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Galileo: Europe's Version of GPS Reaches Key Phase
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."
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.
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).
I'm an American. I love this country and the freedoms that we used to have.
Yes
As we add satellites, even from different systems, the accuracy will get better. It is very cool.
Well, the US did have selective availability enabled for a while. Perhaps European civilians don't want to be affected by US decisions.
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?
Many newer smartphones built in the past 2 or so years can already use GPS and GLONASS combined. I don't see why we wouldn't get Galileo and Compass added to that mix.
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.
That must be exactly what Russians think about us not wanting to use their fine space launch services indefinitely.
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.
Physicists get Hadrons!
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.
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.
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.
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 the EU and Russia, that does not mean it will always be so in the decades to come. I'm not saying we will be in a hostile situation with either in the future, but it's not out of the realm of possibility, either.
The EU is building their own system not because they want to win a "pissing match" with the US or Russia. It would be foolish of them strategically to depend on a GPS that is under someone else's control.
Let's play Four Horsemen of the Apocalypse. I'll be Pestilence.
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
Physicists get Hadrons!
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 would be smart enough to ignore the second point ~50K miles from the Earth.
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.
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
http://www.cmtinc.com/gpsbook/chap5.html
http://www.gpsnuts.com/mygps/gps/technical/ed.htm
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.
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.
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.
The Tao of math: The numbers you can count are not the real numbers.
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).
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 signals. And you really need to stop calling people names, especially when you're so obviously out of your depth.
Yes it is a nose dive. Years back there were rumours about rogue CIA idiots torturing people in Central America - some believed it, but most didn't, and those that did believe mostly didn't think it was standard operating proceedure. Then Baby Bush made it very clear that torture was now fully accepted as a tool and not an atrocity for the first confirmed time in the USA since the American Revolution. That's just one thing, abducting other countries citizens for torture (extreme rendition) is another, so that's a reputation loss that far exceeds Reagan's "bombing starts in 15 minutes" moment or anything Nixon ever did.
It sent a message that the US government could not be trusted to stick to the values it had for the last two centuries, so people wondered if it was going to be as untrustworthy as a place built on "might makes right" such as China or Putin's Russia. Hillary didn't help recently by demanding that agents find out the credit card details of all foreign diplomats in the USA so that they can be easily framed and pressured when needed or whatever she wanted it for.
GPS satellites are already being replaced. So far, three IIF sats have made it into orbit. As usual with US military operations, things happened late and extremely over budget, but things are happening and the chance that the system fails due to not enough operational sats is rather small.
I was promised a flying car. Where is my flying car?
"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
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.
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 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.
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 situation with moving sats and a round planet, you need more data sources to get anything near to accurate results. It may very well be that given enough time, you can get somewhat accurate lat/long information from just 3 transmitters, but you'll be suffering to get altitude positioning correct if they are all in more or less the same horizontal plane. The trajectories of GPS sats are such, that you will in practice need more than three sats to accurately determine your position and still have global coverage with less than 30 sats orbiting the planet.
So in theory, using one location and stationary sats, aided with other navigational means, you may be able to get lat/long with just 2 sats and alt with 3, but in practice, you need 4.
I was promised a flying car. Where is my flying car?
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, so this has nothing to do with the U.S. being less trustworthy. The US-haters and the US-hater-haters are just casting it as a trust issue to troll and have something to argue about
GPS is just a technology which has become so pervasive and useful that it's become essential to modern life. And any country that's capable will want to make sure it controls anything essential which it's dependent upon. If you need a car to get to work, and you neighbor has an extra car which he says you're free to use any time, you're still gonna buy your own car. Not because you don't trust your neighbor, but because you'd feel awfully stupid if you could afford a car but didn't get one, and one day your neighbor through no fault of his own were unable to lend you his car. The exceptions (e.g. Panama and Suez canals) are only where cost or physical constraints limit the number of essentials which can be built.
And all of these systems work as accuracy enhancers and fully capable backups to each other. So it's not like a lot of duplicated effort is being put in with no gain.
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 position of the GPS receiver, that is, x, y and z (we don't care about t).
From the data we get from the satellites, we can make an equation like this for each satellite:
Where c is the speed of light, and i is the number of the satellite (i=1,2,3). This equations simply says that the time it will take for the signal to leave the satellite and reach the receiver, multiplied by the speed of light, is equal to the distance between the satellite and the receiver.
Now, we have 3 equations and 4 unknowns (x,y,z,t). Granted, we're not interested on the value of t if all we care about is position. Still, if you allow t to be free, there are an infinite number of possible values for (x,y,z) that satisfy these equations. Obviously we can discard a lot of silly values for t, like any value less than any of the ti (i.e., the time the signal was received must be later than the time it was sent), and also t can't be much more than any of the ti (i.e., we know the satellites aren't to too far from the receiver). Which means you can get a range of possible values for (x,y,z), but that's not very precise.
If you have 4 satellites, however, there's only one solution (x,y,z,t) for the system of 4 equations.
So, in the end: what's wrong with this reasoning? Or, alternatively, What extra information the GPS satellites could send that would change things?
(By the way, the method I described is -- possibly -- the easiest way to describe the problem, but not the way it's usually stated. See for example this paper for a more realistic approach)
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.
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?
"If you require 3-dimensional coordinates (northerning, easting, and elevation), a minimum of four satellites is needed."
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: "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."
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, 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.
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 belief is stupid. As I say, some Europeans used to have this exact belief - the British, the French, the Germans, but we also eventually learnt that wasn't true.
In contrast, many Americans do actually believe that being American gives you some kind of natural superiority, which simply isn't true. Americans have yet to learn the same lesson that Europeans have learnt in regards to this mindset, because, like the afformentioned European countries at the height of their power, they were blinkered by that power and that resulted in their eventual downfall.
So now that you understand the word 'innate' in this context, perhaps you can go back and re-read the posts in this thread and begin to understand why people are in disagreement with you and why there is no contradiction in what I said, merely a lack of understanding of a rather simple term on your part.
"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 positioning on civilian equipment - it's the laws of physics.
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.
Ezekiel 23:20
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 ionosphere, the propagation velocity of the signals from the satellites can be altered by prevailing "space-weather" conditions; this is actually the major source of error in GPS. As different frequencies are affected differently, using a multi-frequency receiver makes it possible directly to measure this dispersion. Single-frequency receivers must either use a model based upon satellite ascension, or utilise a correction obtained from another source (e.g. DGPS or SBAS).
The galileo service plans to offer a paid-for premium service, offering another signal technically equivalent to the GPS "precise acquisition" signal, and encrypted using commercial cryptography. This service is also expected to provide high-bandwidth downlink for rapid acquisition of satellite ephemeris and differential-correction data (if conventional terrestrial based assisted GPS techniques are not available), as well as a guaranteed level of service, with guaranteed OTA alerts if a satellite malfunction is detecte, allowing a receiver immediately to drop a bad satellite rather than attempt to solve the location problem with bad data. As this signal will be transmitted on a 3rd frequency - triple frequency receivers which could perform an even more precise measurement of atmospheric signal dispersion are possible.