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Accuracy of radio/GPS-like system?

Discussion in 'Electronic Basics' started by Wildepad, Oct 8, 2005.

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  1. Wildepad

    Wildepad Guest

    Thanks, I'll investigate that (my original idea of two position
    readings from different ends of the device apparently won't work, so
    I'm open to new ideas).
    The difference in time that it takes signals from different points to
    reach the device, the same way GPS works.

  2. Wildepad

    Wildepad Guest

    It will be attached to a device that has 8 probes sticking out of it.
    A reading is taken only when the probes have been stabbed about 5
    inches into the ground.

    I termed it 'handheld' because it can't be wheeled around or
    backpacked in. The person using it will be crawling around on their
    knees most of the time, so it has to be small and light.
  3. Right...8 square inches is ambiguous.

  4. ehsjr

    ehsjr Guest

    Um, the word "about" which you left out. The phrase
    "about eight inches square" does not specify that even one edge
    is straight, let alone that the object is square. An old rag
    torn from a tee shirt could be described as "about 8 inches
    As I showed with the diagram, and you recognized below, ambiguity
    made a 90 degree difference. And it can make a difference between
    0 and 180 degrees. The line must be defined, just as you state:

    Pick a straight line --
    Exactly. I drew the reference line, removing the ambiguity.
    And it does completely change things - from the ambiguity of
    no reference line to the precision of a reference line.

    Now that is ridiculous. You won't be orienting text in the field.
    The text represents the handheld unit, as you said. It is not, nor
    does it represent, a reference line.
    It is still not completely described, but at least now we know
    it *is* square. And, since you say we can use an edge as the
    reference line, we know the edge is straight.
    This is what you don't seem to get: *you* are the one who has to
    specify the orientation. We've (responders to your post) have never
    seen the hand held unit. We won't be using it. We don't know
    what it does, other than the function (position, angle) you've
    requested. Obviously that information is used for something, which
    for all we know is also incorporated in the unit. For all any of us
    know, the surface is irregular, making placing an arbitrary line on it
    difficult or impossible. We're trying to consider the engineering
    aspects, which, like it or not, requires a clearer definition
    that you have given. For all we know, there could be some feature
    on the hand held unit that you want to aim at something.
    Um, yes it was. You did not describe a reference line, or state that
    we could pick any line contained within the perimeter of the object we
    wanted. Thus the unit remained the equivalent of a mathematical point
    with respect to determining whether it is parallel to a line, which
    is the context being discussed.

    I have no interest in arguing with you. What has transpired
    can be netted out to the salient point: you want accuracy beyond
    what you can get with the approach you have described, in my
    opinion. To get the accuracy you want, if it is possible within
    your budget, two things will be needed: a different approach and
    precise descriptions. You could google on GPS accuracy to gain an
    understanding of what accuracy to expect from that portion of the
    setup and function (ie position) and also look into determining the
    angle optically or via RF phase detection or perhaps some other
    method to see what kind of accuracy you can get. I also think you
    are doomed to lower accuracy unless you can lock the hand held unit
    in place during the measurement period. You have two axis to worry
    about for determining the angle that the line on the hand held unit
    has to the line formed by towers A-B or B-C or A-C. You have not
    specified that the angle in the vertical plane is of no interest, but
    it *seems* sure that it is not. However, a vertical angle will affect
    distance, just as a horizontal angle will, and if the angle is to be
    determined by trigonometry, the distance affects the computation of
    the angle. We can help you consider some of the factors with which
    you may not be fully familiar. If you won't consider them when
    they are brought up, we can't help you.

  5. The amplitude, yes - but if you're below, say, 175 MHz, the operator's
    presence shouldn't affect the _phase_ enough to make it unreliable. If
    you're using microwaves, then all bets are off, and you'd need four
    directional antennas - and in any case, according to the OP's description,
    the operator will be just more ground clutter.

    Good Luck!
  6. Yes, "8 square inches" is ambiguous, as it doesn't specify the aspect
    ratio - a ribbon 1/4" wide and 32 inches long has an area of "8 square
    inches." "8 inches square" removes any ambiguity as to aspect ratio:
    it's a square, 8 by 8 inches.

  7. Jasen Betts

    Jasen Betts Guest

    hmm, if the antenna is reasonably clear of the ground (or the ground is
    flat) that direction finding antenna scheme could give you the data needed
    to determine position and direction

    the directions to the three transmitters is enough info to uniquely define a
    point inside (or outside) the triangle.

    but I'm not sure how much precision can be had without breaking the bank.

    5 degrees would be pretty simple, 1 degree would be aproaching the practical
    limits afaict.

    You may do better with an optical system.

    put differently pulsed IR (or visible light) sources on the poles and use a
    rotating scanner ( to detect and measure the angle to each one...)

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