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.

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?)

Any constructive answers appreciated.