Gregory L. Weaver

Spacecraft-level verification of the Van Allen Probes' RF communication system

This paper presents the verification process, lessons learned, and selected test results of the radio frequency (RF) communication system of the Van Allen Probes, formerly known as the Radiation Belt Storm Probes (RBSP). The Van Allen Probes mission is investigating the doughnut-shaped regions of space known as the Van Allen radiation belts where the Sun interacts with charged particles trapped in Earths magnetic field. Understanding this dynamic area that surrounds our planet is important to improving our ability to design spacecraft and missions for reliability and astronaut safety. The Van Allen Probes mission features two nearly identical spacecraft designed, built, and operated by the Johns Hopkins University Applied Physics Laboratory (JHU/APL) for the National Aeronautics and Space Administration (NASA). The RF communication system features the JHU/APL Frontier Radio. The Frontier Radio is a software-defined radio (SDR) designed for spaceborne communications, navigation, radio science, and sensor applications. This mission marks the first spaceflight usage of the Frontier Radio. RF ground support equipment (RF GSE) was developed using a ground station receiver similar to what will be used in flight and whose capabilities provided clarity into RF system performance that was previously not obtained until compatibility testing with the ground segments. The Van Allen Probes underwent EMC, acoustic, vibration, and thermal vacuum testing at the environmental test facilities at APL. During this time the RF communication system was rigorously tested to ensure optimal performance, including system-level testing down to threshold power levels. Compatibility tests were performed with the JHU/APL Satellite Communication Facility (SCF), the Universal Space Network (USN), and the Tracking and Data Relay Satellite System (TDRSS). Successful completion of this program as described in this paper validated the design of the system and demonstrated that it will be able to meet all of the Van Allen Probess communications requirements with its intended ground segments.

A Diophantine Frequency Synthesizer for the Examination of High Spectral Purity

Diophantine frequency synthesis (DFS), a number theoretic approach to the design of very high resolution and agile frequency synthesizers was introduced at the IEEE frequency control symposium of 2006, [1]. Since DFS uses frequency addition (and/or subtraction), concerns for the impact of mixing spurs in the spectral purity was raised. Further work has been performed to address this issue and is reported in this paper. The focus has been on basic DFS architecture targeting micro-phase type applications. The design goal has been to achieve 100 dB spurious free dynamic range (SPFD) with minimal circuit complexity. The results of the examination demonstrate that the use of DFS does not impart any extraordinary design constraints to spectral purity from that of other topology choices. In fact, the flexibility of the design technique from its applied math basis allows this demonstration synthesizer to be realized with simple and expedient construction.

The use of multi-variance for likelihood weighted drift estimation in a disciplined oscillator algorithm

Our recent research at JHU/APL has focused on advancing algorithms and software control developments for a next generation, disciplined USO based clock system. This research shows promise that incorporating an in-situ control system for removing USO deterministic frequency error based on multi-variance methods and a Bayesian maximum likelihood estimation (MLE) process could mitigate the burden of post processing spacecraft clock data. We found the effects caused by sudden frequency changes and perturbations could be diminished with a dynamic, multi-variance algorithm used to augment the drift estimation process. We used the complementary aspects of the Allan and Hadamard variance to form an MLE function. We then used the output of the MLE function to drive the weighted decision process of a simulated frequency correction control algorithm. Our paper will discuss the mathematical approach used to formulate the multi-variance MLE and discuss our findings on its implementation in the disciplining algorithm developed from the research. Using laboratory collected USO data, we will show simulated USO timekeeping performance to a maximum time interval error of < 1 µs over 70 days of operation and discuss the impact of frequency anomalies and short term rate changes to the behavior of the multi-variance approach.

Diophantine Frequency Synthesizer Design for Timekeeping Systems

Diophantine Frequency Synthesis (DFS), a number-theoretic approach to the design of very high resolution frequency synthesizers, was introduced in 2006. Further work concerning the impact of controlling mixing products for high-spectral purity was addressed and reported at the 2007 European Frequency and Time Forum. The focus of this paper is on the implementation of nested DFS architectures targeting microphase-type applications for precision timekeeping systems. nWe have shown that DFS does not impart any extraordinary design constraints on spectral purity in comparison to commonly used high resolution frequency synthesis techniques such as DDS or fractional 𝑁 . Here we describe a design approach for n10u2009MHz synthesizers with 1E-13 fractional resolution in consecutive steps ranging ± 1 0 u2009Hz. The synthesizers generate their output from a 10u2009MHz reference standard. Such synthesizers are essential to accomplishing precision frequency correction in timekeeping systems.

Composite USO/CSAC timekeeping system

The recent introduction of a first generation of chip scale atomic clocks (CSAC) offers an opportunity to enhance space-based communication and navigation systems with their unique aspects of frequency accuracy and very low size, weight and power. Nonetheless, CSAC frequency noise levels are likely too high to be used directly as master frequency sources in deep space communication systems. Composite clock systems combine complementary frequency sources to utilize the best accuracy and stability of each source to extend the overall performance of the clock system. We will discuss our research work toward a composite ultra-stable oscillator (USO)/CSAC timekeeping system. Such a system would preserve the low noise and excellent short term stability of the quartz USO while using the CSAC technology as an augmentation for clock drift correction and monitoring for in-situ frequency disturbance during autonomous operation.