KEYWORDS: Transceivers, Receivers, Interfaces, Field programmable gate arrays, Satellites, Doppler effect, Design, Commercial off the shelf technology, Systems modeling, Error control coding
Optical satellite links provide increased throughput at a more competitive SWaP compared to radio frequency links. Due to the nature of the light interaction with atmosphere and the limited pointing accuracy, the propagated optical signal suffers from significant variation of the SNR. This causes error bursts in the received bit stream but can also lead to a temporary loss of synchronization causing further loss of data. Since bit rates in optical communications can reach multiple Gbps, millions of bits are affected by these error bursts and dropouts. Therefore, we propose an additional layer of protection, termed erasure coding, which is applied on top of the PHY layer of the communication system. This approach provides time diversity by long erasure code words, so that PHY interleaving can be abstained from. Therefore, optical transceiver and terminal architectures that were not specifically tailored to optical ground-to-satellite links can be reused with this layered approach. The erasure coding scheme corrects packet losses/erasures due to channel impairments, where the packet size and consequently the length of the code consisting of packets as codeword-symbols can be picked in a flexible way. Since only packet erasures need to be corrected, erasure codes show advantages compared to PHY coding schemes in terms of memory utilization and throughput. In this paper, the proposed erasure correction scheme is explained in detail and a performance analysis for typical scintillation channel models of earth-satellite laser communication links is presented. Furthermore, implementation aspects, as well as the encoding and decoding speed are discussed. In perspective, we aim at reaching a throughput in the order of 100 Gbps.
KEYWORDS: Laser communication terminals, Homodyne detection, Satellites, Receivers, Design, Data transmission, Space operations, Power consumption, Laser communications, Data communications
This paper details the progress in laser communication activities of Tesat-Spacecom. Besides the EDRS program (European Data Relay System) update of in-orbit Laser Communication Terminal (LCT) performance, with more than 81.859 data relay links executed (status Nov 2023), we report on the recent terminal projects, kicked off in 2022/2023. We present recent results of the first intradyne 1064nm coherent laser communication link from space to ground tested between GEO satellite Alphasat and the DLR/TESAT ground station (T-AOGS) at the observatory of Teide in Tenerife, Spain. Furthermore, we share results of intradyne lab measurements. Besides technology demonstrations, we present the new TESAT products in development (SCOT20 and SCOT135), that aim for the Cubesat market on the one hand and the high-performance systems for MEO / GEO satellites with data rates up to 100 Gbps, using commercial COTS technology, on the other hand. Finally, we report on the progress of the SCOT80 terminals delivered for the SDA Tranche 0 program.
KEYWORDS: Laser communication terminals, Quantum key distribution, Satellites, Space operations, Free space optics, Quantum data, Power consumption, Laser communications
This paper details the progress in the laser communication activities of Tesat-Spacecom. The EDRS program, the European Data Relay System, a private public partnership program between the European Space Agency ESA and Airbus Defence and Space ADS, is running flawlessly, until now (Oct 2022) 75.000 data relay links have been executed. We report on the performance of the systems in space and detail on other laser comms related activities of TESAT. Especially the delivery and launch of the first optimized LCTs (Laser Communication Terminals) for LEO data relay, the Smart LCTs. In addition, the delivery of Cubesat LCTs have to be mentioned, and the development, qualification and delivery of the ConLCTs for the SDA Tranche 0 program. Furthermore, TESATs involvement in Quantum Key Distribution (QKD) and Precision Navigation and Timing (PNT) programs will be detailed.
The increasing demand on high speed communication networks has stimulated the development of optical free space data transmission during the last years. TESAT has developed a laser communication terminal (LCT) that fulfills the need of a power efficient system whose capability has been successfully demonstrated at bidirectional space-to-space links and bidirectional space-to-ground links (SGLs) at a data rate of 5.625 GBit/s with a homodyne detection scheme and a BPSK modulation format. In comparison to a direct detection system, the homodyne detection scheme works as a bandpass filter. The transmission is immune to false light and even data transmission with the sun in the receiver field of view (FOV) is possible.
