build a sensor that can calculate distance between itself and sub sensors

simply put all you do is calculate the speed at which the transmission goes, then divide that by the time(this is nanoseconds) this gives you your distance.

 

ok think of it like this.
the sensor prime sends out a little packet of data. the subsensor simply recieves it and sends it back.
when the sensor prime recieves it back it simply does the following equation
speed of transmission / time it took to get there and back / 2 (as it travelled to and from that is twice the distance.)
no matterhow you transmit this will always give you the right answer as long as you calculate the right speed of transmission (I am hoping someone can give the exact speed cos I am lazy tonight lol) and you compute thetime it took.
do not process the signal in the subsensor or anything this will add latency which will affect the time.

 

well if you are using the same length of wire put it on a reel. get one of the countless ways to count how many times you wind it out.
if you use this approach you MUST factor in the fact that the outer layer will be much wider that the inside layer.
edit you could also use a "pulley" and count how many times it spins