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 (later known as "FRS1"), 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.1s (later FRS1), the first of which entered service in 1978.