waterproof sensor dor distance range up to 50 meters

Dear all
I'd like to control a small boat to follow an object ib the water.

What Kind of sensors do you suggest to do that?
Thanks

you really have to tell far more details to get some meaningful answer.

Hi, sorry... you are true.

I try to explain better what i want to do:

i'm a spear-fisher and we use a small buoy floated with a red flag to advise other boats/people that we are somewhere nead the buoy (maybe under the water).

This buoy is connected to the diver with a string of some meters (from 5 to 30... in depends on many thins) and this string is very annoying because it gets around your legs, makes noise, alerts fish, etc etc....

So, what i want to do is to put a small elecrtic egine on it and then control the direction of the buoy with and arduino system, the buoy should follow the diver in the water (not under the water, just follow the diver on the surface of the water), consider that usually the diver stays 1 o 2 minutes under the water (from 5 to 15 meters, in my case) and then takes breath for 2/3 minutes before to dive again.

So, my idea is to have some kind of object / quarz / reflector of signal on myself (must be small and water proof) and then put a sensor on the arduino system to know how far i am, where i'm going and so on.

Is not important to follow the diver even when he is under the water, it is enough to understand how far he is and try to reach him staying in a range of 5-10 meters of the diver.

I have an excellent knowledge of coding and programming with many different languages, i'm not worried about how to code that, but i don't know if exists this kind of sensors.

I've seen that is possibile to use ultrasonic sensor with arduino but i'm worried with this kind of sensors as i will get too many false positive, i need a sensor that can give me the distance (or a noise/analogic level) between the sensor (put on the buoy) and the diver (maybe another sensor / something like RFID but with long-range capabilities?)

Do you have enough information?

Thanks to all in advance

What you want to do is twofold: first 3D localization underwater, then have the buoy steer to a place above the desired position. Given a target position, the steering part is not particularly difficult, but if you browse through this and other forums (or google "3D indoor localization") you will find that the localization problem comes up a lot. It is very difficult. Given a limited budget, by far the best solution to date on land is GPS, but that requires a clear view of the sky and won't work underwater.

I don't like to be negative but in my opinion, what you want to do is just not possible with current technology.

Thanks for your reply but is not clear to me why i need a 3D localiziation underwater.

Maybe my example was not clear: i don't want/need that the buoy goes exacly over my head when i'm under the water: i need only that it stays in a range of 15-10 meters from me.

When i'm under the water the buoy can switch off the motor, as soon as the signal comes back the buoy can start and come near me.

I think that with a "simple" distance sensor this can be implemented, obviously with a GPS information all will be simplest, but i can't carry a gps locator (and a communication system to talk with the arduino) on myself.

For example i've seen that with active RFID you have a mesurement range up to 30-50 meters, isn't there a way to get the distance between the sensor and a specific object?

For example i've seen that with active RFID you have a mesurement range up to 30-50 meters, isn't there a way to get the distance between the sensor and a specific object?

Not easily. Sonar works to get distance under water but radio does not. But how, exactly, would sonar work? If the buoy had a sonar transmitter, how would it distinguish the echo coming from a diver, from any other echo? If the diver transmits a "ping", how would the buoy determine the travel time? Even if the buoy could somehow measure the distance to a diver, it would in addition have to determine which direction to travel in order to keep within a certain range.

The more you think about the problem, the more you will realize how difficult it is to solve.

So, you want this gadget to notice your head, when you come up for air and you are 20 metres away from it.

That is not a particularly simple problem.

The only way I could see to solve this, would be to attach some kind of light to your head. Could be an IR led, or a laser, or some kind of noisy device.

And then build either a rotating sensor or a conical omnidirectional sensor on the buoy, which is going to detect the light on your head and steer towards it.

This seems not impossible , but rather complicated and tricky.

If I was going to try this, I'd start by getting maybe 20 IR photodetector diodes, and glue them in a circle facing outwards, and then try steering towards the one which is picking up the signal. But getting a range more than about 5 metres from them seems to be problematical.

m just wondering about how works The arva system for alpine ski.
It gives both signal level and direction to user.
An extreme solution could be to connect the led of arva as digital input on arduino and use this to control direction and engine