11 ms·

GPS and Relativity

261 points by jcr 12y ago

tgb 12y ago
I don't understand the problem yet. Presumably all the satellite clocks run slower at the same rate. The receiver doesn't have a clock on it (or at least not one of that accuracy) so it can't tell the difference, or can it? Is the problem with the database or satellite positions?
- VLM 12y ago
  Sonic boom analogy will save you? So you tune in and hear "Hi there I'm a rogue air national guard F-16 flying at mach 2.0 precisely over the sears tower at 2000 ft ASL and headed due North at precisely noon" Dude on the ground in .. Racine, lets say, hears that message and then a sonic boom a bit somewhat after noon. So your math gets fed all that stuff, and then you draw a strangely shaped line on the map that predicts at that instant on your clock when you heard the sonic boom, that means you're standing on the ground somewhere along that peculiar shaped line. If you were being constantly overflown by rogue supersonic ANG F-16s from all different directions, your map would rapidly have one intersection point with a whole lot of lines drawn thru it, and thats where you are standing, at least relative to the sears tower, plus or minus your math ability. But what if the broadcasting F-16 was just kidding about mach 2.0 and was actually chugging along at mach 1.9. Your line on the map is going to be all screwed up compared to where you actually are. Worst case isn't being 5% off and you laugh and discard the data, but every overflying rogue F-16 being a tiny little bit off, ruining your standard deviation and requiring a ton more samples be averaged together for the same accuracy. The clock in your GPS is in fact disciplined to the clocks in the satellites, although its noise level is pretty bad (well, relatively, compared to the fancy clocks). If you've ever heard one of us RF guys talking and GPS Disciplined Oscillators or GPSDO that is exactly what we're talking about, an oscillator on the lab bench that is continuously aligned to be in perfect tune with the average GPS satellite, and this works shockingly well. For a couple hundred bucks your RF lab or ham radio station can have a "near atomic clock accuracy" reference clock for its oscillators, spectrum analyzers, freq counters, etc. Ham radio guys can get "hz" level accuracy at 10 GHz microwave freqs without much effort using this tech, for only a couple hundred bucks, which is pretty impressive and handy. All GPS rx have one of these, although performance and convenience vary. So the one in your phone doesn't have a convenient sma connector to feed 10 MHz refclock into my frequency counter... oh well, the innards are exactly the same. The innards of precision surveyor instruments are also lower noise and "better" than the innards of a phone. But the innards are functionally the same, more or less.
- maxerickson 12y ago
  The positions of the satellites are tracked by ground stations that also have atomic clocks. This comment knows more about it than I do: https://news.ycombinator.com/item?id=8041620 https://news.ycombinator.com/item?id=8041620 Those stations have to know the satellite time, so the compensation has to happen somewhere.
  - pdonis 12y ago
    The clocks on board the satellites have their rates corrected so that they run at the same rate as ground clocks--i.e., the satellite clocks' effective rate is not the "natural" rate of a clock at their altitude and moving at their orbital velocity. The ground stations then send corrections to the satellite clocks to compensate for the fact that the rate correction that's built in to the clocks on the satellites isn't exactly right, because the satellites are not in perfectly circular orbits at constant altitude.
rzimmerman 12y ago
GPS receivers also have to correct for things like light traveling slower through plasma in the ionosphere. Receivers on the ground actually solve for their positions in 4 dimensions (they need to solve for time!). GPS is so cool.
- baddox 12y ago
  Any idea if large-scale weather effects (like the jet stream) can cause noticeable accuracy problems?
  - kubiiii 12y ago
    It does, in fact GPS positioning error, derived from fixed GPS stations, is often used as an input for weather forecast models like WRF. The error is closely binded to the atmospheric water content. Using the different azimuths from the GPS constellation you can even build up a 3d representation of the water content on different layers of the atmosphere.
    - kgc 12y ago
      This is possibly the most interesting thing I've read on HN. I knew that a similar principle is used for CT scans, but I never thought of blowing it up to Earth-scale. Thank you.
      - kubiiii 12y ago
        Thanks. This work is interesting http://tel.archives-ouvertes.fr/tel-00444081 http://tel.archives-ouvertes.fr/tel-00444081 Unfortunately the main thesis is in french, however there are included articles in english and lots of references. I am currently working on a project using a high density GPS stations network we deployed in the south east of France. We have had good results for nowcasting and to map in 3d the atmospheric weather content. No publication yet though. 2d maps of rainfall have also been derived from interpolated GSM antennas, using the signal loss.
        12y ago
K2h 12y ago
$30 for a GPS that will log 15 hours of telemetry[1] I just cant get over how much cool math, science, engineering and technology can get crammed into a single concept. in some ways GPS is as revolutionary as the CPU. [1] http://www.adafruit.com/products/790 http://www.adafruit.com/products/790
