Space CSAC: Chip-Scale Atomic Clock for Low Earth Orbit Applications

摘要

Chip-Scale Atomic Clocks (CSAC) promise an attractive alternative to ovenized quartz oscillators (OCXOs) for reduced Size, Weight, and Power (SWaP) timing applications aboard low-earth orbit (LEO) satellites. In this paper, we present measurements and technology advances particular to the radiation requirements of qualifying a terrestrial CSAC, comprised of commercial-grade components in LEO applications. The Symmetricom SA.45s CSAC is manufactured with commercial off-the-shelf (COTS) components with unknown radiation susceptibility. For this development, two standard CSACs were subjected to Total Ionizing Dose (TID) radiation exposure at increasing levels until the point of failure. Both units failed between 10 and 14 kRads, with similar failure modes. Based on this data, alternative SA.45s mu-metal packaging was developed with the intent of providing sufficient shielding for the SA.45s to survive exposure up to 50 kRad (Si) TID. Other than the outer packaging, the Space CSAC is identical to the SA.45s. Five Space CSACs were assembled with the new metalwork, and three were subjected to TID testing. All three Space CSACs remained functional up to 43 kRad except for the temperature-compensated crystal oscillator (TCXO), which failed in all three units at 15 kRad. We conclude that, with the additional shielding provided by the Space CSAC packaging, all SA.45s COTS components, with the exception of the original TCXO, are suitable for use in environments up to TID of 43 kRad. We also note that the damage threshold and failure mode were very consistent during all of the exposure tests; once one unit failed, the others also failed within a 5 kRad window.