Session P5a: Present and Future Clocks for Space

Testing Optical Clock Technologies for Future GNSS on the ISS: The COMPASSO Iodine-Based Frequency Reference
Thilo Schuldt, Klaus Abich, Tasmim Alam, Jonas Bischof, Tim Blomberg, Alex Boac, Andre Bußmeier, Frederik Kuschewski, Markus Oswald, Niklas Röder, German Aerospace Center (DLR), Institute of Quantum Technologies, Department Quantum Metrology; Martin Gohlke, DLR, Institute of Space Systems; Ludwig Blümel, Thomas Zechel, DLR, Institute of Communications and Navigation; Xavier Amigues, Andreas Eckardt, Winfried Halle, Bernd Zender; DLR, Institute of Optical Sensor Systems; Jan Hrabina, Jindrich Oulehla, Institute of Scientific Instruments of the Czech Academy of Sciences; Ahmad Bawamia, Klaus Döringshoff, Markus Krutzik, Christian Kürbis, Andreas Wicht, Ferdinand-Braun-Institut gGmbH; Stefan Oschkera, Syentec GmbH; Maciej Sznajder, PW Sznajder; Michael Jentsch, Salome Schweikle, Christopher Speidel, Airbus Defence & Space GmbH; A. Raja, M. Lezius, Menlo Systems GmbH; Norbert Beller, Christian Dahl, Martin Großmann, Timo Liebherr, Kai Voss, SpaceTech GmbH; Jan Wüst, Claus Braxmaier, DLR, Institute of Quantum Technologies & Universität Ulm, Institute of Microelectronics
Location: Beacon B
Date/Time: Thursday, Jan. 25, 2:35 p.m.

Global Navigation Satellite Systems (GNSS) require high-performance and reliable clocks. While current systems are based on microwave clock technologies, optical technologies have advanced over the last few decades, demonstrating frequency stabilities of 10^-18 for integration times of a few thousand seconds. They surpass microwave clocks’ performance by several orders of magnitude. Ultimate frequency stability is shown using optical ion clock and lattice clock technologies in complex laboratory setups, but lack the rigidity needed for space deployment. Absolute optical frequency references based on Doppler-free spectroscopy of molecular iodine can be realized in space compatible compact and ruggedized setups in a relatively short time [1]. These setups have demonstrated frequency stabilities at the 10^-15 level for integration times between 1 s and 70.000 s – comparable to the active hydrogen maser as currently integrated for the ACES (Atomic Clock Ensemble in Space) mission.
An iodine-based frequency reference – in combination with an optical frequency comb – is a possible clock candidate for future Global Navigation Satellite Systems (GNSSs), e.g. Galileo. Iodine clocks could back-up or replace the currently used microwave clocks, with the potential to improve GNSS position determination due to their lower frequency instabilities. In combination with optical inter-satellite links, optical clocks enable new GNSS architectures, cf. e.g. the proposed Kepler architecture which foresees synchronization of distant optical frequency references within the GNSS constellation using time and frequency transfer techniques [2].
Within the DLR project COMPASSO, optical clock and link technologies will be evaluated in space on the Bartolomeo platform which is externally attached to the Columbus module of the ISS [3]. The system utilizes two identical iodine-based frequency references, a frequency comb, an optical laser communication and ranging terminal (LCRT) as well as a GNSS disciplined microwave reference. While COMPASSO is specifically dedicated to test optical technologies relevant for future satellite navigation (i.e. Galileo), the technologies are also crucial for future missions related to Earth observation and science.
The idea of COMPASSO and the iodine references in particular are based on the developments at DLR within the department of Quantum Metrology at the DLR Institute of Quantum Technologies where prototypes using modulation transfer spectroscopy have been realized in the past, also with respect to applications in space. The COMPASSO iodine reference deploys an external cavity diode laser (ECDL) at a wavelength of 1064 nm as light source, which is provided by the Ferdinand-Braun Institute Berlin. Fiber-optical components are used for beam preparation and manipulation including optical isolator, acousto- and electro-optic modulators, fiber splitter/combiner and second harmonic generation modules. The frequency-doubled laser light at a wavelength near 532 nm is coupled into a free-beam spectroscopy board integrated on a Zerodur baseplate using adhesive bonding technology. This approach ensures high pointing stability, especially for the two counter-propagating laser beams in the 20 cm long iodine-filled gas cell operated in four-pass configuration and offers the robustness needed for space operation. A frequency stability of 3·10^-12/sqrt(\tau), given in root Allan deviation, for integration times \tau between 1 s and 100.000 s is targeted.
Within the COMPASSO payload, the two iodine-based frequency references can be stabilized to the same or to different (nearby) ro-vibronic transitions. Their frequency stabilities are evaluated by comparing both references in the optical frequency range, i.e., near 282 THz (corresponding to a wavelength of 1064 nm). The optical frequency comb can be referenced to the iodine reference and transfers its frequency stability from the optical frequency range to the radio frequency range. Furthermore, the frequency comb can be referenced to the onboard microwave reference and thus enables multiple comparison measurements with which the frequency stability can be evaluated. Employing the two-way optical laser communication and ranging terminal, the performance of the optical references onboard the ISS can additionally be compared to ground-based clocks.
The COMPASSO launch is foreseen for 2026. Currently, the engineering model of the iodine reference, which resembles the flight model in form, fit and function, is realized. The development is accompanied by qualification activities on component level. The mechanical and thermal design of the iodine reference is further developed and a structural thermal model will be realized in 2024.
The two iodine-based frequency references are developed under the lead of the DLR Institute of Quantum Technologies in collaboration with further DLR institutes, the space industry and research institutions. This work is funded by the German Aerospace Center (DLR e.V.) within the project COMPASSO.
[1] Schuldt et al: Optical Clock Technologies for Global Navigation Satellite Systems, GPS Solutions (2021) 25:83
[2] Giorgi et al: Advanced Technologies for Satellite Navigation and Geodesy, Advances in Space Research 64 (2019) 1256–1273
[3] Kuschewski et al: The COMPASSO Mission and its Iodine Clock, accepted for publication in GPS Solutions