IvyMike 12y ago
I seem to remember that the original satellites (which were the first highly accurate clocks in orbit) had a hedge--if the effects of general relativity turned out to not be correct the system could drop that part of the correction. But I'm on janky hotel internet right now so I can't find a reference. Edit: The comment from jcr is correct--this was not in GPS, but in an earlier system. http://www.leapsecond.com/history/Ashby-Relativity.htm http://www.leapsecond.com/history/Ashby-Relativity.htm > At the time of launch of the first NTS-2 satellite (June 1977), which contained the first Cesium clock to be placed in orbit, there were some who doubted that relativistic effects were real. A frequency synthesizer was built into the satellite clock system so that after launch, if in fact the rate of the clock in its final orbit was that predicted by GR, then the synthesizer could be turned on bringing the clock to the coordinate rate necessary for operation. The atomic clock was first operated for about 20 days to measure its clock rate before turning on the synthesizer. The frequency measured during that interval was +442.5 parts in 1012 faster than clocks on the ground; if left uncorrected this would have resulted in timing errors of about 38,000 nanoseconds per day. The difference between predicted and measured values of the frequency shift was only 3.97 parts in 1012, well within the accuracy capabilities of the orbiting clock. This then gave about a 1% validation of the combined motional and gravitational shifts for a clock at 4.2 earth radii.
- jcr 12y ago
  From wikipedia, it seems the keeping of accurate time (i.e. known relativistic effects) was already solved before the first NAVSTAR Global Positioning System (GPS I) satellites were constructed. What you're possibly remembering is one of the GPS predecessor systems like Transit or more likely, Timation. http://en.wikipedia.org/wiki/Global_Positioning_System#Predecessors http://en.wikipedia.org/wiki/Global_Positioning_System#Prede...
hoggle 12y ago
Talk about deployment hell - puts things into perspective: "to counteract the General Relativistic effect once on orbit, they slowed down the ticking frequency of the atomic clocks before they were launched" The mind boggles. It makes me uneasy that we only have this single system though (the effects of a GPS failure would be quite severe I guess). Do the people behind GPS collaborate with the Galileo folks? Thanks a lot for the submit, now I need to find a good book on the history behind the GPS!
- jupiter 12y ago
  There is http://en.wikipedia.org/wiki/GLONASS http://en.wikipedia.org/wiki/GLONASS
- timthorn 12y ago
  There are multiple GNSS platforms - not just GPS & Galileo, but also GLONASS, Compass, IRNSS, and others. Some receivers are multi-system capable, such as: http://www.u-blox.com/en/gps-chips/stand-alone-gps-chips/ubx-g7020-ctktka.html http://www.u-blox.com/en/gps-chips/stand-alone-gps-chips/ubx...
  - tomfanning 12y ago
    And the iPhone, Samsung Galaxy and others, at least assisted GPS + GLONASS http://en.wikipedia.org/wiki/GLONASS#Receivers http://en.wikipedia.org/wiki/GLONASS#Receivers
    - CWuestefeld 12y ago
      Given that 4 satellites are necessary for a fix, are such receivers able to mix-and-match satellites, say, three GPS satellites and one GLONASS satellite?
- agos 12y ago
  The system is a bit more redundant now: there are now 32 satellites, and on average 9 are visibile from any point.
- 12y ago
charriu 12y ago
By the way, Pogge also has recordings of some of his lectures available as mp3. Astro162 was especially interesting, and also goes into more detail on the effects and implications of relativity.
- jcr 12y ago
  Thanks for the info. I totally missed it. Lecture Recordings: http://www.astronomy.ohio-state.edu/~pogge/Ast162/Audio/ http://www.astronomy.ohio-state.edu/~pogge/Ast162/Audio/ Lecture Notes: http://www.astronomy.ohio-state.edu/~pogge/Ast162/index.html#lectures http://www.astronomy.ohio-state.edu/~pogge/Ast162/index.html...
- justin66 12y ago
  I took 162 a really long time ago and these lectures brought back memories. He's a great lecturer. At some point between what's listed as "The Evolution of Low-Mass Stars" and "Supernovae" he had the class on the edge of their seats while describing how stars use up their fuel and the end results. I hope he does some videos at some point.
mer10z 12y ago
Does this mean that the people living on the space station age faster than people on earth, or does this only affect the atomic clocks?
- callesgg 12y ago
  Yes it effects everything. But you got it backwards, they age slower. Edit: or maybe not, now in am confused. Traveling fast slows time down, and going closer to heavy object slows down time so if earth is heavy enough time would speed up by being in orbit. You are right, if that is the case.
  - avz 12y ago
