A report describes the time-transfer system of the Gravity Recovery and Climate Experiment (GRACE), in which information on the distribution of Earth mass is extracted from position and time measurements for two spacecraft about 200 km apart in a circular, nearly polar orbit. Each spacecraft carriers a Global Positioning System (GPS) receiver, a K/Ka-band ranging (KBR) instrument, and an ultra-stable oscillator (USO) that serves as a clock for the GPS and KBR units.
The long-term errors of the USOs are cancelled by use of a technique, called dual-one-way phase measurements, in which the phases of the KBR signals from spacecraft A as measured at spacecraft B are added to the phases of the KBR signals from spacecraft B as measured at spacecraft A. GPS data are used to synchronize time between the USOs to within ≈150 ps as needed to enable the dual-one-way phase measurements: For each spacecraft, the GPS data are used to solve for orbital positions, and the difference between the onboard clocks and a ground clock every 5 minutes. The relative clock rate between the spacecraft is then determined from the difference between the two solutions.
This work was done by William Bertiger, Seen-Chong Wu, Gerhard Kruizinga, Charles Dunn, and Larry Romans of Caltech for NASA's Jet Propulsion Laboratory. For further information, access the Technical Support Package (TSP) free on-line at www.techbriefs.com/tsp under the Physical Sciences category. NPO-40344
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Time-Transfer System for Two Orbiting Spacecraft
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Overview
The document titled "Time-Transfer System for Two Orbiting Spacecraft" focuses on the Gravity Recovery and Climate Experiment (GRACE) mission, which involves two spacecraft launched in March 2002 into a near-polar circular orbit around Earth. The primary objective of the GRACE mission is to measure the Earth's gravitational field with high precision, which is crucial for understanding various geophysical processes, including climate change and water resource management.
The two GRACE spacecraft function as test masses, equipped with ultra-stable oscillators (USOs) that have an Allan Deviation of a few parts in 10^-13 for time intervals ranging from 1 to 1000 seconds. These USOs are integral to the spacecraft's operations, driving both microwave links and GPS receivers. To achieve accurate measurements, the synchronization of time between the two spacecraft is critical, requiring an alignment of about 150 picoseconds. This synchronization is accomplished using GPS data, which helps determine the orbital positions of the spacecraft and the differences between their onboard clocks and a ground reference clock every five minutes.
The document also discusses the technical aspects of time transfer and the challenges associated with maintaining precise timing between the two orbiters. The time delay between transmission and reception is typically less than a millisecond, with the spacecraft separated by approximately 200 kilometers. The second derivative of the clock error is maintained at less than 10^-14 seconds per second squared, ensuring high accuracy in the measurements.
The GRACE mission represents a significant advancement in satellite technology and gravitational field mapping, utilizing micron-level satellite-to-satellite ranging techniques. The results of this mission have broad implications for scientific research and practical applications, including monitoring changes in Earth's water storage, ice mass loss, and other critical environmental factors.
The document acknowledges the collaborative efforts of the entire GRACE project team and highlights the importance of the research conducted under the auspices of NASA's Jet Propulsion Laboratory. It serves as a technical support package, providing insights into the methodologies and technologies employed in the GRACE mission, which has set new standards for precision in gravitational measurements and has potential applications in various fields beyond aerospace.
