Verification of Accuracy in optical time transfer

Abstract

In global navigation satellite systems such as Galileo, the satellites’ knowledge of their orbital states and clock offsets determines the user timing and positioning quality. The German Aerospace Center (DLR) proposed the Kepler concept as a technology enhancement for Galileo that uses optical intersatellite links for two-way time transfer and ranging to support autonomous clock synchronization and orbit determination. Previously, a laboratory demonstrator has achieved sub-picosecond precision in two-way optical time transfer and ranging. However, accurate measurements are required for the Kepler concept. The objective of this thesis is therefore the establishment of an analytical and experimental foundation for ensuring accuracy in optical two-way time transfer and ranging. The analytical work comprises the identification of relevant delay terms, the theoretical development of calibration concepts to determine these delay terms, and the derivation of a correction framework for chromatic dispersion. The relevant ranging delay term is identified to be an internal delay sum whereas the relevant time transfer delay term involves differences between internal delays. The proposed ranging calibration method therefore requires measurements of delay sums. The proposed time transfer calibration methods exploit signal chain symmetries and require measurements of chain delay differences. As part of the experimental work, an accuracy verification testbed is developed that enables controlled delay changes in an optical signal with a precision below 0.1 ps. This testbed is first used to verify the internal scale accuracy of the laboratory demonstrator and to derive measurement uncertainty budgets. The testbed is then used in the laboratory demonstrator to verify the proposed time transfer and ranging calibration methods. For the time transfer calibration methods, an additional reference measurement setup and procedure are developed to measure propagation delays in transceiver transmission chains. The ranging calibration method is shown to enable absolute two-way ranging accuracy, after correction for chromatic dispersion, with a residual error of 0.03(120) ps. For time transfer, both proposed calibration methods are demonstrated at the proof-of-principle level. The demonstration of the second method shows that two-way time transfer accuracy can be recovered after uncontrolled internal delay changes with a residual error of 2.2(15) ps. The thesis concludes by summarizing the results and outlining the required next steps toward operational use of accurate optical two-way time transfer and ranging