    They do age slower. There are two effects here. First, gravitational time dilation slows the on board tick rate deep in the gravity well. As you get out of the well your on board tick rate increases relative to the surface due to decreasing strength of the gravitational field. Second, motional time dilation slows the on board tick rate down as your velocity increases. This effect always slows the on board tick rate down and can never increase it relative to the surface. However, the magnitude of the affect decreases. As you move to higher orbit your orbital velocity decreases and so does the motional time dilation. There are a few useful reference points which help to think about tick rate on board various satellites relative to the tick rate on the surface. 1. On the hypothetical orbit at altitude zero gravitational time dilation relative to the surface has no effect and tick rate slows down solely due to motional time dilation. On board tick rate is lower than for a stationary observer on the surface. 2. On the geostationary orbit motional time dilation has no affect (since relative velocity is zero) and tick rate increases solely due to reduced gravitational time dilation (we're further out the gravity well). On board tick rate here is higher than for a stationary observer on the surface. 3. Somewhere between these two extremes is an orbit where the two effects balance out and the on board tick rate is the same as the tick rate on the surface. This occurs roughly one half of Earth's radius (~3186km) up above the surface. ISS is merely ~423km above the surface and so it's the tick-rate-slowing motional time dilation that matters and their clocks run slower from our point of view. GPS is over 20,000km above the surface and it's the tick-rate-increasing effect of the reduced gravitational time dilation that matters so onboard tick rate is higher from out point of view.
jbert 12y ago
Does the system rely on knowing the position of the satellites to equivalent accuracy? i.e. "millimetres" accuracy for differential GPS. If so, how is this accuracy achieved? (Obviously the satellites themselves cannot use gps to determine their position...). Measurement from the ground plus manouvering? Are there any events (micro debris strike, solar wind?) which cause drift in our estimate of their position?
- Cthulhu_ 12y ago
  From what I understand, GPS receivers synchronize their clocks with those of the satellites; triangulation occurs by comparing the timestamps of three or more sattelites. The deviation, times the speed of light or something, gives the receiver a pretty good indication of how far each satellite is. That should be enough to determine a location.
  - jbert 12y ago
    > That should be enough to determine a location. Yes, if you know where the satellites are to the same degree of precision. That's what I was wondering about.
  - mikeash 12y ago
    That's the basic idea, but note that it takes four satellites, not just three, because you're fixing your position in four dimensions (three space, plus time). If you had an extremely accurate clock locally then you could get away with just three, but without knowing the precise time, you need an extra satellite to fix time as well as space. Basically, GPS doesn't give you distance to the satellites, but the differences in the distances to different satellites. The signal from one satellite gives you nothing much, two satellites fixes your position on a hyperboloid, three satellites fixes your position on a curve, and four fixes you to a point.
- TickleSteve 12y ago
  The orbits of the satellites are known to a certain accuracy allowing prediction of where the satellite is at any given point. The accuracy of this almanac & ephemeris data is only valid for a short time in the future (hours to days) but is continuously updated from measurements of the satellites actual position. The ephemeris & almanac data is what is transmitted to the GPS receivers on the ground along with the timing information allowing the receiver to calculate their position. An assisted GPS (A-GPS) system can transmit predicted ephemeris for up to a week ahead to a device and store it allowing very quick start ups (Time-To-First-Fix) presuming that the data is preloaded (avoids having to wait for an almanac &ephemeris download which can take up to 13 minutes). In answer to your question, the orbits are affected by everything from the atmospheric weather which causes deflections in the signals to phases of the moon and the shape of the earth that it is passing over causing small deflections in the satellites orbit. Many factors combine to make orbital predictions very difficult, akin to predicting the weather.
mschuster91 12y ago
Can one build a GPS reciever chip that uses the signals from GPS, GLONASS, EU and the Chinese navigation satellites? In theory, there should then, assuming the EU/Chinese navigation systems have an equal amount of satellites as GPS/GLONASS, be always 24 or more satellites in view of the user - which should be enough to provide millimetre-precision positioning in real time.
- gambiting 12y ago
  You can already get a centimetre-precision for rather low money: http://hackaday.com/2013/08/05/centimeter-level-precision-gps-for-500/ http://hackaday.com/2013/08/05/centimeter-level-precision-gp... I have a friend who build an automatic lawnmower as his pet project at uni,and he used two GPS receivers to get <10cm accurancy which allowed the lawnmower to make automatic turns around the garden.
- userbinator 12y ago
  Can one build a GPS reciever chip that uses the signals from GPS, GLONASS, EU and the Chinese navigation satellites? Multi-system GPS receivers have been around for some time now, and they're quite cheap: http://www.mirifica.it/store/mediatek-mt3333-gnss-receiver/584-globaltop-gms-b6-gnss-module.html http://www.mirifica.it/store/mediatek-mt3333-gnss-receiver/5... mm-level accuracy is more difficult, but even the accuracy specs of those cheap modules is usually rather conservative.
agarcia-deniz 12y ago
