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On 2005-10-08, Wildepad <noreplies> wrote:
> How accurate could something like this be:
>
> Three antennae on ten foot towers are set up about 100 yards apart.
>
> A handheld unit, no larger than about eight inches square and an inch
> or so thick, uses signals from those to determine its position within
> the triangle (the unit will usually be held fairly parallel to the
> ground, rarely at greater than a 45 degree angle).
> Now it can't just figure out it's basic spot but also what angle it is
> to a baseline -- that is, it isn't enough to say that it's 83 feet
> from tower #1, 212 feet from tower #2, etc., it also has to read out
> that it is 18.42 degrees to the line between towers 1 and 2.
it needs to knows how far off the ground it is, or it needs a fourth
transmitter.
> In other words, on a map with the towers plotted, it must be possible
> to determine exactly where the unit is _and_ where it is pointing.
>
> 'Absolute' accuracy is not as important as repeatable and relational
> accuracy -- that is, it can read 83 feet from tower #1 when it is
> actually 82.4 feet, as long as it always reads the same thing at that
> point and it reads 41.5 feet from that tower when it is exactly half
> as far away (when it is actually 41.2 feet away). ((Does that make
> sense?))
>
> The complete unit has to be no more than $1500, and the towers with
> transmitters no more than $500 each (these do not have to be
> weatherproof -- they will be taken down after each day's use -- pipe
> towers whose 'feet' slip into pipes driven into the ground are
> probably best).
>
> What accuracy could be reasonably reached?
> Within a yard, within a foot, within an 1/8 of an inch?
>
> Perhaps more importantly, what accuracy in the direction it's pointing
> could be reached? (i.e. if there were two such units, and you plotted
> their readings on a map in a computer, could you determine whether
> laser pointers attached to them will cross 250 yards outside the
> triangle?)
that depends how close they are to each other.
>
> Any constructive answers appreciated.
differential GPS will give you centimetre precision (or better) with just
one tower. - should be uner $1000 for the tower.
angle is harder to do, I know there are inertial angle sensors
I have no idea of their cost, precision or operating requirements.
Bye.
Jasen
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