Tobias D. Schmidt, German Aerospace Center (DLR), Institute of Communication and Navigation, Germany; Stefan Schlüter, DLR, Galileo Competence Center, Germany; Thilo Schuldt, DLR, Institute of Quantum Technologies, Germany; Martin Gohlke, DLR, Institute of Space Systems, Germany; Ramon Mata Calvo, DLR, Institute of Communication and Navigation, Germany; Daniel Lüdtke, DLR, Institute of Software Technology, Germany; Matthias Dauth, DLR, Space Operations and Astronaut Training, Germany; Matthias Lezius, Menlo Systems GmbH, Germany; Christian Michaelis, Andrej Brzoska, Tesat-Spacecom GmbH & Co. KG, Germany; Christian Steimle, Airbus Defence and Space GmbH, Germany

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Optical technologies are promising candidates to improve or even revolutionize future Global Navigation Satellite Systems (GNSS). The advantages of optical technologies compared to their classical microwave counterparts are numerous, e.g. better frequency stabilities of optical clocks or higher ranging accuracy using optical links. There are already studies of new concepts for GNSS, architectures which almost completely rely on optical technologies. The most prominent and promising idea is DLR’s Kepler architecture concept, which is based on optical frequency references (OFRs) as well as on bi-directional laser communication and ranging terminals (LCRTs). Compact, highly stable, laser-optical clocks, established by combining optical frequency references with optical frequency combs, will significantly improve future generations GNSSs, such as the European Galileo system. When combined with optical laser links, e.g. via LCRTs, to ground infrastructure and between GNSS satellites, they will result in a higher accuracy of position determination on Earth. At the same time, it will be possible to reduce the complexity and size of GNSS ground infrastructure. In order to initiate the revolution of GNSS DLR prepares an in-orbit verification mission on the Airbus Bartolomeo platform which is attached to the Columbus module of the International Space Station (ISS). The platform can accommodate up to twelve external payloads and offers a unique opportunity for demonstration and verification missions in space, especially as the operational concept includes the return of payloads at the end of their mission period. The primary objective of COMPASSO is to prove the feasibility of in-orbit operation of optical key technologies as well as to pave the way for long duration (10 to 15 years) operation of laser-optical technology, in future generations of the Galileo system and in scientific space missions, e.g. within the Next Generation Gravity Mission (NGGM) program, or the Laser Interferometer Space Antenna (LISA). The project started in 2020 and the envisaged launch is 2025. COMPASSO can be roughly divided into three phases: 1) Payload development and launch: during this phase, the individual payload subsystems will be developed, tested, and integrated onto the Bartolomeo ArgUS Multi-Payload Carrier. A period of about four years is planned for this phase. 2) Experimental phase on the ISS: the planned mission experiments will be performed during this phase. It will last about 18 months. 3) Return and validation: in this phase, the mission will be concluded, and the subsystems will be dismantled, returned to Earth and handed over to DLR for tests and evaluation. The payload of the mission comprises several optical key technologies, i.e. two absolute optical frequency reference systems based on molecular iodine, one optical frequency comb (OFC) and one bi-directional laser communication and ranging terminal. A positioning, velocity, attitude and time (PVAT) system as well as an on-board computing and data storage system completes the core elements of the overall payload. COMPASSO’s optical frequency references are based on Doppler-free spectroscopy of molecular iodine. Both references are using lasers operating at 1064nm which can be stabilized on the same or on different (nearby) hyperfine transitions of molecular iodine near a wavelength of 532nm. An optical frequency comb operating at 1550nm center wavelength with a repetition rate of 100MHz transfers the frequency stability of the two references from the optical to the radio frequency domain. In addition, the frequency comb can be referenced to an on-board microwave reference consisting of a high-performance GNSS disciplined crystal oscillator (OCXO), thereby allowing multiple comparative measurements to assess the frequency stability in different frequency regimes and in the relevant time periods of the references/clocks. A bi-directional LCRT operating at 1064nm enables time and frequency transmission between the stable clock signals on the ISS and on Earth – together with clock synchronization, high-precision ranging (distance measurement) and data communications. By comparing the absolute frequency of the iodine reference operated in orbit with the corresponding value on ground, an analysis of the gravitational red shift can be used as a test of the general theory of relativity. The core components listed above will form the COMPASSO payload once they are integrated and installed on the Bartolomeo ArgUS Multi-Payload Carrier. The physical data connection will be provided via a General-purpose Oceaneering Latching Device 2 (GOLD-2) connector, the standard interface for all Bartolomeo payloads. The payload will be launched pressurized onboard an ISS visiting vehicle, transferred to the outside through the Bishop airlock and installed with the ISS robotic manipulator system. COMPASSO will be operated by the DLR German Space Operations Center supported by the European Space Agency Columbus Control Center and the Airbus Bartolomeo Control Center using the ISS ground segment infrastructure.