Navigational System Project using Trilateration

A. Coley

I'm trying to implement a navigational system using trilateration. I plan on using 4 stationary RF transmitters to send a continuous signal to 4 RF receivers connected to a Javelin Basic Stamp. But I'm not sure how I will be able to determine the time (T) of transmission and reception used to calculate the distance between the transmitter·and·receiver.
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I understand that distance (d) is measured by multiplying the speed (c) of the Radio Wave times the time (T) it takes to send the signal [noparse][[/noparse]d = (c)(T)]. If the speed is to remain constant, this means that the time of transmission will vary, thus determining the distance between the two at a given moment in time. Trying to capture that time (T) is my dilemma. I’m not sure if this is possible with one-way-only RF transmission, but if so any help would be greatly appreciated.
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Thanks,

Franklin

I would work the other way around and transmit from the moving station to the four receivers. You would then have one source to time against rather than four and would not have to determine which signal was from which transmitter.

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- Stephen

sylvie369

(I'm well aware that as soon as I say it can't be done, someone else will chip in with a link to a website showing how someone has done it. Throwing caution to the wind...)

"Trying to capture that time (T) is my dilemma. "

Yes, it is. If you're talking about locating down to just a couple of feet using transmitters located within a reasonable distance (< a mile?) of the object you're trying to locate, the difference in time is going to be pretty miniscule. That means you're going to have to have a device capable of measuring some very short intervals. GPS works in part because the devices are capable of measuring time very precisely, and partly because the transmitters are located somewhere around 10,000 miles away from the receivers.

By my early morning figuring, let's say your device were right next to one of the transmitters, and a mile from another one. The time difference for the signals would be .019 seconds. That's measurable, but we're talking about a big baseline and the best case. Let's suppose you're putting the transmitters just 1/10th of a mile from the device you're tracking. Then the best case time difference is .0019 seconds. You're probably thinking about having the transmitters 50 feet or less from the device you're tracking: at 50 ft, best case difference is about .0002 seconds, or about 1/5000th of a second.

Now, that's best case, so if your hardware can just barely detect 1/5000th of a second differences then with your transmitters located 50 feet apart you're going to have 50 foot resolution.

On top of that, you need a clear line-of-sight so that the signals aren't being received after reflecting off of other objects (which will of course change their travel time and mess up your calculations).

Post Edited (sylvie369) : 4/26/2008 11:44:05 AM GMT

A. Coley

Hmm, ok...that makes sense. I'm not sure if the Javelin would be able to accurately detect the time difference of the signal, because at the maximum distance, the receiver and transmitter would be no further than 20ft from each other.

So then, with that in mind, would be easier to synchronize all of the transmitters and determine object orientation by measure the 'Time-Difference-of-Arrival' (Multilateration)?

Thanks,

Franklin

I saw a group of robots that used a hybrid system. There was a simultaneous burst of RF and US and the other bots timed the difference in time of the two signals to get distance and had a triangle of US sensors for direction to the original pulse. you could expand on this but I would still make the transmitter one point (on the bot)
Found this:

http://video.google.com/videosearch?q=robot+swarm&sitesearch=&start=10

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- Stephen

Post Edited (Franklin) : 4/26/2008 6:26:15 PM GMT

sylvie369

A. Coley said...
Hmm, ok...that makes sense. I'm not sure if the Javelin would be able to accurately detect the time difference of the signal, because at the maximum distance, the receiver and transmitter would be no further than 20ft from each other.

So then, with that in mind, would be easier to synchronize all of the transmitters and determine object orientation by measure the 'Time-Difference-of-Arrival' (Multilateration)?

Thanks,

I think that leaves you with the same issue. I'd think that ultrasonic (as the guy above mentions) would be a lot more practical, simply because the speed of sound is SO much lower than the speed of light (and therefore the time differences you have to measure are much larger).

Take this all with a grain of salt: I haven't ever done any of this, or even tried it. There may be some nice trick that everyone except for me knows about.

Bean

I agree.
For 20 ft range ultrasonics is the way to go.

Bean.

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Did you know that 111,111,111 multiplied by 111,111,111 equals 12345678987654321 ?

www.iElectronicDesigns.com

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sylvie369

Here are some radio-based direction-finding ideas that may be useful in this context:

http://www.utaharc.org/rptr/ark_df_desc.html

"The circuits described on this page are of the "Homing" or "TDOA" type in that they switch two antennas rapidly and, by observing phase changes in the incoming RF signal, allow the user to determine the bearing of the transmitter"

It's going to be a lot more complicated than just reading off some timing numbers and doing some math, but it might do what you want to do.

A. Coley

Ok, thanks....I think I'll·just use an ultrasonic range finder to·determine the distance.