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wireless signal generation device

I'm thinking about starting a project which is essentially based on determining distances(and derivations from that) using triangulation on to fixed and one variable source. I would put a signal generator on the variable source and determine its distance using triangulation by calculating the time it takes for the signal to arrive at the fixed locations and once I know the distances I can recover the speed and do all the things that makes the project interesting.

I assume the method I'm using is pretty much the standard in doing this sorta thing. The issue I'm having is that I do not know anything practical about it. Before I even think about starting this project I need to know how hard it would be to create(or buy) a device that emits a signal that the other two devices can use to determine the distances.

Here are some specs: The distances used are at most a mile but in actuality will probably be a few hundred meters. The devices will be used intermittently with about 1-4 mins continuous use. It needs to be as accurate as possible but not excessively so(I'd like to get millimeter resoluion but centimeter's would be ok or maybe even inches). The device needs to be as small as possible except for the antenna which could be several inches as long as it is not to large and does not weigh to much. Ofcourse cost is a factor but for such a simple device I would imagine the main factors involved in cost are minaturization. The device would need to emit a signal between 10 and 1000hz but I believe that it would need to actually work in a much higher frequency range(ghz) to be practical.

I think thats about the main things. The other parts of the project mainly have similar limitations except size is not as a huge issue.

Basically what I want is something like the transmitter of a cell phone that sends a periodic wave of a few hz that can be picked up about 500-1000 meters away by a device which can then use it to determine the distance of that device away from the reciever. Ofcourse I'll have two of these devices and they will do the same with each other to get the position(2D as I know altitude of the transmitter).

Ofcourse using this method will require that the speed of the electromagnetic wave be constant or approximately. There will be direct line of sight from all the devices except for possible momentary interruptions(sign in the way, etc...).

Just looking for some feedback. Hopefully I gave all the necessary information.

Thanks, Jon

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Jon Slaughter
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since nobody else has replied I mention a few reasons measuring distance in this way with useful accuracy will be troublesome.

Plextek have done something vaguely similar with 10meter range.

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An antenna a few inches long will only work efficiently at UHF frequencys but for a few hundred meters range you can communicate with a short antenna on a portable device and larger antennas on the fixed equipment.

13.56Mhz is probably most suitable for this. VHF/UHF signals tend to bounce off the ground, buildings etc giving multipath reception.

Measuring the absolute time a transmission takes to arrive is not practical with one transmitter and one receiver. You could have a clock at both points and send a transmission exactly once every ten seconds. The speed of light is approximately one nanosecond per foot so you need atomic clocks to get worthwhile accuracy.

Perhaps a scheme where one end receives the signal and retransmits its back on another frequency could be made to work. Transmitting and receiving at the same time requires some careful RF engineering. GSM mobile phones compensate for the speed of light using a "timing advance" value in 350meter steps.

A simple scheme using presence or absence of an RF carrier will have trouble detecting exactly when a transmission starts. When the signal is limited to a narrow frequency band the signal does not go from nothing to full strength at the detector/demodulator instantly, it rises out of the noise over quite a few cycles. It's hard to tell if the start is the real start or some noise that looks like the start. Also the local noise levels at the receivers will vary.

A scheme using an audio frequency signal modulated onto the audio might work or might have enough jitter to make timing measurements useless. A microsecond variation gives you a 1000 foot error.

If you try to use microwave frequencys with 100+MHz bandwidth you have a lot of thermal noise and other noise to contend with. More bandwidth means more noise.

Bob

Reply to
Bob

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