A less useful but,perhaps, more interesting example of relativity at work is the colour of Gold http://en.wikipedia.org/wiki/Relativistic_quantum_chemistry#Color_of_gold_and_caesium http://en.wikipedia.org/wiki/Relativistic_quantum_chemistry#... I believe that this has been posted before
hyperliner 12y ago
Why does the article say that clocks "appear" to be running differently? Are they running differently? Or they only appear to do so? If so, what happens when the two participants get closer? Do clocks start "getting in sync"? If we had an atomic clock with a mechanism to self-destruct when not running at the original "right" time, does the atomic clock self-destruct? Can it "tell" it is running slower? In more biological settings (say, a human being) what are the effects of "aging faster"? Is it possible that the body would "not work correctly" under certain gravitational forces because of the effects of time? (i.e. maybe the blood flows faster, just a terrible example, I hope you get the idea).
- acchow 12y ago
  > Why does the article say that clocks "appear" to be running differently? Are they running differently? Or they only appear to do so? Short answer: they are running differently. Complicated answer: I don't think this question really makes sense in relativity. > If so, what happens when the two participants get closer? Do clocks start "getting in sync"? No. > If we had an atomic clock with a mechanism to self-destruct when not running at the original "right" time, does the atomic clock self-destruct? Can it "tell" it is running slower? There is no "right" time in relativity, only frames of reference. > In more biological settings (say, a human being) what are the effects of "aging faster"? Is it possible that the body would "not work correctly" under certain gravitational forces because of the effects of time? (i.e. maybe the blood flows faster, just a terrible example, I hope you get the idea). Relativistic time dilation will have no effect on chemical and biological reactions. But I'm sure there are aspects to our biology that go beyond chemical reactions and actually rely on quantum effects - time dilation might have some effect here? I don't have the answer for this.
  - pdonis 12y ago
    > Relativistic time dilation will have no effect on chemical and biological reactions. This is not correct. Relativistic time dilation affects all processes. If you flew a chemical reaction and a living organism on a GPS satellite, and then brought them back and compared them with a similar reaction and organism that stayed behind on Earth, the reaction on the satellite would have proceeded further and the organism on the satellite would have aged more. (In practice, the differences would be too small to detect with our current technology, but if we had accurate enough ways of measuring we could detect them.)
bernardom 12y ago
I can't believe nobody posted the obligatory XKCD reference: http://xkcd.com/808/ http://xkcd.com/808/
- tedks 12y ago
  XKCD references don't add to the discussion. Especially since XKCD is itself mostly just references to things technical people often know, a lot of people have the courtesy to keep HN clean of its cheap humor-by-association. Note that the only real connection between this article and the XKCD is that they both contain the words "relativity" and "gps".
debrice 12y ago
GPS Satellites "live" in our future, meaning our present and our future are real. Whatever you do, you are still free to make your own decision but you should know that you already made that decision.
- mihai_ionic 12y ago
  Or, as I prefer to think of it, the state of a distant object is undetermined with respect to your state (in a superposition of all reachable states since your last interaction) until you actually interact with it, at which point both get reconciled into a compatible state. Of course, it's slightly more complicated than that, because this applies to every particle you're made of individually. And then the particles are actually merely excitations of a field. I'll leave the rest to actual physicists. :)
- pdonis 12y ago
  What do you mean by this?
elicash 12y ago
Another way to put this is that we've already built time machines.
nyc111 12y ago
How come everyone here believes without question the statement in the original article that "the engineers who designed the GPS system included these relativistic effects when they designed and deployed the system." The author gives no evidence for this claim. In fact the research was classified and he cannot offer evidence even if he tried. See references in this comment http://science1.wordpress.com/2010/10/11/if-newton-were-to-come-back-to-earth/#comment-1381 http://science1.wordpress.com/2010/10/11/if-newton-were-to-c... which says that "Nevertheless, in practice, neglect of relativity does not now contribute measurably to the GPS error budget, as the OCS software is currently configured. (p.194)" cited in this source GPS AND RELATIVITY: AN ENGINEERING OVERVIEW Henry F. Fliegel and Raymond S. DiEsposti http://tycho.usno.navy.mil/ptti/1996/Vol%2028_16.pdf http://tycho.usno.navy.mil/ptti/1996/Vol%2028_16.pdf I am always amazed how normally skeptical people suspend their skepticism and believe anything a physicist or a learned doctor writes. I would be grateful if you can supply a real evidence that without GR GPS will not work.
axilmar 12y ago
If we can have a common time between two points in space, even if for point A the time is corrected with relativity, doesn't that mean that we can have a common time for the universe? i.e. in other words, simultaneity is real for any two points in space.

