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Seeking 100-150 meter communication solution

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Newbie level 3
Oct 12, 2009
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I apologize in advance, my knowledge of electronics and radio communication is extremely rudimentary and I have no formal education in either, but I'm seeking advice on a project I'm working on, and any input is greatly appreciated.

I'm trying to figure out a solution (or possibly build/have built) for communicating over a relatively small distance (between 100-150 meters) to some sort of very small receiving device, and then have that signal echoed back to the origin. The purpose would be to have the time between sending and receiving measured so that the distance between the two objects can be established. The echoing device will be mobile but the transmitting/receiving end can, and probably would be fixed.

I've read that different frequencies or wireless communication methods have different ranges as well as differing levels of reliability, strength, etc. Are their shorter range signals that would increase the reliability of the signal? Are their other ways to measure the distance between these two points besides a simple 'ping' method? Are their solutions that already exist for determining the direction of the received signal? To refresh the signal data multiple times per second, what what impact would this have on the frequency?

As for the 'echoing' object, what would be required to develop this sort of device? Can you build a small chip, that does not need a power source (or at least a small power source) to accomplish this? What elements are absolutely necessary on the 'echo' end that cannot be located on the transmitter/receiver end?

As for the transmitter itself, where would be a good place to find a pre-build piece of equipment that could be used for this? What's the cost associated with this sort of device? What would be the easiest way to relay this information to a computer?

This project is still at the beginning of the conception stage, so any input or ideas at all are extremely helpful. Oh, and are their any potential pitfalls that I should be aware of with something like this?

Thanks in advance.

This is starting as a hobby project, but depending on how well it works out, might possibly become commercial. Who knows.

Oh, and I received a suggestion about lasers. The system cannot depend on any kind of visual contact.

I've also been given a suggestion of using RADAR technology for this. Where would be a good place to find more information about it? The first thing that pops into my head is that RADAR is a relatively expensive solution... is that right?

Maybe I could get some more feedback if I provide a hypothetical application of what I'm trying to accomplish.

Imagine a person is standing in a crowded area roughly 100x100 meters in diameter and I'm located just outside of this area. This size and location of this area is fixed at all times as is my position. There are a lot of other people walking around inside this area as well as the person I'm trying to track who may also be moving around. So basically, I want to track the distance between myself and this person among a crowd of other people who may or may not be moving. The person designated to be tracked is fixed (for the purposes of this example) and I can give that person something or do something to them (i.e. it was suggested I could use UV paint) that will help me identify and track them. If I give them something or attach something to them it must be very, very small and require almost no power (or none at all, if possible) to function.

Visual solutions would not work, because my fixed location would not always have visual contact with the subject. A wired solution is also not really going to work because of all the movement of the surrounding traffic and of the target.

The absolute goal of the system is to determine, with high accuracy, the distance between myself and the target person in this area.

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