keep building more complex and relatively low-run GPS chips for the military, when what they wanted to do was put them in bombs and things.
By and large though, even the airforce doesn't need GPS elevation data since laser altimeters are faster and very standard on all aircraft.
Re:...Why? (Score:4, Informative)
But I do know just a little about it.
Which seems to be the amount you know about everything.
That's not helpful.
GPS recievers indeed solve a 4D problem (3D space + time) which requires
4 sats. As expensive as the military units are, they may be able to bridge reception
gaps by keeping time by themselves for a while, so may for a while work with
only three sat signals.
But without a local timer with the precision of an atomic clock, that's the digital
equivalent to dead reckoning, and will only get you so far until you need your fourth
sat again.
Of course, if you are the Navy, one of your coordinates is known by default (at least
plus or minus a couple of meters), so maybe their gear is permanently set to a dedicated
2D mode. That would indeed work with 3 signals - but you still need four inputs for a 3D
position, it's just that in this case one of those inputs doesn't come from space, but from
looking out the window.
Maybe you will believe the Los Alamos Labs' GIS unit [lanl.gov]?
"If you require 3-dimensional coordinates (northerning, easting, and elevation), a minimum of four satellites is needed."
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"So some of the military only need 3."
Nice conjecture, but not correct.
3 satellites are all that are necessary to pinpoint you in any 3D position. Period.
(Technically they can only put you in 1 of 2 positions, but the other one is off in space so not very likely.)
The 4th satellite is only necessary for civilian systems to correct for timing that was very much intentionally made ambiguous for civilian receivers, since GPS was made by and for the military.
This is the second time you've brought this up. The US stopped deliberately degrading the signal for civilian uses years ago, mostly because it was simply pointless to keep doing it since most civvy receivers were able to overcome the problem with assisted GPS and other methods.
You are technically correct that you only require three satellites to get a 3D position but *only* if you know your local time extremely accurately. It has nothing to do with only three satellites being "deliberately bad" at position
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No, that's just plain wrong. It is not sufficient to know the exact time at three transmitters to get a 3D fix. You'd also need to know the exact time when the signals are received, which is a very difficult (read: costly) problem. The accuracy of the positioning information is limited by both the precision of the satellite clock signal and the accuracy of the receiver clock signal. The latter can be removed from the equations by using a fourth satellite signal, but not by providing better satellite clock s
Re:...Why? (Score:5, Informative)
Please stop voting this guy up, while simultaneously voting down the numerous posts that are correct.
Four satellites are required because there are four unknowns, and only one measurement per satellite available, irrespective of precision or lack thereof.
Here's some quotes from Global Positioning System [wikipedia.org] so we can all stop agreeing with the loudest person instead of the facts:
"About nine satellites are visible from any point on the ground at any one time, ensuring considerable redundancy over the minimum four satellites needed for a position."
"The receiver uses messages received from satellites to determine the satellite positions and time sent. The x, y, and z components of satellite position and the time sent are designated as [xi, yi, zi, ti] where the subscript i denotes the satellite and has the value 1, 2, ..., n, where n >= 4."
"Although four satellites are required for normal operation, fewer apply in special cases. If one variable is already known, a receiver can determine its position using only three satellites. For example, a ship or aircraft may have known elevation.
The time precision required for a fix of any reasonable accuracy requires atomic clocks. You can't carry atomic clocks in your pocket, they're a tad too big for that. There is no way to know the time on the satellites from the ground, because you don't know where you are, and hence how far the satellites are from you, and hence the delay added to the signals. You can use three satellites to figure out where you are, if you know what time it is, but you don't. Adding a fourth satellite in the mix lets you solve for all four unknowns in the equation. Note the exception in the wiki article applies only in some rare cases, like the GPS units used by ships, not the GPS units handed out to most military personnel.
THIS HAS NOTHING AT ALL TO DO WITH SELECTIVE AVAILABILITY.
The military encryption simply reduces the precision of the solution, it doesn't actually change the number of unknowns and hence the equations in any way. A civilian marine GPS could locate itself with just 3 satellites even with selective availability enabled, as long as it assumes that it's at 0 elevation. In all other cases, four satellites are required, even for military units.
Let's go one satellite at a time... (Score:5, Informative)
Let's go one satellite at a time...
