Improved HAWK or I-HAWK - The original HAWK system had problems engaging targets at low altitude - the missile would have problems picking the target out against ground clutter.
The US army began a program to address these issues in 1964 - the HAWK Improvement Program (HAWK/HIP).
This involved numerous upgrades to the Hawk system:
has a larger 74 kg (163 lb) blast-fragmentation warhead, a smaller and
improved guidance package, and a new M112 rocket motor. The new
warhead produces approximately 14,000 two gram fragments that cover a
much larger 70 degree arc. The missiles M112 rocket motor has a boost
phase of 5 seconds and a sustain phase of 21 seconds. The motor's total
weight is 395 kg including 295 kg of propellant. This new motor
improves the engagement envelope to 1.5 km to 40 km in range at high
altitude, and 2.5 km to 20 km at low altitude, the minimum engagement
altitude is 60 m. The missile was operational in 1971. All US units
had converted to this standard by 1978.