In the context of passive geolocation of High Frequency (HF) transmitters using the Time Difference of Arrival (TDoA) method, the transmitted signal and the propagation channel is unknown. This paper explains the principle of a method termed as "cross-channel sounding" along with its concepts. This method is used to evaluate the propagation duration differences of an unknown signal received by multiple synchronized distributed receivers. The mathematical description for cross-channel sounding is detailed, emphasizing the importance of the nature of the transmitted signal in the estimation of the propagation duration differences. An operational country wide TDoA receiver network capable of capturing HF signals is presented. HF broadcast signals are captured from different transmitters around Europe. Experimental data are used to analyze the different propagation modes from cross-channel sounding and assess the TDoA estimation complexity in the case of multipath channels.