First satellite: You know approximately what time it is because the satellite tells you. You know the position of the satellite, and all of the other satellites, because it tells you in its signal. However, you don't know how far away the satellite is because you don't know the difference in time between when it sent its signal and when you received it. Thus, while one satellite tells you a lot, it does nothing at all to narrow down your position.
Second satellite: Now you know the difference in time between when you heard the two satellites, and thus, you know how much further you are from one of them than you are from the other. So in 3D space, you can use this information to narrow down your position to a point that lies on a sphere. This sphere intersects the earth, forming a circle. Thus, you know a lot of place where you might be, but you still really don't know much.
Third satellite: Now you're able to cut that huge sphere down to a circle. Where this circle intersects the earth, are two points. One point is flying around at high speed, the other relatively stationary. Thus, you kind of know where you are now. ...but only kind of. While the earth is a sphere and we intersected that with a circle to get two points, the places on the earth you might be aren't an infinitely thin mathematical sphere. There's thousands of feet of elevation in which you might exist. ...and worse than that, even if you don't care to know your elevation, the intersection of that circle with the atmosphere isn't straight up and down -- it's at some bizarre and slowly changing angle -- thus you can't ignore it because it isn't just your elevation you don't know, but rather, you're equally uncertain about your latitude and longitude. You know your position to within a mile or so, but if you want to be more accurate than that, you need to either know your elevation or find another satellite.
Fourth satellite: That circle of possible locations is now narrowed down to two points. One is flying randomly through space, the other is near earth. You don't even need to find an intersection with the surface of the earth, unless by some odd chance you're having difficulty figuring out which of those two points is you.
Fifth satellite: No longer any questions, you know exactly which point is you. ...but still, the math is only narrowing you down to about a 10 ft. radius...
Sixth satellite: ...and so it's nice to have some additional data to average together for a slightly more accurate result.
Seventh satellite: ...and it's nice to have some spares for when some become obstructed by trees or tall buildings.
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Bookmarked for future use.
Re:Let's go one satellite at a time... (Score:5, Informative)
It would be a nice explanation. Unfortunately it's not correct. It's all good until this part:
Second satellite: Now you know the difference in time between when you heard the two satellites, and thus, you know how much further you are from one of them than you are from the other. So in 3D space, you can use this information to narrow down your position to a point that lies on a sphere.
The set of points such that the difference between the distances to the two satellites is constant isn't a sphere. How to see this? Any point in space that satisfies the condition can be rotated around the axis through the two satellites. Since this doesn't change the distances to the satellites, the rotated point still satisfies the condition. The connection between the satellites is an axis of symmetry of the point set. There is one point on the axis which satisfies the condition: If the distance between the satellites is d, the point at distance d/2-a/2 from the first satellite is at distance d/2+a/2 from the second satellite and thus satisfies the condition. Since there are other points that satisfy the condition, the suspected sphere can't have radius 0. Together with the symmetry this tells us that there has to be a second point on the axis that satisfies the condition, but there is no such point, so the set of points can't be sphere. (Special case: a=0, you get the same time signal from both satellites at the same time. This puts you on a plane perpendicular to the axis halfway between the satellites.)
So what is the shape of the point set satisfying the condition? It's a hyperboloid. [wikipedia.org]
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Note the phrase: "computed time". Not measured time.
Yes. Computed from at least four sat signals.
Bozo.
Nice sig.
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Simple geometry states you need three satellites for an accurate 2D fix, and four satellites for a 3D fix, not whether you have access to the encrypted P(Y) code.
http://gis.stackexchange.com/questions/12866/gps-positioning-why-four-satellites [stackexchange.com]
http://www.cmtinc.com/gpsbook/chap5.html [cmtinc.com]
http://www.gpsnuts.com/mygps/gps/technical/ed.htm [gpsnuts.com]
Note that the final reference I list is written by someone who is a GPS analyst and has worked for the DOD on the GPS system since 1975.
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If you know the distances between yourself and 3 satellites then you can trilaterate your position. However, with GPS you don't know the distances, or even the transit times (from which the distances could be trivially determined), you only know the time the signals were sent and the time offsets at which you received them (i.e. you know you got satellite A's signal Xmicroseconds after satellite B's). This is not the same as knowing the transit times, since you don't have a clock accurate enough to tell yo
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There is NO geometric problem here... you are getting transmissions from 3 satellites that know their own positions in time and space... therefore you know your position. In full 3D. With only 3 satellites.
