Also known as the Alto, the Derby missile is a BVR, medium-range (~50 km (31 mi)) active radar homing missile. Though technically not part of the “Python” family, the missile is an enlarged version of the Python-4 with an active-radar seeker.

In June 2015, Rafael confirmed the existence of the I-Derby-ER, an extended range version of the Derby that increases range to 54 nmi (62 mi; 100 km), after a “Python 6” version based on an air-launched Stunner missile was abandoned. To achieve greater range, a dual-pulse solid rocket motor is added, where the secondary pulse of energy as the missile nears the target extends flight time. It also combines the seeker and fuse into an integrated sensor and fusing system to make room for the new motor.^[17]

In May 2019, it was reported that India was planning to arm its Sukhoi Su-30MKI fighters with I-Derby ER missiles to replace its R-77 missiles. Previously, in 2018, it had already been selected for the Indian Air Force's HAL Tejas fighter.