Complex sentence: The Royal Air Force's Hawker Siddeley Harrier GR1s had entered service in April 1969; in 1975 the Royal Navy ordered 34 Sea Harrier "FRS.1" s (standing for' Fighter, Reconnaissance, Strike'), the first of which entered service in 1978.
Split into simpler sentences: The Royal Air Force's Hawker Siddeley Harrier GR1s had entered service in April 1969. In 1975 the Royal Navy ordered 34 Sea Harrier "FRS.1" s (standing for' Fighter, Reconnaissance, Strike'), the first of which entered service in 1978.