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Session B5: Trends in Future Satellite Navigation Technology, System Design and Development

Verification of Optical Two-Way Time Transfer Accuracy Through a Closed-Loop Measurement Topology
Manuele Dassié, German Aerospace Center (DLR), Technical University of Berlin (TUB); Gabriele Giorgi, and Grzegorz Michalak, DLR
Date/Time: Friday, Sep. 20, 9:43 a.m.

In current Global Navigation Satellite Systems (GNSSs), the satellites' orbits and their free-running clocks are co-estimated during post-processing in complex Orbit Determination and Time Synchronization (ODTS) processing schemes. The positions and clock offsets are determined, and then predicted for the next few hours. The user receives orbital information in form of ephemeris products and applies the predicted time corrections to estimate its own position and de-synchronization with respect to the system time. The ODTS procedure, highly complex, depends on the monitoring and processing of all satellite signals from a worldwide network of ground stations and it allows the retrieval of satellite orbits at the centimeter level (during post-processing) and satellite clock offsets at the nanosecond level.
Next generation GNSSs are going to take advantage of Inter-Satellite Links (ISLs), connecting satellites across the constellation and providing improved services and robustness. The ISLs not only support data transfer between satellites, but also provide pseudorange measurements, clock comparisons, integrity monitoring, resulting in a number of cascading benefits. Radio-frequency ISLs are already in use e.g. in the BeiDou system [Yang et al. 2019] or are planned as in Galileo G2 generation [ESA 2021]. The recent increased interest in Optical Inter-Satellite Links (OISLs) could result in an additional step forward in the satellite industry thanks to better performance in terms of bandwidth, inherent robustness to jamming, and lack of bandwidth regulations. Additionally, recent advances in optical technology have shown that optical links enable time transfer and ranging capabilities with high precision, in the order of millimeters for ranging and picoseconds for the time transfer [Surof et al. 2022]. This paves the way to the application of OISLs in future GNSS systems. Thanks to the autonomous time transfer enabled by OISLs, future systems can shift from a post-processing synchronization method to an on-board real-time synchronization, as analyzed in e.g. [Trainotti et al., 2022]: from the inter-satellite clock offsets, the satellites can directly compute their own offset with respect to the system time, and accordingly correct the onboard clock. This results in a constellation of synchronized clocks, beating the same time and broadcasting the navigation messages in a highly synchronized manner. The use of OISLs potentially allows for accurate inter-satellite ranging, significantly enhancing orbit determination while diminishing reliance on ground-based infrastructure [Michalak et al., 2021a; Michalak et al., 2021b], and even enabling the potential for in-space autonomous orbit determination [Testa et al., 2023].
Like all emerging technologies, conducting experiments and missions in space to test the performance and showcase the capabilities of OISLs in operational contexts is essential before considering widespread adoption. The DLR COMPASSO mission [Schmidt et al.] is an in-orbit validation mission to demonstrate new optical technologies for future GNSS constellations with a launch scheduled for 2026. A bi-directional optical link will be established between a Laser Communication and Ranging Terminal (LCRT) hosted on the Airbus Bartolomeo platform on the International Space Station and an Optical Ground Station in Oberpfaffenhofen, Germany. One of the primary objectives is the demonstration time transfer stability at the picosecond, as well as ranging at sub-centimeter level precision.
In this work, we present preliminary analyses on a subsequent phase of the validation effort aimed at demonstrating the accuracy of clock offset and range estimates. We consider the following scenario: two Medium Earth Orbit (MEO) satellites, in trailing configuration on the same orbital plane, autonomously establish a two-way laser link whenever the visibility on each other allows for it. The MEO satellites are assumed to be in the same orbital slots of the GALILEO satellites (Walker 24/3/1). On the ground, we assume concurrent operation of two co-located and interconnected Optical Ground Stations (OGSs). This setup would enable the closure of "measurement loops formed whenever both MEO satellites are simultaneously visible from the two OGSs. During such instances, the performance of time transfer and ranging could be validated, potentially demonstrating accuracies of picosecond-level for time transfer and sub-centimeter for ranging. In an ideal scenario with perfect estimates, both the sum of clock offset and the sum of vectorial ranges across the closed loop should converge to zero. However, when biased and noisy estimates of these quantities are obtained, the sum will result in residuals.
As a first step we present a detailed procedure for conducting time transfer and ranging operations based on Inter-Satellite Links (ISLs) and Ground to Satellite Links (GLSs) observables. We begin by defining the observables, which are time-of-flight measurements between the Optical Phase Centers (OPCs) of the linked terminals, and therefore can provide pseudorange measurements. The relationship linking pseudoranges and the more useful intersatellite ranges referred to the satellites’ centers of mass is characterized in terms of clock offset, hardware delays, lever arm, relativistic effects, and measurement noise.
Then we outline a step-by-step methodology to derive relative clock offset and range estimates between optically linked endpoints. The procedure encompasses linearly combining the raw observables obtained from a two-way exchange, estimating positions from ephemeris data, correcting observations for relativistic effects and biases and finally obtain the clock offsets and ranges estimates. We conduct a comparative analysis between performing a combination of two-way time transfer and one-way ranging versus a two-way ranging methodology. By evaluating their respective strengths and drawbacks, we identify the most suitable ranging approach for the specific scenario.
An essential step is the characterization of errors in clock offset and range estimation. To validate the proposed technology, we consider the “closed-loop measurements” scenario mentioned above. We quantify the total error in the closed loop by considering the cumulative estimation errors of individual clock offsets and ranges. We then analyze this quantity and establish the requirements on satellite positions and velocities needed to keep the total error below picosecond-level for time transfer and sub-cm for ranging. The analysis demonstrates that validation of time transfer at such a high level of accuracy is achievable with the use of standard ephemeris products. Conversely, validating inter-satellite ranging proves to be considerably more challenging, given the stringent requirements for orbit determination, which are at the same level of the validation threshold. Finally, we address the estimation or calibration of inter-LCRT offsets and discuss strategies to mitigate the total closed-loop error.
By validating the time transfer and ranging procedures, and characterizing errors, we contribute to the advancement of space-based communication and navigation systems, offering valuable insights for future missions and applications involving optical links.

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