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First modernized GPS satellite Launched 221

Posted by Hemos
from the keeping-up-with-the-neighbors dept.
keen writes "The first GPS 2R-M satellite has launched from Cape Canaveral, Florida on top of a Boeing Delta 2 rocket. The government is now competing with Europe's Galileo system, and has added two additional military channels and one civilian channel, which will increase the accuracy and performance of GPS - as well as increase its resistance to jamming."
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First modernized GPS satellite Launched

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  • by AKAImBatman (238306) * <akaimbatmanNO@SPAMgmail.com> on Monday September 26, 2005 @10:22AM (#13650461) Homepage Journal
    You know, I was about to ask the rocket scientists hanging around here (hi guys!) about how small new generation comsats were going to be. After all, there has been a tremendous increase in miniturization and technology since the original GPS sats were launched. (e.g. better microprocessors, denser batteries, more efficient solar panels, better propulsion, etc.) If we could get these sats small enough, it might be possible to deploy a GPS system for Mars in one or two launches.

    Then I saw the borg cube that assimilated the Willy Wonka Chocolate Factory [lockheedmartin.com] (Mirror [nyud.net]) Excuse me while I pick my jaw up off the floor.

    On another note, the picture makes it look like the design hasn't changed much from the original NAVSTAR [wikipedia.org] configuration. I assume that these satellites are merely sharing the same chassis, and have very different internals?
    • You find a way of fitting a Caesium fountain clock in a smaller case.
    • ;-)

      Seriously, they talk about that in the article. Its a little bit of modernized hardware in essentially the identical satellite.
      • Yeah, I did read it. The part that stuck out to me was that the new sats were about 60 pounds heavier than the old ones. Now I understand that they couldn't have gotten all the new features in for only 60 pounds without modern technology. Still, I can't help but think that it could have been a lot smaller than that.

        Then again, I'd like to see a day when we can create useful PongSats [parallax.com], for this stuff but I supposed that won't be happening anytime soon. (Especially not when you need a large tranceiver!)
        • by cerberus4696 (765520) on Monday September 26, 2005 @11:04AM (#13650752)
          You have to remember that they're overengineering these things by terrestrial standards, because the satellites have to withstand some fairly harsh conditions while in orbit (such as radiation, EM storms from solar flares, etc). I imagine they're also hardened to some degree against human-generated interference, given all the worrying the Air Force has been doing lately about space warfare. Given all that, I'm not surprised that they seem excessivly bulky by the standards of present technology.
        • by GileadGreene (539584) on Monday September 26, 2005 @11:23AM (#13650901) Homepage
          The thing is making a satellite slightly lighter doesn't buy you much. You need a substantial drop in mass in order to get down to a cheaper launch vehicle. So given that you're already constrained to launch on a particular LV, why not pack in as much capability as possible? The Air Force in particular has a habot of keeping upgraded satellite designs at the same (or similar) mass as their predecessors, but adding lots of extra functionality.

          The other thing to keep in mind is that there are many things that contribute to the total spacecraft mass in addition to the electronics. Not all of them have undergone the same kind of Moore's law reductions in mass (or improvements in capability) that electronics have.

    • by jurt1235 (834677) on Monday September 26, 2005 @10:41AM (#13650598) Homepage
      Depending on the rocket which launches the satellite, there will be a general base for the satellite to be build on. You need to be able to mount the satellite on the rocket. Total reuse of the framedesign will save a significant amount of money, so there will be attempts to reuse the frame, solar panels, and general control systems. This ofcourse if the power signature of the new equipment matches with what the frame can deliver.

      One thing is a bit weird though about the first photo. Usually these satellites are assembled in clean rooms with people wearing all kinds of protection against static electricity build up and anti dust covers. So I wonder if the satellite in this picture is just a mockup to make a testfit of the equipment (never trust the drawings).
      • So I wonder if the satellite in this picture is just a mockup to make a testfit of the equipment (never trust the drawings).

        Given the military nature of the project, perhaps it is just a "PR model" for secrecy.

