Several architectures for autonomous optical synchronisation (time transfer) and ranging between satellites have been proposed, which generally require custom transceiver implementations with additional components. These architectures are not well suited or even feasible to the needs of communications transceivers for optical satellite links. In this paper, we analyze the feasibility of integrating time transfer functionality into an optical communications link primarily intended for data transfer. A fully digital system in which all signal processing occurs in the digital domain is presented. The transmitted optical waveform consists primarily of data, as well as a data field containing observables necessary for computing precise and stable two-way time transfer. At the receiver, the signal is detected and demodulated using conventional intradyne signal processing algorithms. The time transfer observables are then obtained at the frame detection by using the timing recovery subsystem to estimate the time of arrival. To validate the feasibility of the proposed time transfer function, a real-time, 5 GBit/s QPSK FPGA-based laboratory demonstrator was developed.

Global navigation satellite systems broadcast signals for positioning estimation and time dissemination to end users. Current systems rely on ground-based observations and predictions. The Kepler system deviates from that by employing bidirectional optical inter-satellite links for time synchronization and orbit determination. This approach increases resilience, reduces ground infrastructure requirements and improves end-user accuracy. Optical two-way time transfer at sub-picosecond level and ranging at sub-millimeter precision are achieved by means of a chip rate of 12.8 Gcps and high-resolution optical correlation tracking. Our laboratory demonstrator verifies this concept. Two-way optical time transfer with a short-term stability below 1.8· 10^-13 was demonstrated over a 30 m free space range. The received optical signal power was scaled to emulate a medium earth orbit inter-satellite distance of over 50,000 km. The hardware is rather similar to the one used in coherent optical communications systems and thus of limited complexity. This confirms that a system architecture based on bidirectional optical free-space links can achieve the required precision in time synchronization and orbit estimation in future global navigation satellite systems.

Coherent optical satellite links enable high-throughput communication and high accuracy ranging to and between satellites. Due to the ever-increasing demand for throughput, wavelength division multiplexing of polarization multiplexed optical signals is being considered as a solution to provide high-speed optical satellite links. Fiber-optic systems solve the implementation scalability problem of these systems by shifting design complexity to integrated circuits, thereby massively reducing the system footprint. As a result of the major advances in complementary metal-oxide-semiconductor (CMOS) technology, the implementation scalability of such systems in terrestrial fiber systems has been solved by shifting the system complexity to digital hardware, enabling intradyne reception and complex signal recovery algorithms. While the use of fiber-optic transceivers provides a fast path to high-speed coherent optical satellite links (OSLs), it requires additional mitigation techniques to combat the effects of both the OSL channel and the space environment. To support future satellite networks with Tbit/s optical links, it will be critical to further minimize the size, weight, and power (SWaP), cost and reliability of the transceivers. Thus, the development of custom intradyne optical transceivers for OSLs is emerging as an attractive option as the demand for throughput in satellite networks continues to grow. This would not only enable the use of a more optimized signal processing chain but also enable the use of radiation mitigation techniques optimized for the signal processing architecture and the use of soft-decision forward error correction (FEC) optimized for OSLs. The signal processing of coherent optical satellite receivers can be divided into three key subsystems: timing recovery, carrier synchronization, and equalization. This paper reviews state-of-the-art digital signal processing for optical communication to identify suitable algorithms for timing recovery, carrier frequency and phase compensation, equalization, and polarization demultiplexing with emphasis on high-throughput optical satellite links. Finally, the performance of different digital signal processing algorithms is assessed by numerical simulations considering different optical satellite link scenarios.

Ultra-precise optical time transfer in space enables global time distribution and enhanced global positioning systems. The feasibility of stable optical time transfer between static terminals has been verified, however the system has to be validated in space to prove its robustness and performance for an application on navigation satellites. Optical links with precise time and frequency transfer from the low earth orbit to ground in frame of the COMPASSO mission shall be performed. We propose a system design which copes with the constraints and performance requirements of the mission. The system is described and preliminary results of the breadboard are shown.

A bi-directional optical link is established over 10.45 km to perform Two-Way Frequency and Time Transfer (TWFT, TWTT) to validate the system design for Lower Earth Orbit (LEO) to ground links through the atmospheric channel. Binary phase shift keying with a homodyne reception scheme is used to recover the carrier frequency. The time transfer is enabled by optical correlation and dynamic tracking of a 25.6 Gcps transmission spread-sequence with a locally generated reference sequence. An additional data transmission of 50 Mbps is performed on the same channel. Optical references and frequency combs on both sides enable the frequency measurements. Ultra-stable oscillators generate Radio Frequency (RF) references for the time transfer. The performance of the system is evaluated under different atmospheric conditions. Frequency transfer with a stability of 5 · 10^?15 at 1 s gate time. Simultaneous two-way time transfer with a stability of 1 · 10^?13 at 1 s gate time prove the feasibility of the system even through the atmospheric channel.

