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Completed TRL 3 (started at 2, targeting 3)
GRACE (Gravity Recovery and Climate Experiment) 2 laser ranging precision is limited by laser frequency stability. The laser reference cavity meets short-term stability requirements, but fails long-term requirements by a factor of 10. ORCLS uses ultra-stable GPS to provide long-term stability readout. No flight missions to date have incorporated a long-term optical frequency reference. The main products will be a TRL-3 proof of concept of laser frequency relative to RF source and a TRL-4 breadboard demonstration of design appropriate for implementation on GRACE 2.
The US/German mission GRACE-FO has been measuring global gravity of the Earth since June, 2018. The surface gravity is inferred by precisely measuring the separation between two satellites in a common orbit about 500 km in altitude. The satellites are about 200 km apart, and their separation is continuously measured using laser beams with extraordinary sensitivity of better than 1 micrometer. GRACE-C will add to the decades long record of Earth gravity measurements established by GRACE-FO and its predecessor, the original such mission, GRACE. Accurate measurement of the satellite separation will require a new instrument, Scale Factor Unit (SFU). The SFU measures the frequency of the laser beams that are sent between the spacecraft. The frequency must be known with precision of about 1 part in 100 million. The SFU being developed at JPL is based on the CIF project completed in 2017. A prototype SFU under test shows the required precision. It works by modulating the frequency of the light on the spacecraft that feeds a specialized arrangement of two mirrors called the reference cavity. The modulation frequency is adjusted so its wavelength is precisely twice the separation of the mirrors, which is about 7 cm. Then the laser frequency is computed by multiplying the modulation frequency by the “mode number.” The mode number, approximately 150,000 is twice the number of laser wavelengths fit between the mirrors, and is precisely measured on the ground and in space.
Demonstrate laser frequency knowledge stability <= 3e-9 fractional = 1 MHz optical over at least 3 days.
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