What's the difference between the top radar and the AESA cheek radar? Is the top for surface targets? Why do a surface radar need 3D?
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The height gives better radar horizon, allows radar to peep down the Earth curvature to detect sea skimming antiship missiles. A lower mounted radar would have a more difficult time in detecting these missiles in advanced. Once these missiles reached a low altitude, the earth curvature would hide them from the main radar.
A 3D radar would give a more precise altitude measurement of the target and it can act as a secondary radar tracking higher flying targets. Also being an AESA, with electronic scanning, it would have a higher and much faster scan rate over the mechanical radar.
In addition, the second radar has a much higher frequency bank that gives better resistance against sea clutter, namely unwanted echoes from the water surface that distracts from the real targets.
The US does a similar thing by installing SPQ-9B radar on top of AEGIS destroyer masts where previously it does not, and you have examples in the Russian Navy where the Pozitiv radar are mounted on a height, such as the Admiral Grigorovich frigates. The last two Sovremenny sold to the PLAN are also equipped with Pozitiv radar on a height, as per PLAN specification, while the first two did not. In the refit of the first two, the Type 364 was added to both.