According to the official description of the SPQ-9B, it is (also) an radar for detecting and tracking air targets, especially low flying ones:

"The AN/SPQ-9B scans out to the horizon and performs simultaneous and automatic air and surface target detection and tracking of low flying Anti-Ship Cruise Missiles (ASCMs), surface threats and low/slow flying aircraft, UAVs and helicopters."
AN/SPQ-9B Radar Set (navy.mil)