High Stability Reference Clock for Small Satellites

Abstract

Small satellites have become capable platforms for a wide range of commercial, scientific and defense missions. Improved onboard clocks would make small satellites a viable option for even more missions, enabling radio aperture interferometry with small satellite constellations, improved radio occultation measurements of scintillating signals, high altitude GPS navigation, and GPS augmentation. Chip Scale Atomic Clocks (CSACs), introduced in 2011, provide exceptional long term time stability for a relatively low cost with minimal size, weight, and power (SWAP), and have been space qualified [1, 2]. However, CSACs also have relatively poor stability for time frames less than 100 seconds, and higher phase noise than less expensive crystal oscillators. Additionally, CSACs alone do not provide sufficient long-term stability for some missions without external synchronization. This paper investigates methods to improve small satellite timekeeping by combining a heterogeneous group of oscillators including multiple CSACs, a GPS receiver, a crystal oscillator, and time error measurement using ground stations. Two methods for combining clocks are studied: Phase Locked Loops (PLLs), and Kalman filters. The performance of the two methods is compared for LEO missions with and without a GPS reference. The goal is to provide an onboard reference clock with excellent long and short-term stability and low SWAP. The results provide insight into the practical limitations expected when implementing an advanced clock system on very small satellites, and include the effects of clock offset measurement resolution and frequency errors due to temperature variation over the orbit. It was found that a combination of a Kalman filter and a PLL provides the best overall performance when using an external GPS receiver. Simulations of this combination demonstrate a reference clock with RMS time errors of approximately half a nanosecond, and estimated power less than one Watt. The simulations also show the importance of the time offset measurement resolution.