Seeking 100-150 meter communication solution

[relic180]

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.

[goldstar]

Many questions, many solutions. Do some research on radar devices as well as distance sensing laser devices.

[Rick Johnston]

Welcome to the forums.

Bushnell makes a laser rangefinders that might fit the bill.

[relic180]

Thanks for the feedback guys.

I should have mentioned that a visual solution (such as lasers or cameras) won't work. In addition, the fixed end will not be manned and manual aiming of any sort of device isn't really what I'm looking for. I'm looking for something that will measure the distance several times per second (5, 10, 20) and can feed this information to a computer to be processed. The measurement process would need to be handled by the fixed end without human assistance... and without the benefit of being able to see the mobile end all the time.

RADAR might work, where would be a good place to find more information on RADAR devices? Is this an expensive solution?

[Rick Johnston]

Like laser, Radar won't work if there's no line of sight to the far end.

A wired device called a Time Domain Relfectometer will work for a fixed point at the far end.

What's this going to be used for? More info may help find a solution.

[relic180]

It would probably help if I give you a hypothetical application of what I'm trying to accomplish.

Imagine a person is standing in a crowded area roughly 100x100 meters in diameter. This size and location of this area is fixed at all times. There are a lot of other people walking around as well and the person I'm trying to track may also be moving. So basically, I want to track the distance between myself (I am not moving) and this person (who may or may not be moving) 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 to carry with them that will help me identify and track them, so long as that item is very, very small and requires almost no power (or none, 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.

[Rick Johnston]

What about a GPS tracker? Found some examples on this web site.

[dobam]

Here's a cheap DIY TDR: TDR Circuit TDRs are normally tens of thousands of dollars with expensive probe sets.