Advancing Inter–Spacecraft Laser Interferometry for Future Gravity Missions

              A decisive step for future space missions in the field of next-generation gravity missions was taken by launching the first and only inter-satellite interferometer aboard the GRACE-FO mission. Improving the LRI for NGGM is the next goal.  Both microwave and LRI instruments provide ‘biased’ ranges with an unknown offset. Thus, absolute ranging that resolves the bias may open new avenues for orbit determination and data processing. Laser ranging involves precise pointing due to the narrow width of the laser beams. This complicates the initial link acquisition which requires the simultaneous alignment of two optical beams with respect to the line of sight, as well as matching the laser frequencies to enable interferometry. Even small changes in the pointing angle lead to enormous movement in the beam spot at the opposite spacecraft several 100 km away. In the LRI, the link acquisition procedure consists of synchronized spatial scans (two per spacecraft) performed by a steering mirror on each spacecraft together with a frequency scan of the laser on the transponder spacecraft. The whole procedure takes about nine hours.

               We investigate alternative strategies to improve link acquisition speed, robustness, autonomy, and compatibility with redundancy schemes. In particular, we evaluate the use of a dedicated non-coherent acquisition sensor, its capabilities and its interaction with the other optical elements. The sensor, likely an InGaAs CMOS camera, will measure the tilt of the incoming beam significantly reducing the initial acquisition time.

              Lastly, laser interferometry can be combined with techniques from telecommunications. With minimal extra hardware, auxiliary functions can be added to the existing laser link. These include the measurement of pseudo-ranges, i.e. combinations of absolute range and clock offsets that can be disentangled to measure the absolute range to centimeter accuracy and clock offsets to nanosecond accuracy. Both may be useful for gravity field recovery data processing. In addition, a data stream of up to dozens of kilobits per second can be transmitted in parallel with the same modulation. This would provide extra redundancy for the ground contacts and/or simplify ground operations, or maybe even enable additional operational features in real-time. Since noise sources and performance limitations directly depend on laser performance and stabilization technique, advanced frequency stabilization based on molecular iodine references may be integrated into the test bed in the future.

           In this poster, we show a LRI test bed comprising two hexapods capable to simulate satellite rotations, initially built to test different acquisition procedures. We address the methodology to improve the 5D link acquisition with realistic hardware and introduce a 3-stage control system for the FSM. Furthermore, we evaluated different optical layouts for NGGM and provide trade-off.