261 points by jcr 12y ago

tgb 12y ago

I don't understand the problem yet. Presumably all the satellite clocks run slower at the same rate. The receiver doesn't have a clock on it (or at least not one of that accuracy) so it can't tell the difference, or can it? Is the problem with the database or satellite positions?

VLM 12y ago
Sonic boom analogy will save you? So you tune in and hear "Hi there I'm a rogue air national guard F-16 flying at mach 2.0 precisely over the sears tower at 2000 ft ASL and headed due North at precisely noon" Dude on the ground in .. Racine, lets say, hears that message and then a sonic boom a bit somewhat after noon. So your math gets fed all that stuff, and then you draw a strangely shaped line on the map that predicts at that instant on your clock when you heard the sonic boom, that means you're standing on the ground somewhere along that peculiar shaped line. If you were being constantly overflown by rogue supersonic ANG F-16s from all different directions, your map would rapidly have one intersection point with a whole lot of lines drawn thru it, and thats where you are standing, at least relative to the sears tower, plus or minus your math ability. But what if the broadcasting F-16 was just kidding about mach 2.0 and was actually chugging along at mach 1.9. Your line on the map is going to be all screwed up compared to where you actually are. Worst case isn't being 5% off and you laugh and discard the data, but every overflying rogue F-16 being a tiny little bit off, ruining your standard deviation and requiring a ton more samples be averaged together for the same accuracy. The clock in your GPS is in fact disciplined to the clocks in the satellites, although its noise level is pretty bad (well, relatively, compared to the fancy clocks). If you've ever heard one of us RF guys talking and GPS Disciplined Oscillators or GPSDO that is exactly what we're talking about, an oscillator on the lab bench that is continuously aligned to be in perfect tune with the average GPS satellite, and this works shockingly well. For a couple hundred bucks your RF lab or ham radio station can have a "near atomic clock accuracy" reference clock for its oscillators, spectrum analyzers, freq counters, etc. Ham radio guys can get "hz" level accuracy at 10 GHz microwave freqs without much effort using this tech, for only a couple hundred bucks, which is pretty impressive and handy. All GPS rx have one of these, although performance and convenience vary. So the one in your phone doesn't have a convenient sma connector to feed 10 MHz refclock into my frequency counter... oh well, the innards are exactly the same. The innards of precision surveyor instruments are also lower noise and "better" than the innards of a phone. But the innards are functionally the same, more or less.
maxerickson 12y ago
The positions of the satellites are tracked by ground stations that also have atomic clocks. This comment knows more about it than I do: https://news.ycombinator.com/item?id=8041620 https://news.ycombinator.com/item?id=8041620 Those stations have to know the satellite time, so the compensation has to happen somewhere.
- pdonis 12y ago
  The clocks on board the satellites have their rates corrected so that they run at the same rate as ground clocks--i.e., the satellite clocks' effective rate is not the "natural" rate of a clock at their altitude and moving at their orbital velocity. The ground stations then send corrections to the satellite clocks to compensate for the fact that the rate correction that's built in to the clocks on the satellites isn't exactly right, because the satellites are not in perfectly circular orbits at constant altitude.

rzimmerman 12y ago

GPS receivers also have to correct for things like light traveling slower through plasma in the ionosphere. Receivers on the ground actually solve for their positions in 4 dimensions (they need to solve for time!). GPS is so cool.

baddox 12y ago
Any idea if large-scale weather effects (like the jet stream) can cause noticeable accuracy problems?
- kubiiii 12y ago
  It does, in fact GPS positioning error, derived from fixed GPS stations, is often used as an input for weather forecast models like WRF. The error is closely binded to the atmospheric water content. Using the different azimuths from the GPS constellation you can even build up a 3d representation of the water content on different layers of the atmosphere.
  - kgc 12y ago
    This is possibly the most interesting thing I've read on HN. I knew that a similar principle is used for CT scans, but I never thought of blowing it up to Earth-scale. Thank you.
    - kubiiii 12y ago
      Thanks. This work is interesting http://tel.archives-ouvertes.fr/tel-00444081 http://tel.archives-ouvertes.fr/tel-00444081 Unfortunately the main thesis is in french, however there are included articles in english and lots of references. I am currently working on a project using a high density GPS stations network we deployed in the south east of France. We have had good results for nowcasting and to map in 3d the atmospheric weather content. No publication yet though. 2d maps of rainfall have also been derived from interpolated GSM antennas, using the signal loss.
      - 12y ago

K2h 12y ago

$30 for a GPS that will log 15 hours of telemetry[1] I just cant get over how much cool math, science, engineering and technology can get crammed into a single concept. in some ways GPS is as revolutionary as the CPU. [1] http://www.adafruit.com/products/790 http://www.adafruit.com/products/790