      • If you've seen the discovery channel show about how the construction of the new James Webb Space Telescope, Deep Space 1, or the (most recent one) NEAR probe, you've probably seen that they build a mockup that includes pretty much every part that will ever be on the actual satellite built, but it's built in a non-clean environment so that as many engineers as possible can work on the hardware without the restrictions that the clean room requires. While the mockup can never be flown (dust, fingerprints, all
    • by lbmouse (473316) on Monday September 26, 2005 @10:42AM (#13650606) Homepage
      Looks like a device for something out of a Dr. Seuss book. Anywho :), here are some specs [lockheedmartin.com].
    • by pr0nbot (313417) on Monday September 26, 2005 @10:43AM (#13650608)
      If we could get these sats small enough, it might be possible to deploy a GPS system for Mars in one or two launches.

      OT... something I've been wondering about, with regard to long-range communication with satellites: we know how to do networks now, why aren't we peppering space with small node probes that travel away from Earth (i.e. aren't orbital satellites) but keep in touch with eachother and so can route the data from real science probes back to us from further and further out?

      I suppose the number of nodes required would grow at the same order as the volume of a sphere (assuming we want to spray them in all directions) but we actually probably only want to send them out in specific directions.

      • by AKAImBatman (238306) * <akaimbatmanNO@SPAMgmail.com> on Monday September 26, 2005 @10:50AM (#13650658) Homepage Journal
        OT... something I've been wondering about, with regard to long-range communication with satellites: we know how to do networks now, why aren't we peppering space with small node probes that travel away from Earth

        1. Nanoprobes wouldn't have a large enough transceiver dish.
        2. We are. Have you heard of the NASA Deep Space Network [wikipedia.org]? Every sat and probe we launch becomes part of NASA's network in space. That's why when they had communications problems with the Mars Rover, they were able to send reset commands from a probe heading elsewhere. As long as they can find a number of sats with the necessary line of sight, NASA can communicate with any probe, anywhere. Even if it's on the other side of the Sun. :-)

        (No, I am not privy to the exact locations of anything. So take this with a grain of salt. We have a lot of hardware up there, but space is a big place.)
        • Go go gadget mesh networking.

          Perhaps what we need to augment this system is a cloud of smaller satellites in orbit armed with short to mid-range tracking so that they can pick out space debris. When something is found which happens to be in the path of something else, it can flag up with mission control so that (if the colliding objects are easily moveable) the offending items can be shunted out of the path temporarily.

          Alternatively, satellites which realise something is wrong with themselves can send out a
        • by ChrisA90278 (905188) on Monday September 26, 2005 @11:56AM (#13651175)
          You've been reading to much science fiction. Yes there are some cases where spacecraft use inderect means of communication through a relay but this is not done ad-hoc using some general purpose capability built into every spacecraft. In every case wherwe relay is used the capabilty is plaanned from the beginning. The idea of selecting some random spacecraft to use as a relay to soe other random spacecraft just can't work. The orbiters currently on mars were design specifically to relay. Closer to Earth TDRSS acts as a relay between low Earth orbit and the ground. Notice (1) that TDRSS is the relay, thaey are NOT sending data between randon spacecraft and (2) the data are passed only between LEO and the ground, not through out the solar system or even to geosync. orbit. http://msp.gsfc.nasa.gov/tdrss/oview.html [nasa.gov]
        • Have you heard of the NASA Deep Space Network? Every sat and probe we launch becomes part of NASA's network in space. That's why when they had communications problems with the Mars Rover, they were able to send reset commands from a probe heading elsewhere.

          Did you even read the Wikipedia article you linked to? The Deep Space Network is an earth based network of large radio dishes that listen to deep space probes. It's not located in deep space. It's almost always easier to talk to a distant probe with

    • On another note, the picture makes it look like the design hasn't changed much from the original NAVSTAR configuration. I assume that these satellites are merely sharing the same chassis, and have very different internals?

