malacay:
First, the beacon sends a RF signal and an ultrasound signal.
Then, the receiver gets the RF signal and start a conunter.
This part is technically sound - BUT the RF24 is not a beacon, it's a communication device. It uses some protocol to make a connection, and send a message, and that by nature makes wireless not time reliable. Only if you can make sure somehow that this error is no more than say 10 µs (about 3 cm in distance) you can use this to time how much longer it takes fro the sound to arrive and have a reasonably accurate measurement.
Wirelessly measuring the distance between two devices accurately is not easy at all.