Shop OBEX P1 Docs P2 Docs Learn Events
Single cm gps on Kickstarter — Parallax Forums

Single cm gps on Kickstarter

W9GFOW9GFO Posts: 4,010
edited 2013-09-05 17:28 in General Discussion
Wow, how did we (meaning all of you + myself) miss this one?

http://www.kickstarter.com/projects/swiftnav/piksi-the-rtk-gps-receiver?ref=live

Do you believe the claims?

Comments

  • GadgetmanGadgetman Posts: 2,436
    edited 2013-09-05 02:00
    I call 'loads of bovine loads' on it.

    True 1cm accuracy can only be achieved by using a reference base at a KNOWN location.
    The best they can achieve with that kit is '1cm in relation to the reference'.
    And that assumes that they get their SW right.

    The kits they're foisting on the backers are 'pre-release' kits that doesn't yet have all the SW.
  • prof_brainoprof_braino Posts: 4,313
    edited 2013-09-05 07:36
    First, in addition to measuring the code phase, an RTK GPS receiver measures the phase of the carrier wave that the code is modulated upon. The carrier has a wavelength of about 19 centimeters. This makes it possible to measure to a much greater degree of accuracy than the 300 meter code, but there is a catch - there are an unknown number of whole carrier wavelengths between the satellite and receiver. Clever algorithms are required to resolve this "integer ambiguity" by checking that the code and carrier phase measurements lead to a consistent position solution as the satellites move and the geometry of the problem changes.

    Ok, I'm no expert, but this seems like it might make sense. Any expert comment?
    Second, an RTK GPS receiver is able to reduce the ionospheric error with the help of an additional reference receiver. The ionospheric delay varies only slowly with location, so with a nearby reference receiver, the delay is almost the same for both receivers and can largely be cancelled out. This is why an RTK GPS system uses two receivers

    So, it this something like getting the position of a cell tower for reference? Those are known, and don't move often. If it relies on cell towers, it would then of course depend on the phone system being up and running. So the government could disable it if there was a need.

    I don't know. Conventional wisdom says this can't be done. But conventional wisdom is a quick to remind us that every problem gets solves eventually, particularly if it is impossible.
  • PublisonPublison Posts: 12,366
    edited 2013-09-05 08:08
    If they are using Differential GPS, what Gadgetman said in post #2 applies.
    The idea is simple. Put the reference receiver on a point that's been very accurately surveyed and keep it there.This reference station receives the same GPS signals as the roving receiver but instead of working like a normal GPS receiver it attacks the equations backwards.
    Instead of using timing signals to calculate its position, it uses its known position to calculate timing. It figures out what the travel time of the GPS signals should be, and compares it with what they actually are. The difference is an "error correction" factor.
    The receiver then transmits this error information to the roving receiver so it can use it to correct its measurements.

    From the Timble site:

    http://www.trimble.com/gps_tutorial/dgps-how.aspx
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-09-05 08:29
    Shouldn't this thread be titled 4 cm gps on Kickstarter?

    The video claims 4 cm precision and the text says "centimeter level precision" as the precision is measured in centimeters not meters. I think it's fair for them to say 4cm precision is "centimeter level precision".

    I'm unconvinced they can reach their stated goal of reaching this precision. My gut (which is often wrong) tells me they are at least legitimately attempting to do what they say.

    I think it's kind of odd how close their name sounds to the new CMUcam5's name Pixy.
  • prof_brainoprof_braino Posts: 4,313
    edited 2013-09-05 09:00
    W9GFO wrote: »
    Wow, how did we (meaning all of you + myself) miss this one?

    $900 is how!
  • PublisonPublison Posts: 12,366
    edited 2013-09-05 09:07
    prof_braino wrote: »
    $900 is how!

    From what I have read, professional Land Surveyors pay over $5000 for the same accuracy, and need to sign some agreement.
  • pjvpjv Posts: 1,903
    edited 2013-09-05 09:12
    Hi All;

    I have messed with RF, and in particular, Code Division Multiple Access (CDMA) signalling which is what GPS uses, quite a bit. Yes, as claimed, clever algorithms are required to sync up to the Pseudo Noise codes quickly, but this is just "standard" GPS technology.... nothing new here.

    As far as directly measuring the phase of the 1500 MHz carrier burried DEEP within the noise at, say, negative 150 dbm..... well I'm a skeptic here.

    So, for the claim of centimeter precision.... I would not even bet a nickel !

    Cheers,

    Peter (pjv)
  • David BDavid B Posts: 592
    edited 2013-09-05 09:38
    Carrier phase GPS is a different thing than differential GPS.

    The idea of solving the location equations by using the centimeter-level carrier phase instead of the meter-level modulated code phase isn't new - it's been done for at least the past 20 years in surveying. My company sent me to take a 5 day course at Trimble around 1992 for a project they were considering, where Trimble explained all about carrier phase locating, most of which was over my head mathematically, but apparently they were able to make it work.

    But way back then, it took hours of post-processing the received GPS signals to be able to get the best final result.

    If someone has figured out how to embed that processing onto a little chip, then that would be impressive.

    http://www.trimble.com/gps_tutorial/sub_phases.aspx
  • prof_brainoprof_braino Posts: 4,313
    edited 2013-09-05 10:40
    David B wrote: »
    Carrier phase GPS is a different thing than differential GPS.
    At the speed of light the 1.57 GHz GPS signal has a wavelength of roughly twenty centimeters, so the carrier signal can act as a much more accurate reference than the pseudo random code by itself. And if we can get to within one percent of perfect phase like we do with code-phase receivers we'd have 3 or 4 millimeter accuracy! Yeeow!
  • W9GFOW9GFO Posts: 4,010
    edited 2013-09-05 12:04
    Duane Degn wrote: »
    Shouldn't this thread be titled 4 cm gps on Kickstarter?

    I changed the thread title (from 1 cm gps...) to reflect this claim;
    An RTK (Real Time Kinematic) GPS system gives positions that are 100 times more accurate - down to single centimeters.
  • Duane DegnDuane Degn Posts: 10,588
    edited 2013-09-05 12:55
    W9GFO wrote: »
    I changed the thread title (from 1 cm gps...) to reflect this claim;

    Fair enough.

    Doesn't the term "single centimeters" seem kind of strange?
  • ercoerco Posts: 20,256
    edited 2013-09-05 17:28
    If it works, robot cars are that much closer. Seems like the government would fear this accuracy. Don't they occasionally reduce gps accuracy (selective availability) when they choose to?
Sign In or Register to comment.