      No, they're substantially different designs. Different manufacturers even (Rockwell vs Lockheed). But if you have a spacecraft performing the same mission, odds are it's going to have a similar configuration. The thing that makes them look most similar is the navigation signal antenna arr

      • But if you have a spacecraft performing the same mission, odds are it's going to have a similar configuration. The thing that makes them look most similar is the navigation signal antenna array (the "Willy Wonka Chocolate Factory").

        Fair enough. I was actually looking at the general shape of the craft, and also was influenced by the story which suggests that the new sats are simply updates of the old ones. On further inspection, it does appear that the two sats are quite a bit different.

        Speaking of which, I
        • by josecanuc (91) on Monday September 26, 2005 @11:42AM (#13651035) Homepage Journal
          Speaking of which, I am wondering what the heck those things on the antenna array are.

          Those *are* the antennas. See the spiral bits on both kinds? That's a conductive strip. It's a helical antenna -- common on satellites. The body that looks like the main bulk is just to give the thin metal something to hold shape.

          • The helical antennae circularly polarize their beams.
          • Some of those antennas also serve to detect nuclear explosions around the globe. For monitoring other countries' tests and as a warning system. Here's a pretty informative overview on the history of GPS [rand.org] (in pdf).

            Man, I dig gps. Used LORAN quite a bit growing up fishing. Useful, but not nearly as easy or informative. Cool.
            J
    • there has been a tremendous increase in miniturization and technology since the original GPS sats

      your statement is naive in that it supposes that mil-spec and rad-hardened technology has advanced at the same rate. Once, it was the military who led the way and consumer devices followed; now, it's the other way round, and in fact the military/space people have big problems with obsolescence, especially with the recent EU rules on Reductions Of Hazardous Substances (often known as "lead free", but actually c

      • Once, it was the military who led the way and consumer devices followed; now, it's the other way round, and in fact the military/space people have big problems with obsolescence

        It's still quite advanced equipment, though. Sure, we're not talking about gigahertz processors and multi-gigabyte memory architectures. Instead, we're talking about old Sparcs, Pentiums, and MIPS from days gone by. That's still pretty advanced stuff. And when it comes to the batteries, LiON batteries were actually developed for spac [nasa.gov]
        • What you say is true. In particular, the "hot new" rad-hardened processor is the RAD750, a hardened PowerPC 750. And yet...
          • When the GPSIIR-M started the design process the RAD750 was nonexistent - major USAF satellites can take on the order of a decade to design and deploy.
          • As I've said elsewhere in this thread, electronics are not necessarily that large a contributor to spacecraft mass.
          • More capable processors may suck down more power, which tends to make other parts of your spacecarft (e.g. the solar a
    • You could say the same for the Model T and Ford's latest Mustang. I mean, they both have wheels, right? They both have engines, right? They both have doors, right? Well, they must have not changed much in the design. Hell, what are we paying these morons for?

      The overall layout of a GPS antenna is not going to change much, you're always going to have to shoot your signal in a certain direction. However, the efficiency of the components and the effectiveness of the materials has all been re-engineered. As wel
  • Compatibility (Score:3, Interesting)

    by slimey_limey (655670) <slimey@limey.gmail@com> on Monday September 26, 2005 @10:24AM (#13650479) Homepage Journal
    Will this improved accuracy come at the cost of compatibility? I already have a GPS reciever, and I don't want to have to buy a new one to make my data more accurate. (Magellan hasn't released new firmware for the SporTrak Basic since 2002, and I'm not holding my breath.)
    • Re:Compatibility (Score:5, Interesting)

      by AKAImBatman (238306) * <akaimbatmanNO@SPAMgmail.com> on Monday September 26, 2005 @10:31AM (#13650533) Homepage Journal
      The new civilian channel is in addition to the existing channel, so your existing equipment should work. As more of these sats go online, you can expect to see gear that provides access to the second civilian channel. (Source [rin.org.uk])

      What I have to wonder, though, is what will they do with the two new military channels? It seems that all the field soldiers tend to use civilian gear because the military gear is too heavy, unfriendly, ugly, and is in short supply. I suppose it would make the missiles hit their targets better, but it would be nice to know that our entire military can use the equipment.
      • When they turn off the civilian channel, you can bet the military is still going to be using their channels.
      • Re:Compatibility (Score:3, Informative)

        by Wyatt Earp (1029)
        In each squad I think there is a military grade GPS reciever, since Afghanistan and more so, Iraq, more and more soldiers are carrying thier own GPS hand held which is good enough for field work.

