B06: Nanometer ranging systems for low Earth orbiters

This project covers the constellation aspects of a future laser-interferometric inter-satellite ranging mission, as opposed to the ‘local’ aspects in A05. It consists of two closely related parts, the study of mission designs including the noise budget breakdown and, on the other hand, experimental investigations on the link acquisition.

For the GRACE Follow-On Laser Ranging Instrument (LRI) with its 80 nm/√Hz noise level, it is known that the most important contributions to the noise budget are due to laser frequency noise and satellite pointing jitter. In order to achieve 10 nm/√Hz for future missions, improvements in many areas are necessary. The overall noise budget must be balanced between many contributors, such as pointing and thermal stability, phase readout, wavefront quality, alignment of optics, shot noise, clock noise etc. This task is intimately linked with the design of the interferometer, e.g. its laser beam divergence, beam routing, pointing mechanism (if any), imaging optics, phase detection scheme etc. In this project the capabilities and constraints of the individual subsystems of the interferometer will be evaluated and joined to a coherent design that optimizes the science return. One output is the expected ranging performance and its frequency dependence which will be used to predict the science return. For this the project team is working in close collaboration with project B05 (High performance satellite formation flight simulator). Another output will be the noise budget breakdown that translates into requirements to the subsystems with respect to their required performance and operation mode. Furthermore the breakdown will be used to steer the subsystem development, e.g. in A05, towards the most rewarding improvements.

The second part of this project is an experimental effort dedicated to the initial link acquisition, which is a challenge in long distance laser interferometry. The issue lies in aligning five degrees of freedom (two angles at each spacecraft and one laser frequency), which all have to be simultaneously within a narrow range to enable operation of the laser link. This can be achieved by synchronized spatial and frequency scans. The acquisition strategy depends on the interferometer design and alignment accuracy, but also on the orbits and satellite attitude control. While the link acquisition needs to occur only once (or infrequently) before science operation commences, it is in many respects the most challenging aspect of interspacecraft interferometry. A well-understood and proven acquisition procedure is not only an absolute necessity for operation, but also implies that the interferometer as a whole is well understood and under control. The output of this part will be robust acquisition procedures tested by simulation and experiment, and a flexible testbed that allows testing of alternative procedures as well as testing of flight-like hardware.