Within this project, we will address fundamental questions regarding the beam that are common in many different applications. Additionally, we will test how our optical simulations can be coupled with the eXtended High Performance satellite dynamics Simulator (XHPS) developed in project Multi-Purpose Space Mission Simulator (B02) in order to even more reliably compute the performance of current and future satellite interferometers.

We will start with the beam characterisation. For this, a few types of sensors and methods are currently available, such as Shack-Hartmann sensors, or beam profilers, however, neither is so far fully sufficient for the high resolution needed today for instance in prototype setups for future space application. We will therefore develop a wavefront sensor and extend an in-house beam fit algorithm and compare the wavefront characterisation performance with other typical methods. We will test our sensors and methods on experimental setups of other projects within TerraQ. Secondly, we will investigate how the presence of higher order modes will affect the interferometry. Here, we will compare computer simulations using a superposition of modes that was found in objective 1 to characterise the corresponding laboratory setup, with experimental results as well as with simulations using different beam models. We will thereby test, how critical the system responds to different types of beam imperfections. For this, we will use again the setups of partner projects within TerraQ, thereby increasing the knowledge and understanding of these setups.

Finally, we will for the first time link the optical simulations performed with our in-house software IfoCAD with XHPS. The satellite and orbit simulator XHPS developed in project B02 will be used to model GRACE-FO and its orbits as well as future mission layouts. On the other hand, IfoCAD is a C++ software library for designing and optimising laser interferometers. It is being developed at the Institut für Gravitationsphysik (IGP) since 2009 and provides methods for ray tracing and wavefront propagation in typical three dimensional topologies and computes interferometer readout signals. Linking these two simulators is therefore a natural step to simulate space-based gravity missions using inter-satellite interferometry like GRACE-FO and successor missions. In the first funding period, the focus will be on instrumental aspects, like exporting and importing data from one simulator into the other, and preparing the grounds for a possible direct interfacing in the future. Additionally, it will be investigated in detail, what simulation aspects can be implemented in what way in the future into XHPS to extend it for interferometric readout signals.