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GPS and Barometric pressure sensor recommendation please

Discussion in 'Embedded' started by Sonoman, Aug 2, 2004.

  1. Sonoman

    Sonoman Guest

    Hi all:
    I am in the process of designing a project for school. This project
    requires a GPS sensor and a barometric pressure sensor to determine
    position in space. I have not much knowledge about sensors yet but I am
    trying. I have spent several hours googling a few terms and so far I
    came up with the Garmin 18 GPS sensor, and I am still not sure which
    barometric pressure sensor I will use. These sensors have to communicate
    to a basic stamp 2pe microprocessor. My question to you guys is: do you
    know of any sensors (GPS and/or barometric) that I should research for
    this project? Or may be you could tell me from where I could scavange
    these sensors. Any help will be greatly appreciated.

    Sonoman, Aug 2, 2004
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  2. To make a cargo bomb, you really need only one of these variables. Why
    do you want to know both?
    Bryan Hackney, Aug 2, 2004
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  3. I am in the process of designing a project for school. This project
    You ARE aware that GPS will give you altitude for free, right?
    Lewin A.R.W. Edwards, Aug 2, 2004
  4. Sonoman

    Paul Burke Guest

    It's less accurate than position, and that's bad enough in many situations.

    Paul Burke
    Paul Burke, Aug 2, 2004
  5. Sonoman

    Rick Merrill Guest

    Bryan Hackney wrote:
    It really is the end of an age of innocence. From now on do we only
    help those who use their Real Name and email address??!! - RM
    Rick Merrill, Aug 2, 2004
  6. johannes m.r., Aug 2, 2004
  7. Sonoman

    Sonoman Guest

    No, I was not aware. As far as I know it will give you longitude and
    latitude which is like the X and Y axis. That sounds very interesting
    and I will look into it. Thanks for the tip.
    Sonoman, Aug 2, 2004
  8. Sonoman

    Sacre Vert Guest

    I use a GPS unit that has a quoted accuracy of 4 metres for 50% of the
    time. This corresponds to the true position being within a ~50 square
    metre area centred on the measurement result. If you consider that
    the surface area of the earth being some 510*10^12 square metres then
    localising your position to 1 part in 10^13 is not bad.

    Sacre Vert, Aug 2, 2004
  9. And, not to forget, an unbelievably accurate clock, too. Physicists
    have been known to use a GPS receiver in experiments weighing
    thousands of tonnes (i.e. which are essentially impossible to move
    even if you were to try rather hard) just because they wanted to know
    the exact time.
    Most "consumer-grade" GPS receivers do indeed work like that. They
    don't trust themselves enough (or can't afford the increased amount of
    processing) to extract altitude, too. Instead, they use a reference
    ellipsoid or terrain altitude information as a constraint to extract
    the (lon/lat) position.
    Hans-Bernhard Broeker, Aug 2, 2004
  10. Vaisala radiosonde might be what you are looking for. It contains
    temperature (two of them), humidity and barometric sensors. RS80 variety
    also comes with 8 channel GPS transiever (it transmits Doppler shifts, so
    you'd need to figure out how to obtain the position from them). They are
    quite cheap on ebay, around 10 quid.

    The links to check:


    Vadim Borshchev, Aug 2, 2004
  11. Sonoman

    Tauno Voipio Guest

    Please note that the sonde requires a special receiver/processor
    to decode the downliked position and weather data. The GPS receiver
    in the sounding system is able to resolve movements, but not
    absolute position, which is not needed in its primary purpose.

    The budget permitting, I think that the solution is a credit-card
    GPS module. They are available from most primary GPS manufacturers.

    There is one more point to take into account, if the altitude is to
    be measured with a pressure sensor: the reference pressure must
    be set to match the current weather situation (called QNH or QFE).

    Tauno Voipio
    tauno voipio (at) iki fi
    Tauno Voipio, Aug 2, 2004
  12. I was a little harsh, but a bit more information about the project would be nice.
    Optional, but so is assistance.
    Bryan Hackney, Aug 2, 2004
  13. IIRC, at sea level a 1 millibar change in atmospheric pressure
    corresponds to about 8 meters elevation change. An uncalibrated
    pressure sensor may have daily elevation drifts of about 80 meters
    with variations in barometric pressure.

    I've been using the UBLOX GPS MS-1E for about a year now. When I have
    5 or more satellites in view, the altitude numbers are usually stable
    to 5 meters or less. However, it is important to realize that GPS
    elevation numbers are actually computed as height above a theoretical
    geoid, and the accuracy of the elevation depends a lot on the accuracy
    of the geoid model---which is generally supposed to be good to within
    60m worldwide, I think.

    For a barometric sensor, you can pick from a number of 5V units
    from Motorola that have 0 to 4.5V output for 0 to ~15PSI. See the
    Digi-Key catalog.

    Of course, everything gets easier and more accurate if you can get
    reference data for a known location.

    Mark Borgerson
    Mark Borgerson, Aug 2, 2004
  14. Sonoman

    Sonoman Guest

    So what you are telling me about the pressure is that I have to
    calibrate the barometric pressure sensor every time before I use it to
    take into account the weather changes? Interesting.

    I did not think that was necessary for my application. I want to know
    the altitude of each point in relation to each other and not against a
    reference (i.e. sea level), unless the weather changes in between
    readings I think I'll be ok without calibration, don't you think? I just
    don't want to complicate my life if I don't have to. Please advise.
    Sonoman, Aug 2, 2004
  15. Sonoman

    Wim Ton Guest

    So what you are telling me about the pressure is that I have to
    Yes, this is standard procedure for mountaineers, who often use barometric
    presure an hight indication. 19th century technology, no batteries needed,
    smaller and lighter than GPS.

    Wim Ton, Aug 2, 2004
  16. A bit confused, the natural 3D solution of GPS is in ECEF
    (earth centered earth fixed) coordinates. So latitude and longitude
    are dependent on various models, as well as altitude.

    The usual 3D solution requires four satellites to solve four unknowns,
    position and time. If there are only 3 satellites available, then
    one can assume an altitude to solve for three unknowns. This is
    normally a fall back feature.
    James D. Veale, Aug 2, 2004
  17. Sonoman

    Alan Balmer Guest

    I haven't seen a consumer-grade GPS which doesn't provide altitude.
    Even the $99 USD Delorme unit calculates altitude.
    Alan Balmer, Aug 2, 2004
  18. No, I was not aware. As far as I know it will give you longitude and
    I dunno about "most". I have a cheap, bottom-of-the-line Garmin eTrex,
    and it solves for altitude. Some of the really old first-generation
    units might not, I guess (IIRC some of them only had three rx channels
    Lewin A.R.W. Edwards, Aug 3, 2004
  19. Sonoman

    Paul Burke Guest

    It is if you need to know whether you'll meet that 20m high cliff at the
    bottom or the top, or where the corner of the house you are going to
    build is NOW, after all, it will be there all the time, not just 50% of
    the time, and 4m away could be on next door's land.

    Paul Burke
    Paul Burke, Aug 3, 2004
  20. Sonoman

    dmm Guest

    Circuit Cellar had an article about an altimeter datalogger in their February 2001 issue.
    It won 3rd prize in their Design2K contest.

    Good luck with your project.

    dmm, Aug 3, 2004
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