Continuous-wave Radar
Modern SARH systems use continuous-wave radar (CW radar) for guidance. Even though most modern fighter radars are pulse Doppler sets, most have a CW function to guide radar missiles. A few Soviet aircraft, such as some versions of the MiG-23 and MiG-27, used an auxiliary guidance pod or aerial to provide a CW signal. The Vympel R-33 AA missile for MiG-31 interceptor uses SARH as the main type of guidance (with supplement of inertial guidance on initial stage).
SARH missiles require tracking radar to acquire the target, and a more narrowly focused illuminator radar to "light up" the target in order for the missile to lock on to the radar return reflected off target. The target must remain illuminated for the entire duration of the missile's flight. This could leave the launch aircraft vulnerable to counter attack, as well as giving the target's electronic warning systems time to detect the attack and engage countermeasures. Because most SARH missiles require guidance during their entire flight, older radars are limited to one target per radar emitter at a time.
The maximum range of a SARH system is determined by energy density of the transmitter. Increasing transmit power can increase energy density. Reducing the noise bandwidth of the transmitter can also increase energy density. Spectral density matched to the receive radar detection bandwidth is the limiting factor for maximum range.
Read more about this topic: Semi-active Radar Homing
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