        The new military channels will be more for JDAM/Cruise Missiles and other targeting systems.
    • Considering that WAAS was announced in 1994 and AOR-W went up in 1997, I don't see much reason for new firmware updates unless there are bugs in the old stuff. There simply hasn't been a lot of change lately.

      That being said, WAAS was specifically designed to be usable by existing Navstar receivers, with only a firmware upgrade. Other improvements, like DGPS and LAAS, require specialized hardware and cooperation from a nearby reference station.

      A new civilian frequency will require new receivers. The clocks o
  • Jamming by whom? (Score:4, Informative)

    by moz25 (262020) on Monday September 26, 2005 @10:28AM (#13650513) Homepage
    As I understand, one of the jamming related problems with GPS is not by criminals/terrorists, but by the government when they see the need. It seems more of a political than a technical nature. That's one of the potential benefits of the Galileo system: to have more than one "supplier" of such information.
    • Re:Jamming by whom? (Score:4, Informative)

      by AKAImBatman (238306) * <akaimbatmanNO@SPAMgmail.com> on Monday September 26, 2005 @10:38AM (#13650580) Homepage Journal
      As I understand, one of the jamming related problems with GPS is not by criminals/terrorists, but by the government when they see the need.

      No, it's more complex than that. Yes, the government can fine tune the results to cause problems for enemies, and even turn off the unencrypted civilian bands if they so choose. However, real methods exist [wikipedia.org] for sending confusing signals that will effectively jam a GPS signal. This jamming can force so called "smart bombs" to rely on internal guidance instead of GPS. The result (hopefully) is that the less precise guidance would cause the bomb or missile to miss the target.

      In practical terms, it seems a bit harder than that to prevent US munitions from reaching their targets. Our guidance computers were well developed prior to the general use of GPS coordinates, and we have the capability to manually deliver ordinance wherever it may be needed. So in the end, this is about keeping the efficiency of our weapons in good order so that we have to risk fewer lives in missed targets and strafing runs.
      • by Dun Malg (230075) on Monday September 26, 2005 @11:04AM (#13650754) Homepage
        However, real methods exist for sending confusing signals that will effectively jam a GPS signal. This jamming can force so called "smart bombs" to rely on internal guidance instead of GPS. The result (hopefully) is that the less precise guidance would cause the bomb or missile to miss the target.

        Problem with active GPS jamming is that it's a "damned if you do, damned if you don't" situation. Any sort of active jamming on the battle field is a huge beacon on the battlefield screaming BLOW ME UP! It then becomes a question of whether or not to turn on the jammer at all, as at most it'll be good for slightly de-accurizing (if that's not a word, it ought to be) one bombing run before being obliterated. If they were cheap enough, maybe, but even still...

        • Re:Jamming by whom? (Score:5, Informative)

          by w42w42 (538630) on Monday September 26, 2005 @11:14AM (#13650817)
          I remember watching a press conference with a military general at the opening of the last gulf war. The press was all lathered up about reports that Iraq was jamming GPS signals - it was assumed the Russians had given them the equipment - and the general commented that whoever was running that equipment had the worst job with the shortest life expectancy in the world at that time. Like you alluded to, any military equipment that relies on an outgoing radio signal instantly becomes a big bright target.
          • Re:Jamming by whom? (Score:3, Informative)

            by nsayer (86181)
            any military equipment that relies on an outgoing radio signal instantly becomes a big bright target.

            Yup [navy.mil].

            And I rather suspect that HARMs are not limited to air defense radar systems...

            • And I rather suspect that HARMs are not limited to air defense radar systems...

              I suggest you remember that the next time you are trying to mooch off my open Access Point ;)

          • Unless that target is another satellite. Say a second generation of GPS that is, strangely, heavier than the first generation.

