range detector anyone?

hi,

I was thinking if there is any way we can build up a cheap device that can detect a range using PIC16F84A?

let say we are using an RTFQ1 to continuously sending data to a moving RRFQ1 (within the 250m range stated in its datasheet),and when that RRFQ1 gets nearer to the RTFQ1, we will know it?

any idea for that?

thanks.