By the end of 1965, Project HARP had fired more than one hundred missiles at heights over 80 km high into the ionosphere. At this point, the project starting planning the launch of the Martlet 4, a projectile that used rocket jets that would ignite mid-flight to send the missile into orbit. For this endeavor, BRL designed the telemetry system that utilized sun sensors to determine the altitude of the projectile. This telemetry system would serve as an early precursor to that of the U.S. Army's Aeroballistic Dynamic Fuze (DFuze).