A paper proposes a method of mutual coherent laser tracking of two spacecraft for detecting gravitational radiation. Each spacecraft would transmit a laser beam to, and receive a similar laser beam from, the other spacecraft. Each spacecraft would also coherently transpond a laser beam back to the other spacecraft. Comparison of the phases of the various transmitted and received signals would yield four sets of tracking data - two sets of one-way and two sets of two-way Doppler shifts that could be partly attributable to gravitational waves. The data would be time-tagged and telemetered back to Earth for analysis.
This work was done by Massimo Tinto of Caltech for NASA's Jet Propulsion Laboratory. NPO-20501
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Two-spacecraft laser tracking for detecting gravitational waves
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Overview
The document discusses advancements in gravitational wave detection through spacecraft-to-spacecraft coherent laser tracking, as presented by Massimo Tinto from NASA's Jet Propulsion Laboratory. Building on previous research (referred to as Paper 1), the study highlights a method to improve the accuracy of Doppler data by deriving a linear combination that minimizes the impact of noise sources such as the troposphere, ionosphere, and mechanical vibrations. This technique significantly reduces frequency fluctuations from onboard clocks, achieving a root-mean-squared noise level of approximately 4.7 x 10^-18 at 10 Hz, assuming calibration for interplanetary plasma-induced fluctuations over a 40-day period.
The main focus of the paper is the extension of the theoretical framework to a configuration where two spacecraft track each other using coherent laser light. The findings indicate that the frequency fluctuations caused by the lasers, which are the primary noise sources in these tracking experiments, can be reduced by several orders of magnitude at specific Fourier components. This capability positions spacecraft-to-spacecraft coherent laser tracking as a viable equivalent to a xylophone interferometer detector for gravitational waves. The document emphasizes that interferometric measurements of gravitational radiation can be conducted with just two free-falling particles, as the transfer functions of gravitational wave pulses differ from those of laser frequency fluctuations when the wavelength of the gravitational wave is shorter than the distance between the spacecraft.
The paper also notes the growing interest from NASA in multi-spacecraft missions to explore the solar system. If suitable optical payloads are integrated into these missions, they could facilitate searches for gravitational waves through the proposed laser tracking method. Additionally, the data analysis techniques discussed could serve as a backup for the future LISA (Laser Interferometer Space Antenna) mission, which is a space-based Michelson interferometer designed to detect gravitational waves and is being considered for launch by the European Space Agency (ESA) and NASA in the coming decade.
Overall, the document presents a significant step forward in gravitational wave detection technology, with implications for future space missions and our understanding of the universe.
