“The AN/MPQ-4A counterbattery radar used a Foster scanner, the large conical object in the middle of the image. This produced a horizontally-scanning signal, which was sent forward by the large rectangular reflector.” (Wikipedia)
“The Foster scanner, or Variable Path scanner, is a type of radar system that produces a narrow beam that rapidly scans an area in front of it. Foster scanners were widely used in post-World War II radar systems used for artillery and mortar spotting.” (Wikipedia)
If you are curious to learn how it works, feel free to download the operating manual (Download PDF), kindly made available via Google, and read Chapter 1 for a detailed explanation.
Below is a short video of this counter mortar radar screen rotating at the US 1st Infantry Division’s base:
University of Cape Town