How it Works

The experimental Laser Ranging Interferometer (LRI) is a technology demonstration that uses laser interferometry instead of microwaves to measure fluctuations in the separation between the two GRACE-FO spacecraft. This is the same measurement made by the MWI, but the LRI offers the potential to improve the precision of range fluctuation measurements by a factor of at least 10, largely due to the laser wavelength being 10,000 times shorter than the microwave wavelength. These improvements will enable the satellites to detect gravitational differences at smaller scales. The LRI will demonstrate precision inter-spacecraft laser interferometry for future GRACE-like missions. GRACE-FO LRI data are for technology demonstration purposes only and will not be the mission’s data of record for use by the science community.

The LRI was developed jointly by the United States and Germany. JPL managed the development of the laser, laser frequency stabilization reference cavity, and interferometer readout and control electronics, and supported spacecraft integration. Germany provided the optical components of the LRI (optical bench steering mirror, triple mirror assembly) and supported spacecraft integration. The German contribution was managed by the Max Planck Institute for Gravitational Physics Albert Einstein Institute (AEI) in Hannover, with implementation by SpaceTech (STI) in Immenstaad.

The components of the LRI include:

• An Optical Bench Assembly, which routes, detects and points the laser optical beams. The Optical Bench Assembly was developed by STI. The steering mirror was developed by Airbus. The photoreceivers were developed by the German Aerospace Center (DLR) in Aldershof.
• Optical Bench Electronics, which provide power to the steering mirror and photoreceiver and signal conditioning between the photoreceiver and laser ranging processor. The Optical Bench Electronics was developed by Apcon Aerospace and Defence in Neubiberg/Munich.
• A Triple Mirror Assembly, which routes the beam around the MWI. The Triple Mirror Assembly was developed by STI and Hensoldt Optronics in Oberkochen.
• Optical Baffles, which prevent obstruction of the laser beams and control scattered light effects in the interferometer. The baffles were developed by STI.
• A Light Path Closure, which protects the LRI during spacecraft integration and covers the triple mirror assembly mirrors to avoid contamination from the spacecraft. The Light Path Closure was developed by STI.
• A Laser Ranging Processor, which measures the phase of the laser interferometer signal from the photoreceiver as representative of fluctuations in the separation between the two orbiters, provides control of the laser frequency, and commands the steering mirror angle and implements the search to establish the optical link. The Laser Ranging Processor was developed by JPL.
• The laser, which provides the light used for laser interferometry and emits approximately 25 milliwatts of light at 1064 nanometers. The laser is based on a commercial spaceflight unit developed by Tesat Corporation for inter-satellite laser telecommunications, and successfully flown on several projects, including the USAF Near Field Infrared Experiment (NFIRE) and German TerraSAR-X projects.
• An Optical Cavity, which stabilizes the laser light wavelength. The Optical Cavity was delivered by Ball Aerospace in Boulder, Colorado. The phase-modulator for the cavity was delivered by Photline, part of iXblue in Saint-Germain-en-Laye, France.
• The LRI’s laser frequency stabilization and laser ranging processor are based on prototypes developed under NASA’s Instrument Incubator Program by Ball Aerospace and JPL.
• Optical fibers for the LRI were delivered by Diamond USA Inc. and Diamond SA in Losone, Switzerland.
• Optical ground support equipment for the LRI was developed by the German Aerospace Center (DLR) Institute of Space Systems in Bremen, The AEI and JPL.