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

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.

Thanks!

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. Tom MacIntyreGuest

Right...8 square inches is ambiguous.

Tom

4. ehsjrGuest

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
square".
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

Ed

5. Rich, Under the AffluenceGuest

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!
Rich

6. Rich, Under the AffluenceGuest

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.

Cheers!
Rich

7. Jasen BettsGuest

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...)

Bye.
Jasen