Compared to common RF transmission which is implemented on spacecrafts for data transmission, optical transmission provides not only higher transmission rates (factor 10) but also shows excellent security features since the laser beams directivity making it immune to interception.
KEYWORDS: Digital signal processing, Receivers, Phase shift keying, Modulation, Signal detection, Homodyne detection, Astronomical imaging, Personal digital assistants, Optical design, Data communications
Due to higher data rates, better data security and unlicensed spectral usage optical inter-satellite links (OISL) offer an attractive alternative to conventional RF-communication. However, the very high transmission distances necessitate an optical receiver design enabling high receiver sensitivity which requires careful carrier synchronization and a quasi-coherent detection scheme.
KEYWORDS: Digital signal processing, Receivers, Filtering (signal processing), Homodyne detection, Signal to noise ratio, Signal detection, Optical filters, Phase shift keying, Modulation, Satellites
We present simulative and experimental investigations of different coherent receiver designs for high-speed optical intersatellite links. We focus on frequency offset (FO) compensation in homodyne and intradyne detection systems. The considered laser communication terminal uses an optical phase-locked loop (OPLL), which ensures stable homodyne detection. However, the hardware complexity increases with the modulation order. Therefore, we show that software-based intradyne detection is an attractive alternative for OPLL-based homodyne systems. Our approach is based on digital FO and phase noise compensation, in order to achieve a more flexible coherent detection scheme. Analytic results will further show the theoretical impact of the different detection schemes on the receiver sensitivity. Finally, we compare the schemes in terms of bit error ratio measurements and optimal receiver design.
Karen Saucke, Christoph Seiter, Frank Heine, Mark Gregory, Daniel Tröndle, Edgar Fischer, Thomas Berkefeld, Mikael Feriencik, Marco Feriencik, Ines Richter, Rolf Meyer
Tesat together with Synopta have built a Transportable Adaptive Optical Ground Station (TAOGS) under contract of German Aerospace Center DLR for communication with the 1st and 2nd generation of Tesat’s spaceborne Laser Communication Terminals (LCTs), which employ coherent homodyne optical communication with 1064 nm and binary phase shift keying (BPSK) modulation. The TAOGS is able to communicate with space segments on low earth orbit (LEO, high pointing and tracking dynamics, 5.625 Gbps), and with space segments on geostationary orbit (GEO, low pointing dynamics, up to 40,000 km distance, optical data rate of 2.8125 Gbps and user data rate of 1.8 Gbps). After an alignment and testing phase at the location of Izana, Tenerife, using the TDP1 LCT on geostationary Alphasat as counter terminal, the TAOGS is now fully functioning. Several up-links, down-links and bi-directional links have been performed. Experimental results of some of these links are presented. An outlook to further activities is given.
Tesat is performing inter-satellite links (ISLs) for over 5 years now. Besides the successful demonstration of the
suitability of coherent laser communication for high speed data transmission in space, Tesat has also conducted
two major satellite to ground link (SGL) campaigns during the last 3 years. A transportable ground station has
been developed to measure the impact of atmospheric turbulence to the coherent system. The SGLs have been
performed between the Tesat optical ground station and the two LEO satellites TerraSAR-X and NFIRE, both
equipped with a Tesat LCT. The capability of the LCTs of measuring the signal intensity on a direct detection
sensor and on a coherent sensor simultaneously makes the system unique for investigating the atmospheric distortion
impacts. In this paper the main results of the SGL campaigns are presented, including BER performance for
the uplink and downlink. Measured scintillation profiles versus elevation angles at different weather conditions
are illustrated. Finally preliminary results of an adaptive optics system are presented that has been developed
to be used in the transportable adaptive optical ground station (T-AOGS) acting as the counter terminal for the
LCT mounted on Alphasat, a geostationary satellite of the European Space Agency (ESA), in autumn 2013.
Laser communication terminals with data rates far above 1 Gbps have been in operation in orbit since January 2008, and the links established between two low Earth orbit (LEO) satellites have demonstrated error-free communication. Bit error rates better than 10−11 have been achieved without data encoding. Signal acquisition can be reproducibly achieved within a few seconds. After adaptation to larger link separation distances these laser communication terminals will be used in the low earth orbit-geosynchronous satellite (LEO-GEO) link of European data relay satellite (EDRS), the GEO European data relay system. LEO-to-ground and ground-to-LEO links have examined the impact of the atmosphere on such optical links. In the future, high data rate GEO-to-ground links will require ground stations equipped with adaptive optics, which are currently under development.
