Last year during a Russian wireless ‘capture the flag’ (CTF) competition one of the goals was to reverse engineer a remote controlled toy tank, and then to control it with a HackRF. One of the Russian CTF teams has posted a thorough write up on the reverse engineering process that was used on the toy tank (the link is in Russian, but Google Translate works okay).
The write up first shows the reception of the signal from the wireless controller, and then moves on to show how to receive it in GNU Radio and obtain a time domain graph of the digital signal. From the pulses it is simple to visually work out the binary string. Next an instruction decoder is created in GNU Radio which automatically obtains the binary string from the signal directly. Then once the codes for back, forward, left and right were obtained it was possible to write another GNU Radio program to transmit these codes to the RC toy tank from the HackRF.