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Cheapest DGPS receivers

Discussion in 'Electronic Design' started by linnix, Jan 12, 2007.

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  1. linnix

    linnix Guest

    How about this:

    If we have all day to track 100 positions and
    looking for the cheapest solution.

    We install 100 remote receivers (micro + RF) and
    sample the IF (20 to 30 MHz) for 10 to 20 seconds
    (100 MBytes)? We can transmit this with a slow
    serial link to a central station.

    We can then process this 10G data in DGPS mode,
    using the Philips's software solution.

    Will this work?
  2. linnix

    linnix Guest

    OK, I guess we need to use a mobile rover,
    using the new L2C signals.
    Someone has done this without the L2C signal,
    they claim to have cm accuracy.
    There are currently 2 or 3 birds with L2C,
    so we need to time the daily sampling windows.

    The plan is to have two sets of dual frequencies
    RF front ends (L1=1.5GHz L2=1.2GHz).
    Using a micro, we can sample and store a couple
    of seconds of the 1Mhz sub-carrier signals.
    Differential measurements can be taken with
    both mobile and stationary (fixed reference position)
    samples together.

    The samples would be WiFi'ed to a central computer
    for post processings. We can even archive the 1G bytes
    samples on DVD (3 to 4 days of samples), in case we
    need to go back and recalibrate the data.

    Any suggestions or comments?
  3. Ted Edwards

    Ted Edwards Guest

    Any WAAS enabled GPSR would give you 2 meter accuracy on a good day if
    you averaged for a couple of minutes. This assumes that that is good
    enough (you don't sat what accuracy you need) and that you are in an
    area with a decent sky view and can receive WAAS (you give no indication
    where you are). Waypoints will store the location(s).
  4. linnix

    linnix Guest

    Yes, we have WAAS enabled. The sky is clear.
    We want 0.1 meter accuracy. Someone did a terrain
    mapping within a couple of cm with DGPS,
    using L2 (military band) sub-carrier corrections.
    I think that's how they make survey grade GNSS.

    I believe we can make use of the new L2C (civilian band) codes
    for similar result. We don't need to process the data immediately,
    just to log enough information for later processing. We can tap
    into the L1 signal from a standard GPS. My question is whether
    we can use a different crystal and get the L2 signal?
  5. Iwo Mergler

    Iwo Mergler Guest

    Yes. In fact, if you chose your frontend wisely,
    you only need to sample about 10MBits/sec - 2 bits
    @ 5MHz. It's not an easy job though. You may want
    to talk to NXP (Ex-Philips Semiconductors) first,
    before investing a lot of work in this.

    However, unless you are planning to make thousands
    of mobile devices, it's probably cheaper (and lower power)
    to find a complete GPS chipset which outputs

    Kind regards,

  6. linnix

    linnix Guest

    The problem is finding a chipset for L2 (1.2G).
    Some internal filters tuned to L1 (1.5G) would not work.

    In additional, we want to use the NIST chip atomic clock (E-15
    instead of the TCXO crystal (E-10 precision). This would elimate the
    carrier phase unknown and simplify the PLL.

    The atomic clock chip contains laser tuned cesium particles in silicon
    It does not seem too difficult to make. A Differential mode Atomic
    Positioning System (DA-GPS) would be perfect for us.
    Is anyone working on the atomic clock, other than the NIST reseach

  7. MassiveProng

    MassiveProng Guest

    One big issue would be the band you intend on "transmitting to a
    central location" with.

    You'd better get a license, and get informed about what is
    available, and at what power levels. Each station needs a power
    supply too. Solar.
  8. linnix

    linnix Guest

    One idea is to charge solar cells on stations with a high power laser.
    Since the critical logging time is in bad weather, without sun light.
    A laser (with warnings) could also deter unwanted visitors.

    ISM at 900MHz should be more than enough bandwidth.
    Initially, just a mobile rover to power up
    the sensors and download the log. Hardwiring the stations at 115K
    serial link could work, if we can get enough atomic clocks. They said
    the atomic clocks can be battery powered and with precision of a
    of a picosecond. All the atomic clocks could still be out of phase,
    but at
    a constant.

    My question is whether anyone has reproduced the NIST result, and at
    what cost?
  9. If you are using lasers, then why all the BS about DGPS. You can use
    the lasers directly to determine the distances between points to at
    least mm accuracy. And no, if you are using a non-eye-safe laser, then
    there is no way in hades you will be able to use them in a public
    accesible place, even with warnings...

  10. linnix

    linnix Guest

    OK, laser does not make sense at all. With laser measurements, you
    have to do lots of manual works. The site is actually dangerous
    to keep the public away, except for the maintenance people.
    Of course, they can turn off the power while working.
    I guess high power voltage lines would be fine.
  11. Why dangerous? Talk to some of the geologists who watch volcanos and
    earthquake faults. They set up laser monitoring stations all the time
    that have to be unmanned but measured in mm accuracy. You should be
    able to buy this stuff 'off the shelf' somewhere!

  12. linnix

    linnix Guest

    That's all relative. Mudslide is no big deal for geologists and us,
    but it's dangerous enough for the public and especially teenagers
    fooling around in the area.
    To reposition the laser automatically over multiple points is not a
    simple matter.
    Same for GPS equipments, if you have the money to spend.
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