A hydrogen maser for long-term operation in space

We describe the design of a space-worthy hydrogen maser capable of four or more years of continuous operation in space. This maser system can be adapted for several future space applications, such as very long baseline interferometry, high precision time transfer, and tests of relativistic gravitation. The maser clock system is described, including details on its design for surviving very high levels of launch stress, operating with frequency stability, /spl sigma//sub y/(/spl tau/)=1/spl times/10/sup -15/ for /spl tau/=24 hours over ambient temperature variations from 14 to 25/spl deg/C, and magnetic field variations of /spl plusmn/0.5 gauss. These specifications are intended to allow it to be used on a wide range of launch vehicles and space platforms. Tests of this maser clock system for approximately 1 year duration will be made in a low orbiting spacecraft by making short pulsed-laser time comparisons with a maser monitored via a common view GPS technique located in a laser-equipped earth station. Operation of the space-borne maser system is controlled and monitored using an on-board microprocessor that implements a number of automated procedures for controlling the maser and testing its operational parameters while in orbit. The entire system, including two reflector-detector-event timer modules, operates on less than 95 watts. The package, as configured for mounting on the European Space Agency's EURECA spacecraft, weighs less than 110 kg.<>