IvyMike 12y ago

I seem to remember that the original satellites (which were the first highly accurate clocks in orbit) had a hedge--if the effects of general relativity turned out to not be correct the system could drop that part of the correction. But I'm on janky hotel internet right now so I can't find a reference. Edit: The comment from jcr is correct--this was not in GPS, but in an earlier system. http://www.leapsecond.com/history/Ashby-Relativity.htm http://www.leapsecond.com/history/Ashby-Relativity.htm > At the time of launch of the first NTS-2 satellite (June 1977), which contained the first Cesium clock to be placed in orbit, there were some who doubted that relativistic effects were real. A frequency synthesizer was built into the satellite clock system so that after launch, if in fact the rate of the clock in its final orbit was that predicted by GR, then the synthesizer could be turned on bringing the clock to the coordinate rate necessary for operation. The atomic clock was first operated for about 20 days to measure its clock rate before turning on the synthesizer. The frequency measured during that interval was +442.5 parts in 1012 faster than clocks on the ground; if left uncorrected this would have resulted in timing errors of about 38,000 nanoseconds per day. The difference between predicted and measured values of the frequency shift was only 3.97 parts in 1012, well within the accuracy capabilities of the orbiting clock. This then gave about a 1% validation of the combined motional and gravitational shifts for a clock at 4.2 earth radii.

jcr 12y ago
From wikipedia, it seems the keeping of accurate time (i.e. known relativistic effects) was already solved before the first NAVSTAR Global Positioning System (GPS I) satellites were constructed. What you're possibly remembering is one of the GPS predecessor systems like Transit or more likely, Timation. http://en.wikipedia.org/wiki/Global_Positioning_System#Predecessors http://en.wikipedia.org/wiki/Global_Positioning_System#Prede...

hoggle 12y ago

Talk about deployment hell - puts things into perspective: "to counteract the General Relativistic effect once on orbit, they slowed down the ticking frequency of the atomic clocks before they were launched" The mind boggles. It makes me uneasy that we only have this single system though (the effects of a GPS failure would be quite severe I guess). Do the people behind GPS collaborate with the Galileo folks? Thanks a lot for the submit, now I need to find a good book on the history behind the GPS!

jupiter 12y ago
There is http://en.wikipedia.org/wiki/GLONASS http://en.wikipedia.org/wiki/GLONASS
timthorn 12y ago
There are multiple GNSS platforms - not just GPS & Galileo, but also GLONASS, Compass, IRNSS, and others. Some receivers are multi-system capable, such as: http://www.u-blox.com/en/gps-chips/stand-alone-gps-chips/ubx-g7020-ctktka.html http://www.u-blox.com/en/gps-chips/stand-alone-gps-chips/ubx...
- tomfanning 12y ago
  And the iPhone, Samsung Galaxy and others, at least assisted GPS + GLONASS http://en.wikipedia.org/wiki/GLONASS#Receivers http://en.wikipedia.org/wiki/GLONASS#Receivers
  - CWuestefeld 12y ago
    Given that 4 satellites are necessary for a fix, are such receivers able to mix-and-match satellites, say, three GPS satellites and one GLONASS satellite?
agos 12y ago
The system is a bit more redundant now: there are now 32 satellites, and on average 9 are visibile from any point.
12y ago

charriu 12y ago

By the way, Pogge also has recordings of some of his lectures available as mp3. Astro162 was especially interesting, and also goes into more detail on the effects and implications of relativity.

jcr 12y ago
Thanks for the info. I totally missed it. Lecture Recordings: http://www.astronomy.ohio-state.edu/~pogge/Ast162/Audio/ http://www.astronomy.ohio-state.edu/~pogge/Ast162/Audio/ Lecture Notes: http://www.astronomy.ohio-state.edu/~pogge/Ast162/index.html#lectures http://www.astronomy.ohio-state.edu/~pogge/Ast162/index.html...
justin66 12y ago
I took 162 a really long time ago and these lectures brought back memories. He's a great lecturer. At some point between what's listed as "The Evolution of Low-Mass Stars" and "Supernovae" he had the class on the edge of their seats while describing how stars use up their fuel and the end results. I hope he does some videos at some point.

mer10z 12y ago

Does this mean that the people living on the space station age faster than people on earth, or does this only affect the atomic clocks?

