How does GPS know where you are?


We take for granted how easy it is to get around these days. A decade ago, when my friends and I were turning 16 and getting our driver’s licenses, a big, fat Thomas Guide was the gift of choice — I mean, how else would you find your way around the city? Then TomTom, Magellan and Garmin came around. And now, with the proliferation of smartphones, maps of the entire world are in the palm of our hands.

But even when maps go awry, we tend to take the technology for granted. Here’s an explanation of how GPS navigation works — so you can appreciate it a little more the next time it gets you where you need to be.

It really is a wondrous and remarkable piece of technology.

  • impaler

    It is a great piece of tech, but not without flaws. It transmits over L-Band, over a very weak signal, and is subject to degredation. Also, GPS really doesn’t actually know where YOU are…you’re triangulated but it’s a passive service.

  • the_other_stevejobs

    Not sure exactly what’s wrong with us transmitting over L-band… its unaffected (mostly) by water, so it is all weather – which seems like a plus to me.

    Also, GPS is currently running at some of the lowest user range errors ever – in 2011, GPS provided an average user estimated range error of 0.7629 meters – with a requirement of 6 meters, that means GPS is operating at almost a magnitude better than it needs to. This year, its still going strong: The new IIF’s are providing insanely great UREs in the very low sub-meter range. As we launch more IIFs, along with the good performing IIR-Ms (okay, except SVN 49), things should actually get better – assuming we can launch more satellites at a steady rate.

    The mashable article mentioned ionosphereic errors – GPS provides users with a fairly good correction level, and many users with high-accuracy receivers that listen to both the L1 and L2 bands are able to completely wipe out ionosphere errors. When the new control system comes online for GPS, L2C will be a simple, modern navigation message (MNAV) that not only will allow all users to get navigation data on two separate frequencies (now, civilians only get nav data on L1 C/A), but the L2C will be much more accurate, and it suffers less multipath (when the signal bounces off of buildings) because of its advanced binary offset carrier signaling.

    Adding on WAAS in the US, EGNOS in Europe, QZSS in Asia, and other GNSS systems to augment GPS, users should be able to get crazy accurate location and time anywhere on the earth – in all weather.

    … or something. 😉

    • the_other_stevejobs

      btw: the first version of the Chinese system, BeiDou does know when you are – as you send it a signal, and it does the calculation of your location for you, then sends it back.

      now, how that could be a bad thing in the hands of a totalitarian, closed society, i couldn’t tell you. 😉

  • Yet another benefit of federally-funded pure research.

    • the_other_stevejobs

      GPS was conceived and built to be one of the most important military systems ever created by the US to enable war fighters to be effective in their efforts to kill and destroy enemies of George Bush and Barack Obama.

      Thanks to GPS, US and allied warfighters can not just place a MK82 onto a target from miles above the earth onto the target building that has no idea they are moments from being atomized, but they can pick which window pane they want to put the bomb through to send the 7th century, boy-raping scum of the earth to their 72 virgins.

      Oh, and ditto what you said too.

      • Wow. Got something you want to get off your chest, there?

        • the_other_stevejobs

          Your comment is tangential and confusing.

          Its obvious your comment didn’t take into account the real purpose of something that your humanistic religious belief system led you to worship as “pure”.

          When one beatifies governmental work, its useful to understand in full what one is giving a religious affection toward.

          As for getting things off one’s chest, I am totally cool with turning the products of madrassas into grease spots using the insanely accurate navigation solutions provided to weapons by the GPS as this will reduce the numbers of women and children maltreated by backward religious beliefs.

          The real question is, are you so inclined?

          • I’m inclined to avoid making blanket generalizations about people I’ve never met. Yourself included.

          • Its obvious your comment didn’t take into account the real purpose of something that your humanistic religious belief system led you to worship as “pure”.

            Lighten up, Francis.

  • As long as it doesn’t care why I’m there, it still okay.

  • Sorry Shawn but that Mashable article is really bad.

    Satellites, 186,000 miles per hour, triangulation, atomic clock, foliage, signal bouncing off of large buildings – there I “explained” it with less words. The delight in that article? “chances are it doesn’t work underground”. No shit, Sherlock.

    The Wikipedia entry for GPS is about a hundred times more informative than this dribble. What’s worse is that it also gets information wrong.

    Sorry to be this harsh but the sheer fact alone that GPS triangulation is PASSIVE, meaning that your GPS does NOT, I reapeat NOT communicate with the satellites (communication implies sending and receiving) makes this not a good thing to link to.

    • Agreed. This is a terrible article full of inaccuracies and spends two paragraphs at most on how GPS actually works, the rest is just dribble. Just more noise in the blogosphere

  • mycroftxxx

    Lots of technical errors in the article, but one of the biggest is the statement that you can do GPS triangulation with only tracking 3 satellites. Only if you have an atomic clock in the GPS receiver! A time error of, say, 1 millisecond (0.001 sec), which is a pretty damned accurate clock for most purposes, is a position error of nearly 200 miles; so you have to track a minimum of four satellites so that you can determine three positions (latitude, longitude, altitude) PLUS time.