            There aren't many countries in the world that can quickly and accurately take out a satellie that is beaming a jamming signal.
          • Well, I'm guessing that the guy who sets them up doesn't hang out to watch them work. I'm also guessing they get activated remotely, because I wouldn't want to be the guy driving away from a jammer after it goes live, either.
        • They become suicide bombees instead of suicide bombers.
        • Can they not use multiple stations to make it appear the signal is coming from a place where it is not?
          • Re:Jamming by whom? (Score:3, Interesting)

            by Dun Malg (230075)
            Can they not use multiple stations to make it appear the signal is coming from a place where it is not?

            No, all that would do is present multiple individual targets. Modern direction finding equipment uses such advanced digital processing that it can separately identify two transmitters right next to each other based on subtle differences between them caused by things like inherent manufacturing variations in the transmitters' modulation circuitry.

    • by RebornData (25811) on Monday September 26, 2005 @11:35AM (#13650997)
      I use a laptop-integrated GPS in my car, and I drive by the Pentagon regularly for work. The GPS goes nuts on certain roads that pass near the building... the "position" of the vehicle jumps all over the place. Same thing happens near the capitol building. No suprise of course...

      -R
      • How effective would that jamming really be.... since anyone smart enough to build a cruise missile that could use GPS for guidance would be equally intelligent enough to build a cruise missile to simply home in on the jamming single it's self. Or even with out that could simply free fly the last little distance around the jammer and still hit extremely close to the target.
        And if your talking about a suicide attacker in a plane; how exactly is losing your GPS location when your within visual range of your t
        • How effective would that jamming really be.... since anyone smart enough to build a cruise missile that could use GPS for guidance would be equally intelligent enough to build a cruise missile to simply home in on the jamming single it's self. Or even with out that could simply free fly the last little distance around the jammer and still hit extremely close to the target.

          1) He might not know it is there
          2) The jamming dish (or array of jammers) is presumably far away from any critical installation. If the m
          • I wonder if GPS signals are authenitcated in any way? If not, the Iraquis could do the same thing (redirecting bombs). Of course, US bombs would use the encrypted military channel, so that would prevent Iraqi spoofing (although the US could spoof those bombs if they desired).
    • Phrack magazine awhile ago had an article about building a Low Cost and Portable GPS Jammer [phrack.org].
  • by jurt1235 (834677)
    Step plan to GPS signal jamming profit:
    1. Launch GPS satellites and sell lots of GPS devices
    2. Launch jamming satellite (last week news)
    3. Launch new GPS satellite system which is less prone to jamming
    4. Sell new receivers => profit!
  • Its about time (Score:3, Insightful)

    by dj245 (732906) on Monday September 26, 2005 @10:36AM (#13650568) Homepage
    The sattelites up there are fairly old. Some of the newer ones were launched only a couple years ago, but some have been up there since the early 90's or before. We've had the math equations and the computing technology to be able to put up satelites with around 1m accuracy and better signal strength for a couple years now. Forget about the better jamproofing; with the newer eqipment you can sum the error of your integrals with newer algorithms and faster and determine position that much better less error-prone initial conditions.
    • Re:Its about time (Score:2, Insightful)

      by spankus (140336)
      Do you upgrade your computer every time a new processor comes out????

      Same reasoning (sorta) applies to GPS. Why throw away a $100,000,000 satellite when it hasn't died yet?

      The newer satellites do have some expanded capabilities, but don't plan on seeing those operationally for 5 to 8 years. (It's a long story of governmental mismanagment and strife)
      • by gunnk (463227)
        Do you upgrade your computer every time a new processor comes out????

        Bad analogy, there, you're posting on Slashdot after all...

    • Actually, the satellites themselves have no "accuracy" to speak of. It's entirely up to the receiver. With a good carrier-phase differential receiver, you can get accuracies of ~1cm from the existing GPS satellites.
  • by Anonymous Coward on Monday September 26, 2005 @10:40AM (#13650588)
    " The government is now competing with Europe's Galileo system "

    Lets see :
            Galileo has not launched yet.
            Galileo will not be free.
            The 2R-M was planning before Galileo was anounced.
            Galileo operational capibility is not planned until 2008.

