Complex sentence: The 100 kg Gravity Probe A spacecraft housed the atomic hydrogen maser system that ran throughout the mission, and a microwave repeater to measure the Doppler shift of the maser signal.
Split into simpler sentences: The 100 kg Gravity Probe A spacecraft housed the atomic hydrogen maser system that ran throughout the mission. Along with the hydrogen maser, a microwave repeater was also included in the probe in order to measure the Doppler shift of the maser signal.