The Gravity Recovery and Climate Experiment (GRACE) has demonstrated that low–low satellite-to-satellite tracking enables monitoring the time variations of the Earth’s gravity field on a global scale, in particular those caused by mass-transport within the hydrosphere. Due to the importance of long-term continued monitoring of the variations of the Earth’s gravitational field and the limited lifetime of GRACE, a follow-on mission is currently planned to be launched in 2017. In order to minimise risk and the time to launch, the follow-on mission will be basically a rebuild of GRACE with microwave ranging as the primary instrument for measuring changes of the intersatellite distance. Laser interferometry has been proposed as a method to achieve improved ranging precision for future GRACE-like missions and is therefore foreseen to be included as demonstrator experiment in the follow-on mission now under development. This paper presents the top-level architecture of an interferometric laser ranging system designed to demonstrate the technology which can also operate in parallel with the microwave ranging system of the GRACE follow-on mission

The Laser Ranging Interferometer (LRI) instrument on the Gravity Recovery and Climate Experiment (GRACE) Follow-On mission has provided the first laser interferometeric range measurements between remote spacecraft, separated by approximately 220 km. Autonomous controls that lock the laser frequency to a cavity reference and establish the 5 degree of freedom two-way laser link between remote spacecraft succeeded on the first attempt. Active beam pointing based on differential wavefront sensing compensates spacecraft attitude fluctuations. The LRI has operated continuously without breaks in phase tracking for more than 50 days, and has shown biased range measurements similar to the primary ranging instrument based on microwaves, but with much less noise at a level of 1 nm/ √ Hz at Fourier frequencies above 100 mHz.

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We report on the first demonstration of time-delay interferometry (TDI) for LISA, the Laser Interferometer Space Antenna. TDI was implemented in a laboratory experiment designed to mimic the noise couplings that will occur in LISA. TDI suppressed laser frequency noise by approximately 10 9 and clock phase noise by 6 × 10 4 , recovering the intrinsic displacement noise floor of our laboratory test bed. This removal of laser frequency noise and clock phase noise in post-processing marks the first experimental validation of the LISA measurement scheme.PACS numbers: 04.80. Nn, 07.60.Ly, 95.55.Ym The Laser Interferometer Space Antenna (LISA) [1] is a joint National Aeronautics and Space Administration (NASA) and European Space Agency (ESA) gravitational wave observatory. LISA will observe gravitational radiation from: massive black hole mergers out to a distance of z=20; black hole, neutron star, and white dwarf inspirals into massive black holes; and white-dwarf binary orbits throughout the galaxy [2].LISA will measure the relative motion of three dragfree spacecraft (SC) separated by 5 × 10 9 m with a oneway resolution of 2×10 −11 m/ √ Hz (4×10 −21 / √ Hz strain sensitivity). Laser light is passed between SC and the interference phase between the local and distant laser (oneway) phases recorded. The design sensitivity is dominated by shot noise from the laser light for frequencies above 3 mHz and by spurious forces on the proof masses at lower frequencies.Orbital motion of the SC Doppler shifts the laser beams by up to 20 MHz, giving rise to heterodyne signals upon interference with a local oscillator on each SC. The phase change of these beat note signals is proportional to the change in path length between SC. Gravitational waves also cause a displacement between SC, phase-shifting the beat note. The challenge for LISA is to measure these phase shifts with µcycle accuracy in the presence of slowly varying Doppler shifts, millions of cycles of laser frequency noise and variations in the clock sampling frequencies.The LISA arm lengths will neither be matched (the mismatch will be up to 75,000 km) nor static, introducing sensitivity to laser frequency noise. LISA will use a technique called time-delay interferometry (TDI), combining local and inter-spacecraft phase measurements in postprocessing, to form configurations equivalent to Michelson and Sagnac interferometers [3]. TDI suppresses noise from laser frequency fluctuations by many orders of magnitude, yet preserves the gravitational wave signal. TDI consists of linear combinations of the phase measurements recorded at specific times determined by the light travel time between SC. TDI will also correct for phase noise of the ultra-stable oscillators (or clocks), which provide the phase measurement references on each SC.Although TDI has been extensively studied theoretically, there have not previously been any experimental demonstrations of the key aspects of the signal processing chain. This paper reports results from the first demonstration of TDI in a labor...

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Abstract:We experimentally demonstrate an inter-satellite laser link acquisition scheme for GRACE Follow-On. In this strategy, dedicated acquisition sensors are not required-instead we use the photodetectors and signal processing hardware already required for science operation. To establish the laser link, a search over five degrees of freedom must be conducted (± 3 mrad in pitch/yaw for each laser beam, and ± 1 GHz for the frequency difference between the two lasers). This search is combined with a FFT-based peak detection algorithm run on each satellite to find the heterodyne beat note resulting when the two beams are interfered. We experimentally demonstrate the two stages of our acquisition strategy: a ± 3 mrad commissioning scan and a ± 300 µrad reacquisition scan. The commissioning scan enables each beam to be pointed at the other satellite to within 142 µrad of its best alignment point with a frequency difference between lasers of less than 20 MHz. Scanning over the 4 alignment degrees of freedom in our commissioning scan takes 214 seconds, and when combined with sweeping the laser frequency difference at a rate of 88 kHz/s, the entire commissioning sequence completes within 6.3 hours. The reacquisition sequence takes 7 seconds to complete, and optimizes the alignment between beams to allow a smooth transition to differential wavefront sensing-based auto-alignment.

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