Sensor Node Localization

Hello Everybody,

I would like to implement a sensor node localization through the use of a PIC16F877 microcontroller and ultrasonic waves to calculate the distance.

Here are steps of what I would like to implement:
1. I have 4 nodes, each having an ultrasonic rx/tx, a radio frequency rx/tx and a PIC16F877 which serves as the brain to each node . 1 node is interfaced in the personal computer w/c could be called as the base node or the beacon. The other 3 nodes are called the listeners.
2. Here is what I would like to happen; When the 3 listener nodes are relocated, the computer then displays the real-time orientation of the nodes. The computer has this data through the base node which is interfaced with the computer.

My question then is, how am I be able to measure the distances between each of the nodes?

Thank you very much in advance...

you have to make four fixed beacon, each one capable of interrogating your listeners, then process those four distances gathered to calculate relative positions.

Ahhh...ok, i get your point...But I want my implementation to have 1 beacon/base node which serves as the reference point or the origin. The whole system would only localize if at least 3 listener nodes are placed all together on the arena. The base is the one to calculate estimated local coordinate system of the 3 listener nodes...I know this is possible 'coz I've seen a very similar scenario but was not implemented with a PIC16F877 and yes, it was a very expensive approach.Could you help me on how to calculate estimated distances using TDOA or other similar approaches?
Anywayz, thank you very much kind sir....