That's not how I learned it, but I want to change my mind on this if you're really right. But if what you're saying is true, then what exactly is wrong with the following reasoning:
The GPS receiver gets signals from 3 satellites telling the exact time and position of each satellite at the exact moment the signal was sent. Let's call (xi,yi,zi,ti) the position ant time of satellite i (i=1,2,3), and (x,y,z,t) the position and time of the GPS receiver when the message was received. We want to find out the pos
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Erm, I messed up the equation. It should be:
That doesn't change the reasoning, though.
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How about if you require t1 = t2 and t2 = t3 ? Then you have five equations and four unknowns.Maybe this would break due to special relativistic effects.
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Oh, right, ti is the time at which the signal was sent, sorry.
Assume you know the difference in time between the receipt of the signals, you can solve for the time at which the second satellite sent the signal.
The receipt time difference
dt1i = ti' - t1'
Where t1', ti' is the time when you received the signal i, on some arbitrary clock.
Then this is the equation for t2:
t2 = c*sqrt( (x-xi)^2 + (y-yi)^2 + (z-zi)^2 ) - c*sqrt( (x-xi)^2 + (y-yi)^2 + (z-zi)^2 ) + dt12.
This second order equation in lets you solve fo
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Now do the same for satellites 1 and 3, and 2 and 3, and you have two more equations.
Only two equations. Strike that "2 and 3"
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You can't set constraints to these values, they're determined by the relative positions between the satellites and the GPS receiver.
You will only have t1=t2 when the distance between the satellite 1 and the GPS receiver is the same as the distance between the satellite 2 and the GPS receiver.
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How about if you require t1 = t2 and t2 = t3 ? Then you have five equations and four unknowns.Maybe this would break due to special relativistic effects.
It does. The clocks in the satellites themselves have to be continually adjusted to account for relativistic effects. That's what the GPS master ground station is for. You can't assume they are all the same.
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"Simple geometry states you need three satellites for an accurate 2D fix, and four satellites for a 3D fix, not whether you have access to the encrypted P(Y) code."
Um... Duh... simple geometry doesn't apply here, because these are TRANSMITTERS THAT KNOW THEIR OWN LOCATION IN SPACE AND TIME.
Get that?
They know their location in space and time, but they cannot tell you what it is before it changes. The speed of light is not instant.
You have four unknowns - x, y, z and time. You can't solve those with only three equations, unless you can make a reasonable guess about one of them, thus either you need an extremely accurate clock (an atomic one) or you need to know either x, y or z to high accuracy - laser altimeter, being at sea level, knowing a point due to a separate beacon that knows its position (eg, a
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No, the obfuscation layer on GPS has been deactivated for almost a decade now. Before then you just needed a terrestrial correction signal which any GSM tower could send. The un-obfuscated signal of GPS is still a lot less precise than Galileo, though GPS planed for upgrades that would put them on par.
Re:...Why? (Score:5, Informative)
While "selective availability" (the intentional degradation of civilian signals to roughly 100m accuracy) has been disabled for a while and the new satellites don't have the capability for implementing it, the military does indeed have separate signals for civilian and military users.
Referring to the wikipedia [wikipedia.org], the civilian signal ("C/A") is only transmitted on the L1 band at 1575.42 MHz. The encrypted precision codes (for the military) are transmitted on both L1 and L2 at 1227.60 MHz. The military signal is indeed quite a bit more accurate than the civilian signal: by itself, the civilian GPS signal is only accurate to around 3 meters. The military signal is accurate to around 30cm.
With current civilian signals only transmitted on a single frequency, receivers cannot correct for ionospheric conditions (which composes a major part of the current uncertainty in measurements) as doing so requires two frequencies. Military signals are transmitted on two frequencies so receivers can correct for ionospheric delay. The military signal is also transmitted at a much higher rate (10x the civilian rate), yielding proportional increases in accuracy.
Currently there are systems like WAAS (North America) and EGNOS (Europe) that provide augmentation in the form of corrections for ionospheric delays (and some other information, like current "health" status of the satellites). This can improve accuracy even more (my handheld civilian unit is able to compute position with an uncertainty of 2 meters). EGNOS also provides an internet feed of the augmentation data so one doesn't have to have a clear view of the geostationary satellites that provide the augmentation. WAAS only augments GPS, but EGNOS augments GPS, GLONASS, and Galileo (for what it's worth at present).
