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Accuracy of radio/GPS-like system?

W

Wildepad

Jan 1, 1970
0
Put two loop antennas on the handheld, set the transmitters at the
towers to three different frequencies, and have three receivers
in the box, which can resolve the relative phase of the two loop
antennas' signals (quadrature detector?); this will give you relative
angles to the three towers, then just trig it out.

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).
With just radio, there's no way to determine distance, unless you're
using some kind of radar.

The difference in time that it takes signals from different points to
reach the device, the same way GPS works.

Thanks!
 
W

Wildepad

Jan 1, 1970
0
the only problem I see with this is that the operator holding the device
will distort the readings.

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.
 
E

ehsjr

Jan 1, 1970
0
Wildepad said:
Um, what part of "square" is missing from "eight inches square"?

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".
What difference could that possibly make?

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 --
front edge, side, diagonal, whatever -- as long as you don't change
what is defining the orientation of the unit between readings, how the
'angle of the unit' changes relative to the baseline at consecutive
readings is being acquired.




I was asking what accuracy is possible. Adding that I need 1/4 degree
expresses my disappointment at there not being anything readily
available.




You added four different lines, completely changing the situation!

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.

The orientation of the text, which is how you originally represented
the unit, is parallel to the A-B 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.
But if you want to consider that the orientation of the unit is
determined by the side, fine -- it can't possibly matter since such
information is only used when determining how the angle has changed
between readings.

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, no, it wasn't. I described a three dimensional object -- width and
length (eight inches square) and height.

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.

Ed
 
R

Rich, Under the Affluence

Jan 1, 1970
0
the only problem I see with this is that the operator holding the device
will distort the readings.

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
 
R

Rich, Under the Affluence

Jan 1, 1970
0
Right...8 square inches is ambiguous.

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
 
J

Jasen Betts

Jan 1, 1970
0
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.

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
 
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