I think a lot more information is required so we can answer this What are you actually trying to do what have you tried What do you have (hardware ) and how is it connected.
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+1. Do as Rick suggests and you'll get much more useful answers.
Use a repeater xbee and measure time laps differences like any proper sonar. You will need a faster processor then arduino to describer the time differences. A
So you want to use an arduino and a pair of xbees to calculate distance based on signal strength? While this can be done it is very inaccurate. I did a bit of searching online with this but haven't really found any finished project with this concept. It seems you will need to get the RSSI values from the xbees and figure out what distance relates to what signal strength. Problem comes when you have to consider all the other factors in the equation. For instance air temperature can affect these values. Also for best results you need a clear line of site between the 2. Any objects in the way will effect signal strength. If you want to make a decent range finder with an arduino then look into using a laser or ultrasonic sensor.
Many xbee type modules have analog inputs on them - you configure them beforehand to just 'send the values on device A pin x to device B and mimic or digitize that value on pin x' Not just a radio any more! How? You'll have to read the manual.