they just do their best, but there is no guarantee of dectecting a threat in time, so they need help in decting missiles and ships.Air defence radars must be capable of detecting low flying missile to. A surface ship is a much bigger target than a anti-ship missile flying at or below 20m.
here are some data
AN/SPS-48: ACs(400KMs), ships (-), missiles (30KMs)
AN/Spy-1: ACs(320KMs), ships (85KMs), missiles (40KMs)
The Navy operates the AN/SPS-49(V) as a shipborne radar on board approximately 115 ships and shore installations. Operation in the 902-928 MHz band is critical because it offers unique propagation characteristics that permit detection of small, fast moving targets over water, referred to as sea skimmers. A sea skimming missile or aircraft poses a particular problem, since at normal target tracking frequencies in smooth sea conditions there is a tendency for the radar return to be reflected back off the sea surface, causing confusion to the radar resulting in gross errors in assessment of speed and range. This problem has been largely overcome by using frequencies in the 902-928 MHz band. The Navy maintains that continued access to the 902-928 MHz band is essential to meet national defense requirements.
traditionally, air-search radars work at the frequency(L-Band) but not all of them, US SPY-1 and UK type996 work at 2.4GHz(S-Band).