Intersatellite clock synchronization and absolute ranging for space-based gravitational-wave detectors

authored by

Jan Niklas Reinhardt

supervised by

Gerhard Heinzel

Abstract

Space-based gravitational-wave detectors extend the observation band of ground-based detectors into lower frequencies. One such mission is the Laser Interferometer Space Antenna (LISA), which is designed to investigate the gravitational universe between 0.1 mHz and 1 Hz. LISA performs single-link interferometric measurements across three widely separated satellites to detect picometre variations in their distances caused by gravitational waves (GWs). A priori these single-link measurements are swamped by laser frequency noise, which surpasses any GW signal by more than eight orders of magnitude. To address this, a post-processing method known as time-delay interferometry (TDI) combines appropriately delayed versions of the single-link measurements to synthesize equal-arm interferometers, thus effectively reducing the laser noise. TDI requires estimates for the intersatellite signal propagation delays, i.e., it requires absolute interspacecraft ranging. The primary ranging scheme in LISA is based on pseudo-random noise (PRN) codes. PRN ranging needs further treatment as it involves ambiguities, offsets, and noise. The LISA interferometric and ground-based measurements can be combined to form three further absolute ranging observables: clock-sideband ranging, TDI ranging, and ground-observation-based ranging. This thesis introduces a ranging sensor fusion that combines the four LISA ranging observables to generate accurate and precise estimates for the absolute intersatellite ranges. Besides interspacecraft delays, onboard delays due to onboard signal propagation and processing constitute non-negligible parts of the synthetic equal-arm interferometers in TDI. In this thesis, we develop compensation schemes for onboard delays in TDI. We hereby focus on onboard optical delays before the combining beam splitters, which require adjustments in the TDI algorithm. Furthermore, this thesis addresses intersatellite clock synchronization in the LISA mission. Since the satellite clocks are not actively synchronized, the synchronization process must be carried out in on-ground data processing. The necessary information regarding the desynchronization between the satellite clocks is obtained from the previously mentioned ranging measurements. To be precise, these measurements capture pseudoranges rather than actual ranges, i.e., they combine the intersatellite light travel times with the clock desynchronizations between the sending and receiving satellite clocks. To achieve intersatellite clock synchronization, it is necessary to isolate the differential clock desynchronizations, i.e., to disentangle the pseudoranges. Pseudorange disentanglement further requires two ground-based LISA observables: the orbit determination (OD) and the MOC time correlation (MOC-TC). This thesis introduces and simulates these ground-based measurements. It then demonstrates their application in the pseudorange disentanglement process. For pseudorange disentanglement, this thesis develops an algorithm based on a nonstandard Kalman filter (KF). This algorithm is specifically designed for clock synchronization in systems where pseudorange measurements are taken in different time frames. It achieves intersatellite clock synchronization with nanosecond accuracy and estimates the intersatellite light travel times with submeter accuracy, thus meeting the TDI requirements for laser frequency noise suppression.

Organisation(s)

PhoenixD: Simulation, Fabrikation und Anwendung optischer Systeme

Type

Dissertation

No. of pages

174

Publication date

02.07.2025

Publication status

Veröffentlicht

Electronic version(s)

https://doi.org/10.15488/19219 (Access: Offen)