I got advise from an expert as follows;
You can make a 300 MHz counter with a CPLD (a programmable logic device) that costs ~$20.
The counter (18-bits) would run continuously.
The transmitter pings every millisecond.
Each receiver would generate one COUNT reading per ping.
The computer would take all the COUNT readings and crunch them to calculate the transmitter position.
Does anyone know of any text books or online tutorials that may assist in building this. Also would I be able to use a RS-232 to USB adaptor for this device as my laptop has only USB ports.
The devil is in the word "crunch" That is not a command in most computer languages.
The difficulty is not in getting hardware to send and receive signals. It is getting the signal to be sufficently stable to give you readings to the required accuracy.
For books, look in amazon under "programable logic devices".