We describe our investigation into the performance of low-power heterogeneous timing systems for small satellites, using real GPS observables from the GRACE Follow-On mission. Small satellites have become capable platforms for a wide range of commercial, scientific and defense missions, but they are still unable to meet the needs of missions that require precise timing, on the order of a few nanoseconds. Improved low-power onboard clocks would make small satellites a viable option for even more missions, enabling radio aperture interferometry, improved radio occultation measurements, high altitude GPS navigation, and GPS augmentation missions, among others. One approach for providing improved small satellite timekeeping is to combine a heterogeneous group of oscillators, each of which provides the best stability over a different time frame. A hardware architecture that uses a single-crystal oscillator, one or more Chip Scale Atomic Clocks (CSACs) and the reference time from a GPS receiver is presented. The clocks each contribute stability over a subset of timeframes, resulting in excellent overall system stability for timeframes ranging from less than a second to several days. A Kalman filter is used to estimate the long-term errors of the CSACs based on the CSAC-GPS time difference, and the improved CSAC time is used to discipline the crystal oscillator, which provides the high-stability reference clock for the small satellite. Simulations using GRACE-FO observations show time error standard deviations for the system range from 2.3 ns down to 1.3 ns for the clock system, depending on how many CSACs are used. The results provide insight into the timing performance which could be achieved on small LEO spacecraft by a low power timing system.

我们使用GRACE后续任务中的真实GPS观测值描述了对小型卫星低功率异构定时系统性能的调查。小型卫星已经成为执行各种商业，科学和国防任务的有力平台，但仍无法满足需要精确定时（几纳秒数量级）的任务的需求。改进的低功耗车载时钟将使小型卫星成为执行更多任务的可行选择，从而实现无线电孔径干涉测量，改进的无线电掩星测量，高空GPS导航和GPS增强任务等。提供改进的小型卫星计时的一种方法是组合一组不同种类的振荡器，每个振荡器在不同的时间范围内提供最佳的稳定性。提出了一种使用单晶振荡器，一个或多个芯片级原子时钟（CSAC）以及来自GPS接收器的参考时间的硬件体系结构。每个时钟在一个时间范围的子集上都具有稳定性，从而使时间范围从不到一秒到几天不等，从而具有出色的整体系统稳定性。卡尔曼滤波器用于根据CSAC-GPS时间差估计CSAC的长期误差，改进的CSAC时间用于控制晶体振荡器，从而为小型卫星提供高稳定性的参考时钟。使用GRACE-FO观测值进行的仿真显示，对于时钟系统，系统的时间误差标准偏差范围为2.3 ns至1.3 ns，这取决于所使用的CSAC数量。