hi everyone,
i was wondering what sensor can record distance?
Depends on how big the distance is, and what (water, air, rock, vacuum...) is in the way, and how fast and accurately you need to know.
It also depends on what you are measuring the distance to (or from)
Check the sensor section of e.g. sparkfun - Sensors - SparkFun Electronics -
keywords include: rangefinder ultrasonic IR proximity
thanks everyone for the reply! i want to create a robot(underwater) that when moved can go back to its original position...so i want to know if theres anything that can record its distance traveled in order for it to go back to its original position.
when moved can go back to its original position.
How far?
Accelerometers and gyros could work, but a tether is more certain.
maybe around 1meter...uhmmm the robot is autonomous so its needs to know for itself...im thinking of using an accelerometer...is this a good idea?
ranzdelfin:
maybe around 1meter...uhmmm the robot is autonomous so its needs to know for itself...im thinking of using an accelerometer...is this a good idea?
Typically in water sound is the only option.
sir i dont want to know the distance of any object from the robot...i want to know the displacement OF the robot...how far he traveled in x,y and axis...thank you! 
Probably your best first project would be a 2 wheel differential drive robot with quadrature encoders. This represents the least difficult autonomous robot platform, but is by no means easy. There are a variety of readily available platforms to start off with as well, or you are welcome to build your own from individual components.
Quadrature encoders will provide the least errors for calculating robot location as well.
When you find out how to do it with a submerged craft be sure and tell the government, they would love to know.
The only way you are going to even approach this task is if you use accelerometers and integrate the readings to provide an inertial dead reckoning system. However, with the sensors available for a price less than your house, this is going to be subject to drift and will require a lot of calibration.
It is not something that you can expect a forum member to write straight off, it requires months even years of experimentation.
Good luck.
I guess I should have better clarified what I was trying to say.
If you haven't successfully built ANY sort of autonomous vehicle, your best first project should be what I outlined. Attempting any other sort of autonomous vehicle, with all the additional complexities and hurdles they introduce, without having at least that experience under your belt is simply biting off more than you can chew.
The government has been on the LORAN system for positioning ships but is phasing it out in favor of satellite systems such as GPS. In order to tell position, you need an infrastructure, satellites for GPS or streets and address for a mailman, no exceptions. You can't just build A sensor to find x,y,z from it, unless there is ether in water of course. Your best bet is the encoder. If you want a good accurate system, use sonar. With the idea of LORAN, set up 3 sonar emitters under water at fixed locations and emission sequences, you can easily determine the position, but again, getting the sonars to do that is another story. I don't know how to.
Positioning using sonar beacons/transponders (about the size of an oil-drum) is quite common, but very, very expensive (oil/gas industry expensive), and I doubt really appropriate to locating a (presumably small) robot in the low metres range.
I didn't really know. 
Just thinking about cheap sonic rangers and thought these underwater transponders would be cheap.