The GRACE Follow-On (GFO) mission, launching this year, will include the first interspacecraft laser interferometer. Measuring the displacement between the two GRACE satellites with laser interferometry will improve upon the sensitivity of the original GRACE by orders of magnitude, allowing more precision in the measurements of Earth’s Geoid. Considerable work was required to adapt lab-based interferometry techniques for space with a couple of challenges that had to be overcome during the development of the GFO laser ranging interferometer including link acquisition, laser frequency noise and rotation of the satellites coupling into the displacement measurement.

While working at the Department of Quantum Science I have been developing an interferometer architecture which I call multi-link interferometry. Using multiple interspacecraft link measurements and high-speed signal processing, the interspacecraft displacement can be synthesised in post-processing. By forming linear combinations of the individual link measurements with different weights any displacement error from rotation-to-pathlength coupling can be cancelled. Since the weights used in the linear combinations can be adjusted in post-processing, any rotation-to-pathlength coupled errors resulting from the positioning and alignment of the interferometer can be cancelled.  This allows the requirements placed on the alignment and positioning of the interferometer to be relaxed. 

In this presentation I will give an overview of multi-link interferometry, focusing on three experiments I performed to verify the technique could be useful in a future interspacecraft laser interferometry mission.