    I'm failing to see the link to the vaporware...
  • which # (Score:2, Insightful)

    by allelopath (577474)
    Is there a way to know which # (1..24) this one is replacing?
    Just curious...it would be fun to know when i turn on my GPS receiver.
    • Re:which # (Score:3, Informative)

      by Ossifer (703813)
      From TFA:

      GPS 2R-M1 will assume the Plane C, Slot 4 position, taking over for the GPS 2A-20 craft launched in May 1993.
    • Re:which # (Score:2, Interesting)

      by Toaste (892190)
      From the article: "GPS 2R-M1 will assume the Plane C, Slot 4 position, taking over for the GPS 2A-20 craft launched in May 1993."

      From the designation of the old satellite, I presume that this position is number 20 on GPS receiving equipment. Just a guess.

      By the way, does anybody know how they plan to move the old one out of the way? According to info found here [astronautix.com] the origional was a 3-axis stabilized NAVSTAR, but I doubt it will be able to move significantly with only its thrusters.

      Another interesting

  • by qwp (694253) on Monday September 26, 2005 @10:45AM (#13650624) Homepage Journal
    I would have been able to post first post,
    had i known about this great achievement. The problem is
    I was suck in my car cause I took a wrong turn due to my
    dam'ed gps navigator. Maybe they haven't turned it on yet..
  • Specs? (Score:3, Interesting)

    by adminispheroid (554101) on Monday September 26, 2005 @11:14AM (#13650818)
    Anybody know what's on the new civilian channel? e.g. is it the same kinda stuff as the two existing channels, on a new carrier? Or is it a new code?
    • Re:Specs? (Score:4, Interesting)

      by Anonymous Coward on Monday September 26, 2005 @11:31AM (#13650966)
      The new civilian code is probably a longer signal that repeats less often. This gives better accuracy but takes longer to determine position when first switched on. Using the lower accuracy signal would allow for a rapid first estimate of postion while the GPS would become more accurate once the position using the new signal was calculated. The military GPS systems use the civilian signal to estimate their position more quickly.
    • Anybody know what's on the new civilian channel?

      New episodes of "Lost".
    • Re:Specs? (Score:3, Funny)

      by stienman (51024)
      Anybody know what's on the new civilian channel? e.g. is it the same kinda stuff as the two existing channels, on a new carrier? Or is it a new code?

      According to one of the press conference questions:

      "We are not going to confirm the content of the new channel at this time. We can state, however, that due to recent FCC regulations affecting our public broadcast, we will not be including Howard Stern for the initial lineup."

      I hope this helps.

      -Adam
  • Humm, I'll start stealing slashdot's content for our new slashsite! :-)

    Want to discuss GPS stuff or anything related to geospatial like GIS and Remote Sensing, visit the brand-new http://slashgisrs.org/ [slashgisrs.org] website. Ad-free and non-for-profit.

    It has just launched (last friday afternoon), so plenty of low uid still available ;-)
  • Russians using GPS (Score:3, Interesting)

    by Thagg (9904) <thadbeier@gmail.com> on Monday September 26, 2005 @11:48AM (#13651081) Journal
    I was fairly astonished to see in the recent issue of Aviation Week that Russia is now building GPS-guided bombs. Presumably this is just using the civilian signal, which could be disabled or degraded in a conflict theater -- but still, it was an fairly amazing development. I suppose that it's conceivable that AvWeek got the facts wrong, and that it was a GLONASS-guided bomb, but they're usually pretty good about that sort of thing.

    Thad Beier
    • by CiXeL (56313)
      they figure they wont turn it off in time before they strike.
    • I was fairly astonished to see in the recent issue of Aviation Week that Russia is now building GPS-guided bombs. Presumably this is just using the civilian signal, which could be disabled or degraded in a conflict theater -- but still, it was an fairly amazing development.

