# Create a small range (10 mm to 1m) GPS by triangulation of distance

Discussion in 'The Projects Forum' started by Tiago Costa, Mar 13, 2015.

1. ### Tiago Costa Thread Starter New Member

Mar 13, 2015
2
0
Hey guys,
I am working on a school project, where I need to design a boat that will follow a certain path, avoiding obstacles.
My idea is to map the obstacle circuit and then use a homemade small scale GPS system to determine the position of the boat in the map. (The control of the movement of the boat is done already).
To accomplish this, I was thinking about setting up 3 transmitter towers around the circuit (a 2-meter-diameter round pool) and a receiver on the boat.
Each transmitter would calculate its distance to the sensor on the boat, and send it to the computer. The computer would then analize the distances and find the exact position of the boat on the map.
I have already programmed a software, thorugh matlab, to do this calculations. But I am having trouble with the sensors.
I need a precision of about 10 to 20 mm (i.e. the distance measured by the transmitter can have an error of 20 mm max). And my budget is not that high.
Does anyone know a good sensor that I could use? What can I use for the transmitters and the receiver?
I was maybe thinking ultrasound, but I did not find one with the function I needed.
Any suggestions? It will be gratfully appreciated.

Thanks.

2. ### MikeML AAC Fanatic!

Oct 2, 2009
5,451
1,066
Radio pulses travel at 3ns per m, so to measure time of flight over distances of 10m or so, your measurement resolution must be measured in ps (pico seconds), which is very difficult to do.

Much better to do such a small area using Ultrasound (tens of kHz), like the backup sensors in modern cars.

I would put one transmitter on the boat, and then have multiple receivers.

Last edited: Mar 13, 2015
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3. ### Tiago Costa Thread Starter New Member

Mar 13, 2015
2
0
Yeah, I was really thinking of using Ultrasound. Do you have any suggestions of brand/model that could perform like that??
I am not too familiar with ultrasound sensors.
Thanks!

4. ### John P AAC Fanatic!

Oct 14, 2008
1,638
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Actually over a short range and with clear line-of-sight (line-of-hearing?) you might be able to use sound in the audio range. Say you had 3 speakers around the rim of the pool, and you make each of them in turn emit a beep with a different frequency but an exactly known number of cycles (50 cycles of 800Hz, 1KHz and 1.2KHz, say). On the boat you have a microphone connected to suitable circuitry and a microcontroller. Instead of looking for "a tone" you actually respond to the cycles of each wave, and calculate the phasing of each versus a known timebase. That wouldn't give you distance directly, because you don't know the time taken from speaker to receiver, but it would tell you the difference between A and B, between B and C, and between C and A. From that and knowing the distances between the speakers, you can derive the actual distances.

I said put the receiver in the boat because it's the boat that needs to move. But you might reverse the setup and have one speaker on the boat and microphones around the pool, and that would let you have a computer do the calculations, which are certainly quite complex. But then you'd have to send commands to the boat about which way to move. And that leads to yet another complication--how does the boat move? It seems very tricky, given that the boat can rotate, so you can't just say "go north". You'd most likely need a compass on the boat and maybe two propellers, so it can orient itself to a fixed directional frame. Then commands about which direction to move would be possible.

5. ### #12 Expert

Nov 30, 2010
16,685
7,327
Look on the bright side: You don't have to compensate for water currents in a wading pool.