Introducing optical inter-satellite links in a global navigation satellite system enhances orbit determination and offer frequency synchronization across the constellation. To show proof-of-concept and system capabilities a laboratory demonstrator has been developed and first tests in laboratory environment have been taken out. The demonstrator is buildup of two reciprocal units interconnected via bi-directional free-space optical links. Each unit is composed of opto-mechanical terminal, optical communication system and digital signal processing implemented on a field programmable gate array. Ranging and time transfer is realized utilizing a 25.55 Gigachip-per-second binary phase shift keying modulation scheme. Correlating the received ranging sequence with a local generated reference, pseudo range determination within the order of 100 µm accuracy is achieved. In addition, a 50 Megabit-per-second data signal is multiplexed onto the ranging sequence for exchanging satellite and time information as well as user data. Frequency transfer is realized using a cavity stabilized laser as carrier transferring its stability onto the phase locked local oscillator at the receiver side. Global synchronization at system level is achieved by means of synchronizing the optical carrier and spread sequence to a highly stable clock reference as well precise ranging and information distributed through the data channel. Using on-satellite ultra-stable optical oscillator and a frequency comb for radio frequency reference generation short-term frequency stability in the order of 10-15 s/s (Allan deviation at 1 s gate time) is achieved. The aim of this paper is to show the current status of the laboratory demonstrator development and present first measurements of the entirely setup system. A general overview of the digital signal processing is given and data transmission as well as time transfer are discussed in particular. The laboratory demonstrator setup and mechanical construction is presented. Clock stability transfer between high-accuracy radio frequency reference and the digital signal processing system is shown. Further, optical frequency transfer experiments are performed demonstrating successful synchronization between laboratories.

The adoption of optical inter-satellite links in future evolutions of Global Navigation Satellite Systems could enable key capabilities such as intra-system communication, time-transfer and precise ranging. This work reports on the development of a laboratory demonstrator designed to verify the performance of two-way optical links in a controlled environment. First test results on the performance of bi-directional frequency transfer and single directional ranging over a 30 m free-space link are reported.

Broadband internet access has become a vertex for the future development of society and industry in the digital era. Geostationary orbit (GEO) satellite can provide global broadband coverage, becoming a complementary solution to optical fiber network. Low-earth-orbit (LEO) constellations have been proposed in the last years and they may become a reality soon, but still based on radiofrequency for the ground-to-satellite links. Optical technologies offer multiple THz of available spectrum, which can be used in the feeder link. The DLR's Institute of Communications and Navigation has demonstrated Terabit-per-second throughput in relevant environment for GEO communications, in terms of the turbulent channel. In 2016 DLR set the world-record in free-space communications to 1.72 Tbit/s, and in 2017 to 13.16 Tbit/s. Two terminals, emulating the satellite and the ground station have been developed. Bi-directional communications link with single-mode-fiber coupling at both ends was demonstrated. Adaptive optics for the downlink and uplink (pre-distortion) improved the fiber-coupling in downlink and decreased signal fluctuations in uplink. A 80 Gbit/s QPSK system based on digital homodyne reception was also developed, demonstrating the use of coherent communications under strong turbulence conditions. These activities were performed in the frame of two internal DLR projects, THRUST and Global Connectivity Synergy project. Several measurement campaigns took place in the last years in a valley-to-mountain-top test-link. Turbulence has been monitored at both ends and the point-ahead-angle has been emulated by separating the downlink beacon from the receiving aperture. An overview of the system and the main results will be presented.

Optical inter-satellite links are proposed in the Kepler constellation to connect satellites in a Global Navigation Satellite System (GNNS) constellation for optical ranging, time transfer and data transmission [1]. A laboratory demonstrator is being developed to verify all three aspects. The demonstrator is constituted by two terminals, performing a bidirectional free-space optical link in the laboratory, with single-mode fiber coupling in the receivers at both sites. The optomechanics is based on commercial off-the-shelf (COTS) components. The optical terminal includes a point-ahead assembly, which compensates for the point-ahead angle (PAA) between the two linked satellites. The absence of the PAA under laboratory conditions allows this mirror to be used for pointing jitter emulation instead, i.e. to emulate the expected satellite platform angular vibrations. The ranging is performed by using a 25.55 Gc/s binary phase shift keying (BPSK) phase modulation of the optical carrier. This high modulation-rate allows ranging accuracy in the order of 100 mu m. The data-communication channel is multiplexed to the ranging signal at a rate of 50 Mb/s and allows exchanging satellite and timing information. Both the optical carrier and the spreading sequence are synchronized to the on-board reference and a reference clock input for the sequence generator Field Programmable Gate Array (FPGA) is generated. Thus, the information exchanged though the data communication channel and precise ranging are used to support highly accurate two-way time transfer between the linked satellites. The objective of this paper is to present the hardware developments of optical terminals, which will demonstrate range measurements and communications. The first characterizations of the main terminal components are shown, namely the real-time Digital Signal Processing (DSP) system based on FPGA for ranging and data transfer and the optical phase-locked-loop (OPLL) for locking and tracking the incoming optical carrier. Further, static homodyne ranging estimation is evaluated and the impact on the frequency stability of the optical assembly and other main components is shown.

The paper presents the laboratory demonstrator design of the bidirectional inter-satellite optical link for the Kepler system. Communication and ranging subsystem serves for data transmission at 50 Mbit/s and providing highly precise sub-millimeter ranging accuracy. The preliminary back-to-back verification based on data and ranging signal transmission confirms this. For interconnection of the two systems, an optical terminal has been designed and is currently being integrated and evaluated in the laboratory. Furthermore, it will be used for emulation of channel impairments caused mainly by mechanical platform jitter.