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The two largest users of the Harrier were the Royal Air Force and the United States Marine Corps (USMC). The exported model of the aircraft operated by the USMC was designated the AV-8A Harrier, which was broadly similar to the RAF's Harrier GR.1.[69] Changes included the removal of all magnesium components, which corroded quickly at sea, and the integration of American radios and Identification Friend or Foe (IFF) systems; furthermore the outer pylons, unlike the RAF aircraft, were designed from delivery to be equipped with self-defence AIM-9 Sidewinder heat-seeking air-to-air missiles.[38] Most of the AV-8As had been delivered with the more powerful Pegasus engine used in the GR.3 instead of the one used in the earlier GR.1.[69] Two-seat Harriers were operated for training purposes; the body was stretched and a taller tail fin added.[70] The RAF trained in the T.2 and T.4 versions, while T.4N and T.8 were training versions the Navy's Sea Harrier, with appropriate fittings.[71] The US and Spain flew the TAV-8A and TAV-8S, respectively.[72][73]
All RAF GR.1s and the initial AV-8As were fitted with the Ferranti FE541 inertial navigation/attack suite, but these were replaced in the USMC Harriers by a simpler Interface/Weapon Aiming Computer to aid quick turnaround between missions. The Martin-Baker ejection seats were also replaced by the Stencel SEU-3A in the American aircraft.[74][75] The RAF had their GR.1 aircraft upgraded to the GR.3 standard, which featured improved sensors, a nose-mounted laser tracker, the integration of electronic countermeasure (ECM) systems and a further upgraded Pegasus Mk 103.[35][36] The USMC upgraded their AV-8As to the AV-8C configuration; this programme involved the installation of ECM equipment and adding a new inertial navigation system to the aircraft's avionics. Substantial changes were the Lift Improvement Devices, to increase VTOL performance; at the same time several airframe components were restored or replaced to extend the life of the aircraft.[37] Spain's Harriers, designated AV-8S or VA.1 Matador for the single-seater and TAV-8S or VAE.1 for the two-seater, were almost identical to USMC Harriers differing only in the radios fitted.[76]
https://en.wikipedia.org/wiki/Hawker_Siddeley_Harrier