This is the easiest of all (Space) RADAR. Forward Scatter RADAR is reflection of the RF Energy from distant transmitters facilitating the monitoring of only the space over a line between a transmitter of convenience and the receiver, not nearby local activity. This technique generally requires Carrier Wave reception with a dedicated Receiver for Audio detection and/or the Spectral display of a Software Defined Radio of over the horizon illumination sources (Transmitters) typically requiring custom-built high gain antennae pointing above the horizon at the transmitter of interest. Forward Scatter reflections are effectively dimensionless, consisting of narrow Doppler modulation of the reflected transmitter's energy over time. Being that the energy from the forward scatter is arriving at the receiving antennae from above the horizon, the antennae may be physically shielded from local terrestrial RF with several different techniques, some applied together:
Placing the Antennae where a local obstruction like a building or thick forest attenuates local RF sources.
Placing a Faraday Fence of at least 1.5 times the wavelength of interest one half wavelength from the antennae, between the antennae, perpendicular to the offending source.
Placing the Antennae below the level of the ground, behind a berm, hill, or mountain between the antennae and the offending RF source.