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."
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).
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.
Re:...Why? (Score:5, Informative)
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?
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: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:...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.
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."
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."
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:...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.
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]