Passive interference

Searching radars for low-flying and ground targets are subject to strong interference from the terrain. The radio wave is reflected not only from the targets, but also from the terrain and local objects.    The lecture deals with the physical processes of the occurrence of passive interference and their influence on the search for targets.

  Powerful reflections from the terrain can mask the goal, partially or completely destroy useful information.

  In non-coherent radars, amplitude detection results in vectorial signal addition and interference. Useful information has the nature of amplitude modulation interference Doppler frequency. If the interference does not reach the receiver limit level, then the amplitude modulation is preserved. There is only a masking signal interference. And in some cases, the signal can be detected. If the interference is powerful, then it reaches the limit of the receiver. Then all the information about the goal is completely lost (target suppression).

  In coherent radars, even with a deep limitation, phase modulation is partially retained. It can be successfully used to detect a signal against the background of passive interference.

  The sounding signal is reflected from the terrain and vegetation formations as well as from the target. Therefore, using the radar equation, it is easy to calculate the level of interference at the receiver input. For this it is necessary to calculate the EPR of passive interference S _m .

  The vegetative formations and the earth's surface, “illuminated” by the antenna, can have the same distributed structure.    Calculate the value of S _{m is} not difficult if you have information about what is the EPR of one square meter of interference. Such data are shown in the table of the figure.

  Further, it is necessary to calculate the total surface area illuminated by the antenna beam and multiply it by the specific EPR of a specific type of terrain.

  In addition to evenly distributed formations, there may be other local objects in the “illuminated” surface: trees, houses, etc. Their ESR should also be taken into account by adding distributed noise to the EPR.