The GPS upgrades will add more detailed signals (the civilian signal will be broadcast on L1 and L2 as well as safety-of-life signal on L5 at 1176.45 MHz). The military codes will also get an upgrade as well, but that won't really matter for civilian users. With the civilian signals being transmitted on a total of three frequencies it will be possible for receivers to account for ionospheric delay and other factors. Overall, things will get considerably more accurate.
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The "high precision" part of the GPS system was opened up to the public by Clinton back when he was president. Birds launched since then lack the capability to restrict civilians to the lower quality fix.
Re:Chicken::egg. (Score:5, Insightful)
They'll arrive. There are already devices that can receive both Soviet GLONASS and GPS (e.g. Galaxy SIII) to get better positional accuracy. Soon the new devices will receive Galileo as well, for triple redundancy and improved accuracy.
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In Asia China's Beidou/Compass should be interesting now, and we'll see how quickly the next phase comes along.
And Japan's QZSS is easily usable by GPS receivers with the right software.
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They'll arrive. There are already devices that can receive both Soviet GLONASS and GPS (e.g. Galaxy SIII) to get better positional accuracy. Soon the new devices will receive Galileo as well, for triple redundancy and improved accuracy.
It's already happened. There have been modules for GPS+GLONASS+Galileo on the market for quite some time.
Re:Chicken::egg. (Score:5, Informative)
Are there any consumer gear that can receive Galileo?
I don't see how this could possibly be called a Chicken and the Egg type problem, as the satellites are are already in space to support consumer devices. They obviously didn't need consumer device support to get things started at all.
Re:Multibillion pissing contest (Score:4, Insightful)
Re:Multibillion pissing contes (Score:2, Insightful)
The EU sees the US as far less trustworthy than you do, and expects to come into conflict with it again - war is unlikely but economic and policitical spats are quite common between the two. In addition to that galilleo lets them have greater accuracy than the US will allow with GPS, and ensures that they don't have a strategic dependency on the US in space.
Strange how myopic and solipsistic the view from the US is sometimes.
Re:Multibillion pissing contest (Score:5, Insightful)
When your international reputation is at an all-time low you should expect things like this... Galileo was started way before the US popularity took a nose dive, but the last decade or so are only going to make projects like this *more* likely. To the civilised world, the US is not one of the good guys any more, they're not in "bad guy" territory yet, but they're sure headed there fast.
Basically the world no longer trusts the USA. Simple as that.
Simon.
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they're not in "bad guy" territory yet
Just speaking for myself here: they most certainly are, although discussing the reasons is out of scope and would take ages.
I get the impression that many people see the USA as a necessary evil. Apparently everybody thinks that there has to be domination by one nation over the others; in this perspective, the USA are the least bad option (alternatives such as China or Russia aren't that exciting).
Re:Multibillion pissing contest (Score:5, Interesting)
Jesus, tunnel vision much ?
Look it's nothing to do with GPS vs Galileo. It's to do with the USA, a nuclear power, declaring war left, right and center, & invading other countries basically because it can. No-one likes that; international reputation suffers, trust is lost, and consequences ensue. There's no point in getting pissy about it, you brought it on yourselves.
I don't think Europeans are innately superior. I think people are just people, wherever you are. I'm married to an American woman, whom I love dearly. I do think the USA is fucked though, the society is (IMHO) past the tipping point and heading down, and I can't see myself staying around much longer, as I've said before on this site. At some point, the money just ain't worth it.
Simon
Re:Multibillion pissing contest (Score:5, Interesting)
"Hm, what was the last time Congress declared war? Or are we just making shit up now?"
Note the "invading countries left, right, and centre" part of his argument too, the US has been doing this basically without a single break since World War II in one way or another, whether it's drone strikes in Pakistan, or the CIA pulling off defacto coups across the world. His point is that America spends far too much time and far too much money meddling with other nations, rather than keeping to itself, and that often leads to greater instability. Case in point, by ousting Saddam, the US removed the only credible counterbalance in the middle east to Iran, and since then Iran has been able to carry out proxy attacks everywhere from Iraq, to Lebanon, from Afghanistan, to the Philippines. They couldn't do this shit when Saddam was around, because Saddam would then be given the international blessing he needed to do the exact same thing in Iran proper. By trying to make things better for the oppressed minorities in Iraq, the US ended up making things worse for everyone in Iraq, and people in many other countris too. So when you stop avoiding the point he was making by focussing on a specific intentionally mis-used part of that, tell me, are you disagreeing that America consistently meddles in the dealings of other nations?