callesgg 12y ago
Yes it effects everything. But you got it backwards, they age slower. Edit: or maybe not, now in am confused. Traveling fast slows time down, and going closer to heavy object slows down time so if earth is heavy enough time would speed up by being in orbit. You are right, if that is the case.
- avz 12y ago
  They do age slower. There are two effects here. First, gravitational time dilation slows the on board tick rate deep in the gravity well. As you get out of the well your on board tick rate increases relative to the surface due to decreasing strength of the gravitational field. Second, motional time dilation slows the on board tick rate down as your velocity increases. This effect always slows the on board tick rate down and can never increase it relative to the surface. However, the magnitude of the affect decreases. As you move to higher orbit your orbital velocity decreases and so does the motional time dilation. There are a few useful reference points which help to think about tick rate on board various satellites relative to the tick rate on the surface. 1. On the hypothetical orbit at altitude zero gravitational time dilation relative to the surface has no effect and tick rate slows down solely due to motional time dilation. On board tick rate is lower than for a stationary observer on the surface. 2. On the geostationary orbit motional time dilation has no affect (since relative velocity is zero) and tick rate increases solely due to reduced gravitational time dilation (we're further out the gravity well). On board tick rate here is higher than for a stationary observer on the surface. 3. Somewhere between these two extremes is an orbit where the two effects balance out and the on board tick rate is the same as the tick rate on the surface. This occurs roughly one half of Earth's radius (~3186km) up above the surface. ISS is merely ~423km above the surface and so it's the tick-rate-slowing motional time dilation that matters and their clocks run slower from our point of view. GPS is over 20,000km above the surface and it's the tick-rate-increasing effect of the reduced gravitational time dilation that matters so onboard tick rate is higher from out point of view.

jbert 12y ago

Does the system rely on knowing the position of the satellites to equivalent accuracy? i.e. "millimetres" accuracy for differential GPS. If so, how is this accuracy achieved? (Obviously the satellites themselves cannot use gps to determine their position...). Measurement from the ground plus manouvering? Are there any events (micro debris strike, solar wind?) which cause drift in our estimate of their position?

Cthulhu_ 12y ago
From what I understand, GPS receivers synchronize their clocks with those of the satellites; triangulation occurs by comparing the timestamps of three or more sattelites. The deviation, times the speed of light or something, gives the receiver a pretty good indication of how far each satellite is. That should be enough to determine a location.
- jbert 12y ago
  > That should be enough to determine a location. Yes, if you know where the satellites are to the same degree of precision. That's what I was wondering about.
- mikeash 12y ago
  That's the basic idea, but note that it takes four satellites, not just three, because you're fixing your position in four dimensions (three space, plus time). If you had an extremely accurate clock locally then you could get away with just three, but without knowing the precise time, you need an extra satellite to fix time as well as space. Basically, GPS doesn't give you distance to the satellites, but the differences in the distances to different satellites. The signal from one satellite gives you nothing much, two satellites fixes your position on a hyperboloid, three satellites fixes your position on a curve, and four fixes you to a point.
TickleSteve 12y ago
The orbits of the satellites are known to a certain accuracy allowing prediction of where the satellite is at any given point. The accuracy of this almanac & ephemeris data is only valid for a short time in the future (hours to days) but is continuously updated from measurements of the satellites actual position. The ephemeris & almanac data is what is transmitted to the GPS receivers on the ground along with the timing information allowing the receiver to calculate their position. An assisted GPS (A-GPS) system can transmit predicted ephemeris for up to a week ahead to a device and store it allowing very quick start ups (Time-To-First-Fix) presuming that the data is preloaded (avoids having to wait for an almanac &ephemeris download which can take up to 13 minutes). In answer to your question, the orbits are affected by everything from the atmospheric weather which causes deflections in the signals to phases of the moon and the shape of the earth that it is passing over causing small deflections in the satellites orbit. Many factors combine to make orbital predictions very difficult, akin to predicting the weather.

mschuster91 12y ago

Can one build a GPS reciever chip that uses the signals from GPS, GLONASS, EU and the Chinese navigation satellites? In theory, there should then, assuming the EU/Chinese navigation systems have an equal amount of satellites as GPS/GLONASS, be always 24 or more satellites in view of the user - which should be enough to provide millimetre-precision positioning in real time.

gambiting 12y ago
You can already get a centimetre-precision for rather low money: http://hackaday.com/2013/08/05/centimeter-level-precision-gps-for-500/ http://hackaday.com/2013/08/05/centimeter-level-precision-gp... I have a friend who build an automatic lawnmower as his pet project at uni,and he used two GPS receivers to get <10cm accurancy which allowed the lawnmower to make automatic turns around the garden.
userbinator 12y ago
Can one build a GPS reciever chip that uses the signals from GPS, GLONASS, EU and the Chinese navigation satellites? Multi-system GPS receivers have been around for some time now, and they're quite cheap: http://www.mirifica.it/store/mediatek-mt3333-gnss-receiver/584-globaltop-gms-b6-gnss-module.html http://www.mirifica.it/store/mediatek-mt3333-gnss-receiver/5... mm-level accuracy is more difficult, but even the accuracy specs of those cheap modules is usually rather conservative.

