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The Australian National University

Technology development

The GRACE Follow-on mission will supplement the original GRACE microwave measurement with a new laser ranging system. The laser's shorter wavelength and excellent wavelength stability will improve measurement accuracy by a factor of 25. Flying a laser system in addition to the existing microwave system provides this improved measurement capability yet retains continuity with original GRACE measurements. The microwave system will remain the primary instrument, with the laser system to act as a technology demonstrator instrument.

Previous funding under NASA's Instrument Incubator Program resulted in a prototype interferometric laser transponder, developed jointly by Ball Aerospace, the University of Colorado, and JPL. This transponder is appropriate for GRACE Follow-on; however, there remain three key components to be developed to complete the laser ranging system: a beam routing subsystem, a laser frequency stabilisation subsystem, and a laser beam steering subsystem.

The objectives of the technology development project are to develop the three remaining subsystems of the laser ranging system.

  • The beam routing subsystem (a.k.a. racetrack) is a core component of the inter-spacecraft laser measurement. The racetrack consists of three mirrors precisely positioned to form an ultra-stable retro-reflector that routes the laser beam around the microwave ranging instrument. We will design and fabricate a beam routing subsystem, comprised of optical components and a mechanical structure, suited to the space environment and demonstrate that it meets GRACE Follow-on performance requirements.
  • Laser frequency stabilisation subsystem: Laser frequency noise is a limiting factor in laser ranging measurements. We will design and fabricate a laser frequency stabilisation subsystem based on an optical cavity. We will develop laser modulators, electronics, and digital signal processing algorithms for locking a laser to the cavity. To reduce the risk of not meeting the GRACE Follow-on performance requirements, we will also evaluate two alternative stabilisation schemes.
  • Laser beam steering subsystem: Laser beam steering is required to maintain the laser link between spacecraft in the presence of spacecraft rotation. We will develop and test a laser beam steering prototype based on an optical phased-array. An optical phased-array is a powerful technology for beam steering without mechanical actuation.

When combined with commercially available space-qualified lasers (Tesat-Spacecom Co.) and the interferometric ranging transponder previously developed, this project will complete the technology development for the laser ranging system for GRACE Follow-on.

Updated:  14 February 2013/Responsible Officer:  Project Director /Page Contact:  Web Admin