Researchers have used data from the GRAIL mission to the Moon to make the first in-flight verification of ultra-stable oscillators (USOs) with Allan deviation below 10–13 for 1-to-100-second averaging times. USOs are flown in space to provide stable timing and/or navigation signals for a variety of different science and programmatic missions.
The Gravity Recovery and Interior Laboratory (GRAIL) mission is flying twin spacecraft, each with its own USO and with a Ka-band crosslink used to measure range fluctuations. Data from this crosslink can be combined in such a way as to give the relative time offsets of the two spacecrafts’ USOs and to calculate the Allan deviation to describe the USOs’ combined performance while orbiting the Moon. Researchers find the first direct in-space Allan deviations below 10–13 for 1-to-100-second averaging times comparable to pre-launch data, and better than measurements from ground tracking of an X-band carrier coherent with the USO. Fluctuations in Earth’s atmosphere limit measurement performance in direct-to- Earth links. In-flight USO performance verification was also performed for GRAIL’s parent mission, the Gravity Recovery and Climate Experiment (GRACE), using both K-band and Kaband crosslinks.
This work was done by Daphna G. Enzer, William M. Klipstein, Rabi T. Wang, and Charles E. Dunn of Caltech for NASA’s Jet Propulsion Laboratory. NPO-48705
This Brief includes a Technical Support Package (TSP).
Measurements of Ultra-Stable Oscillator (USO) Allan Deviations in Space
(reference NPO-48705) is currently available for download from the TSP library.
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Overview
The document is a technical support package from NASA's Jet Propulsion Laboratory (JPL) detailing the performance measurements of Ultra-Stable Oscillators (USOs) in space, specifically focusing on the Gravity Recovery and Interior Laboratory (GRAIL) mission. GRAIL, launched on September 10, 2011, aims to map the lunar gravity field and understand the moon's internal structure and thermal evolution. The mission utilizes two spacecraft, GRAIL-A and GRAIL-B, equipped with USOs that allow for precise distance measurements between them, crucial for the mission's objectives.
The document outlines the significance of USOs in providing stable timing and navigation for various scientific missions. Typically, USOs are tested on the ground to a precision of part in 10^13, but their performance can only be verified in space due to atmospheric interference when signals are transmitted to ground stations. GRAIL represents a unique opportunity to compare the performance of two independent USOs in space without the degradation that occurs when signals travel through the atmosphere.
Key measurements include the Allan deviations (ADEV) of relative time offsets between the two USOs, which are critical for assessing their performance. The document discusses the methodology for measuring these deviations, including the use of ka-band data and the impact of factors such as signal travel time and ionospheric phase shifts.
The GRAIL mission has extended its science phase beyond the initial 90 days, allowing for further data collection and analysis. The document emphasizes the importance of the data obtained during the mission, which not only contributes to lunar science but also provides valuable insights for the clock and timing community.
In summary, this technical support package serves as a comprehensive resource on the performance of USOs in the GRAIL mission, highlighting their role in advancing our understanding of the moon's interior and the technological advancements in space-based timing systems. It underscores the collaboration between scientific research and technological innovation, showcasing NASA's commitment to exploring and understanding celestial bodies.