agarcia-deniz 12y ago

A less useful but,perhaps, more interesting example of relativity at work is the colour of Gold http://en.wikipedia.org/wiki/Relativistic_quantum_chemistry#Color_of_gold_and_caesium http://en.wikipedia.org/wiki/Relativistic_quantum_chemistry#... I believe that this has been posted before

hyperliner 12y ago

Why does the article say that clocks "appear" to be running differently? Are they running differently? Or they only appear to do so? If so, what happens when the two participants get closer? Do clocks start "getting in sync"? If we had an atomic clock with a mechanism to self-destruct when not running at the original "right" time, does the atomic clock self-destruct? Can it "tell" it is running slower? In more biological settings (say, a human being) what are the effects of "aging faster"? Is it possible that the body would "not work correctly" under certain gravitational forces because of the effects of time? (i.e. maybe the blood flows faster, just a terrible example, I hope you get the idea).

acchow 12y ago
> Why does the article say that clocks "appear" to be running differently? Are they running differently? Or they only appear to do so? Short answer: they are running differently. Complicated answer: I don't think this question really makes sense in relativity. > If so, what happens when the two participants get closer? Do clocks start "getting in sync"? No. > If we had an atomic clock with a mechanism to self-destruct when not running at the original "right" time, does the atomic clock self-destruct? Can it "tell" it is running slower? There is no "right" time in relativity, only frames of reference. > In more biological settings (say, a human being) what are the effects of "aging faster"? Is it possible that the body would "not work correctly" under certain gravitational forces because of the effects of time? (i.e. maybe the blood flows faster, just a terrible example, I hope you get the idea). Relativistic time dilation will have no effect on chemical and biological reactions. But I'm sure there are aspects to our biology that go beyond chemical reactions and actually rely on quantum effects - time dilation might have some effect here? I don't have the answer for this.
- pdonis 12y ago
  > Relativistic time dilation will have no effect on chemical and biological reactions. This is not correct. Relativistic time dilation affects all processes. If you flew a chemical reaction and a living organism on a GPS satellite, and then brought them back and compared them with a similar reaction and organism that stayed behind on Earth, the reaction on the satellite would have proceeded further and the organism on the satellite would have aged more. (In practice, the differences would be too small to detect with our current technology, but if we had accurate enough ways of measuring we could detect them.)

bernardom 12y ago

I can't believe nobody posted the obligatory XKCD reference: http://xkcd.com/808/ http://xkcd.com/808/

tedks 12y ago
XKCD references don't add to the discussion. Especially since XKCD is itself mostly just references to things technical people often know, a lot of people have the courtesy to keep HN clean of its cheap humor-by-association. Note that the only real connection between this article and the XKCD is that they both contain the words "relativity" and "gps".

debrice 12y ago

GPS Satellites "live" in our future, meaning our present and our future are real. Whatever you do, you are still free to make your own decision but you should know that you already made that decision.

mihai_ionic 12y ago
Or, as I prefer to think of it, the state of a distant object is undetermined with respect to your state (in a superposition of all reachable states since your last interaction) until you actually interact with it, at which point both get reconciled into a compatible state. Of course, it's slightly more complicated than that, because this applies to every particle you're made of individually. And then the particles are actually merely excitations of a field. I'll leave the rest to actual physicists. :)
pdonis 12y ago
What do you mean by this?

elicash 12y ago

Another way to put this is that we've already built time machines.

nyc111 12y ago

How come everyone here believes without question the statement in the original article that "the engineers who designed the GPS system included these relativistic effects when they designed and deployed the system." The author gives no evidence for this claim. In fact the research was classified and he cannot offer evidence even if he tried. See references in this comment http://science1.wordpress.com/2010/10/11/if-newton-were-to-come-back-to-earth/#comment-1381 http://science1.wordpress.com/2010/10/11/if-newton-were-to-c... which says that "Nevertheless, in practice, neglect of relativity does not now contribute measurably to the GPS error budget, as the OCS software is currently configured. (p.194)" cited in this source GPS AND RELATIVITY: AN ENGINEERING OVERVIEW Henry F. Fliegel and Raymond S. DiEsposti http://tycho.usno.navy.mil/ptti/1996/Vol%2028_16.pdf http://tycho.usno.navy.mil/ptti/1996/Vol%2028_16.pdf I am always amazed how normally skeptical people suspend their skepticism and believe anything a physicist or a learned doctor writes. I would be grateful if you can supply a real evidence that without GR GPS will not work.

axilmar 12y ago

If we can have a common time between two points in space, even if for point A the time is corrected with relativity, doesn't that mean that we can have a common time for the universe? i.e. in other words, simultaneity is real for any two points in space.