A) Radio direction finding
Radio direction finding involves using antennas and receivers to determine the location of an RF source. The are many different techniques:
- Directional antennas - Using highly directional antennas such as yagi's, loops, etc. you can determine the direction of the signal.
- Pseudo-doppler - antennas are arranged in a circular array and then sequentially sampled to simulate the effect of a rotating antenna. This modulates the received signal and the phase of this modulation can be used to determine the direction of the signal.
- Phase interferometer - the phase difference between two spacially separated antennas is used to determine the direction of the signal.