Since the transmitter can broadcast to multiple receivers over longer distances than RS-232, and they all receive identical data, it's more than likely the transmission protocol is RS-422.
A simple way to find out would be to take a RS-422 to RS-232 converter, connect the RS-422 side to the Davis transmitter and the RS-232 side to a com port on a PC, set the baud rate, parity and stop bits on the PC to match the Davis and run HyperTerminal or something else that would show ASCII text. If you see the "data" transmitted from the Davis as readable characters then you will know it's RS-422.