      I would be very very very surprised if the Russians haven't been able to get their hands on a military GPS locator and picked it apart. Sure, the US can still turn it off but then it's lights out for everyone. Isn't the civilian signal ra
    • The Russians have their own GPS satellite network, GLOSNASS.

      -everphilski-
    • by ran-o-matic (667054) on Monday September 26, 2005 @01:00PM (#13651705) Homepage
      GLONASS is a GPS (global positioning system), so Aviation Week is right. One of the first examples of Russian GPS-guided bombs is the KAB-500S-E [kanwa.com] with a 1500 lb device also available.
    • ... to a guided missle.

      He's built a few generations of self-propelled ballistic miss...^R^R^R^ errr.. spacecraft.

      The fact that for 50k dollars, most anyone with enough garage space and basic eletronics/metal working can build a small rock with GPS guidance, makes me glad places like the Pentagon and the Capital Building randomly fuck with civilan GPS channels.

      Question is, if the Russians were not in a war with us, and using smart-munitions which used our GPS system, by not disabling it are we in fact aiding
  • I read that the improved accuracy from Gallileo will be because, apart from the low-orbit sats, there will also be geostationary ones helping the low-orbit ones to determine their own position. If this is correct, just a technology upgrade should not improve accuracy that much. Anybody any numbers?
  • by Animats (122034) on Monday September 26, 2005 @12:31PM (#13651456) Homepage
    We've recently been struggling with a Novatel GPS that receives Omnistar High Precision corrections, and supposedly provides 15cm accuracy. The problem is that it needs to see at least five GPS satellites for Omnistar HP to work. Regular GPS requires only four, but the ionospheric corrections for Omnistar require some redundancy. Five sats are the minimum; six are better.

    Unless you're in a very flat area, in the air, or on an ocean, you won't see five or six sats 100% of the time. 70-80% is more like it. If one of the sats is down (which happens; PRN #5, plane B, slot 4, wss down for 8 days recently [uscg.gov]), the outages are longer.

    GPS uses six rings of four satellites each, with all rings in polar orbit. The four satellites in each ring are 90 degrees apart. So, when a satellite in a ring is near the zenith, it's usually the only one visible in that ring. The original design called for more satellites per ring; with six per ring, you'd always have at least two satellites visible per ring, as long as you could see to within 30 degrees of the horizon. But there was a budget cut in the early days of GPS.

  • How soon? (Score:3, Interesting)

    by Ced_Ex (789138) on Monday September 26, 2005 @12:39PM (#13651508)
    How soon do we realize the benefit of this new satellite? Should we be able to see results right away just from one satellite? Or will we have to wait for 2 more satellites and hope that our GPS connects to the 3 newest ones in order to get the better resolution?
  • Television as GPS (Score:2, Interesting)

    by tocs (866673)
    Why do you need newer and fancier GPS satellites when you can just use satellite television signals [gpsworld.com].
  • As with everything we're moving towards a kind of distributed mesh of systems. I think soon there will be devices that not only pickup GPS, GLONASS (whats left of it) and when it comes, Galileo, but will also have other fall-backs such as databases of cell-phone networks, TV and radio transmitters and other satellites such as TV, net and phone. Hell it could even go as far as using a camera to read the stars, and with a built-in detailed map of the entire earth you'll most likely be able to rely on terrain
  • GPS satelites work essentially the same was as celestial navigation (sailing). You note the time and angle to a star. That produces a very large circle of possible locations where that star could be seen, at that time. Then you do the same with another star, which will also give a large area of location solutions - but there will be a narrow overlap where someone could see both stars, at the given angles, at the given time. Now, you do the same with a third star. You've narrowed your position on the earth
    • This is not really how it works.
      With GPS you don't measure any angles. The satellites send the current time, and information about their location (in the form of ephemerides).
      The position is determined by examining the time differences between the received signals. That is why you need 4 satellites: there are 4 unknowns (your position in 3 dimensions, and time) and to solve the set of equations you need 4 equations.
      With 3 visible satellites you need to make 1 assumption (e.g. constant elevation) to determ

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