Optical on–off keying data links for low Earth orbit downlink applications

Abstract

Optical free-space links will shape the high-speed communications technology landscape for space missions substantially in the next years. The dramatically reduced signal spread-as compared to any radio frequency (RF) technology-provides a variety of advantages: increased power efficiency, the avoidance of interference and thus spectrum regulation issues, the inherent tap-and spoof-proofness and, most of all, the vastly increased data rates (DRs) will make this technology a 'game changer' comparable to the introduction of glass fibre instead of copper cables previously used in the global communication infrastructure. As one use case of optical space links high-speed geostationary data-relays for the repatriation of low Earth orbit (LEO) observation satellite telemetry have been tested and are currently implemented operationally by various space agencies [1-4]. Deep space missions will also boost their DRs by several orders of magnitude by sending their data to large optical receiver telescopes, NASA is currently transforming its Deep Space Network to an optical DNS, and we also see European developments in optical deep space communications [5-7]. In order to connect very high-throughput communication satellite systems to the Tbps-regime (Terabit-per-second), optical uplinks can solve the spectrum bottleneck that RF links would otherwise encounter [8]. In the LEO regime (inter-satellite, as well as optical LEO downlinks-OLEODL), distances are way shorter, allowing very high data rates while, at the same time, reducing the requirement for high system sensitivity (where complexity and thus costs generally increase with sensitivity). Instead, components and technologies that are close to commercial-off-the-shelf (COTS) from terrestrial fibre communications can be used, allowing both very high throughputs and moderate-to-low system costs. Using COTS components in inter-satellite as well as downlinks is also supported by the shorter life time of LEO missions, implying less radiation exposure of these components. In the last years, several demonstrations of OLEODL have been performed by various agencies [9-15], and its commercialisation will be seen in near future.

Downlink communication experiments with OSIRISv1 laser terminal onboard Flying Laptop satellite

“…Feasibility of lasercom downlink has been demonstrated at high data-rate through the atmosphere in various missions: From first demonstrations at 1024 Mbps with an 830nm data signal in the 1990s [2], and 50 Mbps at 847nm in the 2000s [3], lasercom has been demonstrated successfully at higher data rate up to 5.625 Gbps bidirectional and with coherent phase-modulation between the NFIRE satellite and an Optical Ground Station (OGS) at 1064 nm [4]. Recently the 1550nm wavelength is employed for several small downlink terminals [5] [6]. Despite of the great potential of lasercom, its performance in satellite-to-ground links is still limited by the adverse effects of the atmospheric channel (absorption, scattering and index-ofrefraction turbulence [7]), by intermediate cloud blockage, and by the challenges of extremely precise pointing during a link pass with high angular speed.…”

Atmospheric absorption and scattering impact on optical satellite‐ground links

“…In other words, all downlinks above 5° elevation will see elevations between 5° and 15° for 50% of the time. There exists no analytical solution for the CDF of elevation, however, numerical analysis confirms that the shape of the normalized distribution function is similar for different ground station locations when the orbit height stays constant 5,36 . This confirms 5°–15°–90° as the typical minimum–average–maximum elevation angles for any downlink contact of Earth‐observation satellites.…”

“…Space‐ground free‐space optical (FSO) communication links—employing modulated laser beams—are currently developed to replace radio frequency (RF) 1,2 communication links in high‐bandwidth application areas that have a point‐to‐point link characteristic, as typical for Earth‐observation data repatriation or satellite communication constellations 3–8 . To date, various experimental downlinks have been performed by research agencies and industries 9–12 .…”

A Laser Link From Lunar Surface Employing Line-of-Sight MIMO

“…Free-space optical (FSO) communications at carrier frequencies of several hundred terahertz is also affected by the atmosphere, but comes with the advantage of significantly higher bandwidths and drastically lower free-space loss (FSL) than radio frequency (RF) communications (Agrawal, 2002;Hemmati, 2006;Henniger and Wilfert, 2010;Giggenbach et al, 2018). Hence, laser-based communication terminals (LCTs) are compact, lightweight and low-power alternatives to classic RF equipment and have demonstrated their performance at the lunar distance (Boroson et al, 2014).…”