For the first time, laser communication terminals will be operational in a commercial service, EDRS, the European Data
Relay System. Future earth observation satellites call for satellite-to-ground links with high availability to make their
data immediately available to the user. High availability is only achieved by GEO relay links, i.e. communication links
from LEO satellites to a geostationary relay satellite which transmits the data to the ground. While RF communication
limits the GEO relay's data rate to roughly 1 Gbps optical communication will extend its capacity into the 10 Gbps
range.
Today, laser communication terminals operated in LEO-LEO, LEO-to ground and ground-to-LEO links, all at a data
rate of 5.625 Gbps, are applied in campaigns with scientific objectives and for technology demonstrating purposes.
Adapted to the larger link distance of the LEO-to-GEO link the laser communication terminals will be applied in the
GEO relay EDRS operated for the time being at 1.8 Gbps. Terminals will be delivered for the accommodation on
Alphabus, EDRS-A and EDRS-C, European GEO-satellites, and on Sentinel-1a and Sentinel-2a, European LEO
satellites.
5.625 Gbps bidirectional laser communication at 1064 nm has been demonstrated on a repeatable basis between a Tesat
coherent laser communication terminal with a 6.5 cm diameter ground aperture mounted inside the European Space
Agency Optical Ground Station dome at Izana, Tenerife and a similar space-based terminal (12.4 cm diameter aperture)
on the Near-Field InfraRed Experiment (NFIRE) low-earth-orbiting spacecraft. Both night and day bidirectional links
were demonstrated with the longest being 177 seconds in duration. Correlation with atmospheric models and preliminary
atmospheric r0 and scintillation measurements have been made for the conditions tested, suggesting that such coherent
systems can be deployed successfully at still lower altitudes without resorting to the use of adaptive optics for
compensation.
Optical links, based on coherent homodyne detection and BPSK modulation with bidirectional data transmission of 5.6
Gbps over distances of about 5,000 km and BER of 10-8, have been sufficiently verified in space. The verification results
show that this technology is suitable not only for space applications but also for applications in the troposphere.
After a brief description of the Laser Communication Terminal (LCT) for space applications, the paper consequently
discusses the future utilization of satellite-based optical data links for Beyond Line of Sight (BLOS) operations of High
Altitude Long Endurance (HALE) Unmanned Aerial Vehicles (UAV). It is shown that the use of optical frequencies is
the only logical consequence of an ever-increasing demand for bandwidth. In terms of Network Centric Warfare it is
highly recommended that Unmanned Aircraft Systems (UAS) of the future should incorporate that technology which
allows almost unlimited bandwidth. The advantages of optical communications especially for Intelligence, Surveillance
and Reconnaissance (ISR) are underlined. Moreover, the preliminary design concept of an airborne laser communication
terminal is described. Since optical bi-directional links have been tested between a LCT in space and a TESAT Optical
Ground Station (OGS), preliminary analysis on tracking and BER performance and the impact of atmospheric
disturbances on coherent links will be presented.
Since February 2008 coherent laser communication links are operational in-orbit. Transmitting data at a rate of
5.625 Gbps they verify the capability of laser communication exemplarily in LEO-LEO and Ground-LEO
constellations. The LEO-LEO links run with a bit error rate of 10-11. Acquisition typically is closed within seconds. On
the basis of these results laser communication terminals are adapted to LEO-GEO links with a still scalable user data
rate of 1.8 Gbps. The terminals will be applied in the European data relay system.
Today, coherent optical inter-satellite links are operational in LEO-LEO constellations for more than two years. With
data transmitted error free at rate of 5.625 Gbps and links established within a few seconds they prove a performance
well suited for commercial applications. For the first time coherent LEO-to-ground links have been built up, too. They
allow to investigate the atmosphere's impact on optical space-to-ground links for later optimization of optical ground
stations.
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