"Well, a lot of your countrymen do"
Well, ignoring the fact Europe is a country, what Europeans thinks is not that they have any kind of innate superiority - that's simply not in the European mindset- Europeans are simply much more rational than that, they recognise their fallibility in part because they have thousands of years of history of it to learn from. Some European nations did have this mindset- the French and British at the height of their empires for example, but as their empires fell they realise it was simply a load of bullshit. Ironically, the reason you most likely claim it is because it IS something that's in the US mindset, there's even a term for it - "American exceptionalism", it's something America hasn't, like Europe, grown out of yet.
What many Europeans do believe however, that you're getting confused with, is the fact that currently, Europe is at least governing itself just a little bit more sanely than the US, and that is what Europeans are happy to point out to you. The terms you mention are nothing more than banter, and if you believe any use of them implies some perceived superiority then it simply demonstrates that you, as an American, haven't got out of this absurd mindset that some nations and their people are simply inherently superior to others. Europeans know full well they have their problems, and this is ironically why the current global economic instabilities focus on the Eurozone's issues - because Europe is the only one really openly admitting they have a big problem and trying to deal with it, in contrast to for example America's insanely massive deficit, see here for example, and sort by worst to best, note how insanely large the US figure is in the negative compared to even the closest member on the list?
http://en.wikipedia.org/wiki/List_of_sovereign_states_by_current_account_balance [wikipedia.org]
The only reason you believe there's some kind of belief about inherent superiority is because you have that mindset yourself, until you lose that you wont be able to get over this stupid idea that Europeans think they are innately superior. Believing they are doing some things better that lead to for example, lower infant mortality, longer life expectancy, higher levels of personal happiness, etc. does not in any way imply this is because of some innate superiority or belief in such.
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For the record, I don't really suvbscribe tothat rationality argument, eithar. At the very least I don't think we are inherently more rational, although we might be effectively more rational. Our advantage is that we're not a superpower. Given
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Oh dear, I think you're having real difficult with the rather simple word "innate".
In this context the word innate would mean that there was a belief that Europeans are born with some kind of natural superiority, the fact that I stated that Europeans tend to be much more rational and not believe in such hogwash doesn't conflict with this, because the rationality spoken about in case is a learnt trait taught into European culture through are many lessons from history we can draw on to understand why that bel
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*facepalm*
I'm sure you knew what I meant. That'll teach me for being pedantic about the GP's post!
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Right, I'm talking about a cop executing a handcuffed helpless suspect [youtube.com] [warning. Graphic.] and the lack of respect for the law that "peace officers" show; about the TSA, just the very fact of its existence; about the demagoguery that passes for news and its knock-on effects on society; about the constant military action taken to divert attention from problems; about the massive debt and crippled economy; about the shameful lack of a decent for-all healthcare system; about the proliferation of lethal weapons
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Speak for yourself. I don't trust either.
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Your comment is short-sighted.
GPS, whether American, Russian, or EU, is first and foremost, a military asset for their respective owners.
The US military can elect to disable or cripple civilian GPS service to all devices other than their own when they deem it necessary to prevent its use by hostile forces. Presumably, GLONASS and the EU systems have the same capability.
History repeatedly shows that international political alliances vary over time. Just because we currently are at relative peace with t
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Article did not answer: If Europeans don't have GPS yet, how do their existing sat-navs work? And why is this new system only for Europe?
The Europeans do have GPS, it's publicly available and "global" is in the name. There is also the Russian GLONASS, which is basically the same. The EU doesn't have it's own network, though, which is important for political reasons (and only political reasons). Although there are some technical improvements Galileo makes over GPS, they certainly don't justify a new system. It's also available for everyone, or at least will be, if it ever gets operational (again, global).
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I love how position is often triangulated by connecting points to form something other than a triangle.
I thought the "triangulation" name came from forming a triangle between yourself and each remote point (and depending on the accuracy you desire, there could be many remote points)
It's not that you're drawing one big triangle on a map, you're using many (or at least several) triangles to known points.