What is the operational principle of passive homing missiles?

Passive homing missiles operate by detecting and homing in on energy that is radiated by the target. This energy typically comes from the target's own radar, infrared, or electronic emissions. The missile's onboard systems analyze the characteristics of that energy, allowing it to lock onto and follow the target without needing to emit any signals itself. This characteristic makes passive homing missiles particularly effective for stealth applications, as they do not give away their position by transmitting signals.

In contrast, the option regarding gravity suggests a reliance on natural forces and does not pertain to guided weaponry. Ground-controlled adjustments imply an external command guidance system, which is not a feature of passive homing systems. The option mentioning the emission of radar signals describes active radar homing, where the missile emits its signals to detect the target, which is in direct opposition to the principle of passive homing that relies entirely on the target's emissions.