Optical link from ground to space is susceptible to turbulence induced fades that can last from a fraction of millisecond to longer than 10ms. Unmitigated, the fade can lead to frequent drop outs and unacceptable link performance. In this paper, we describe the system design tradeoff for various fading mitigation approaches, and concluded that channel coding is the most viable approach to provide reliable data transmission. The paper then discusses the design trades for a channel coder/interleaver for an optical ground-space feeder link.
Within the scope of the FALCON project Mynaric Lasercom GmbH, in collaboration with Facebook Inc., has built two laser terminals for optical communications: One airborne terminal MLT-70-ATG and one optical ground station GS-200. Both terminals are designed to establish communications between the stratosphere and ground. The athermalized design of the MLT-70-ATG, its efficient temperature management system and an optimized dynamic behavior for high-altitude platforms qualify Mynaric’s system to be easily integrated into carriers that fly up to tens of kilometers. The GS-200 achieves and sustains fast and reliable free-space data transfer links between the airborne segment and ground. It is designed for outdoor operations and is mounted on a stationary stable platform. A flight campaign executed by both companies has demonstrated a 10 Gbps bidirectional error-free link between the airborne laser communication terminal and the optical ground station in a representative scenario. The optical link was acquired successfully in a few seconds and both terminals maintained a steady link. Limitations in the line of sight between the communication partners, due to the flight patterns followed by the aircraft, triggered reacquisitions that were handled by the terminals autonomously. Bidirectional data transmission with maximum data throughput has been achieved. Even for strong fluctuation conditions, which were experienced during ground-to-ground tests, the link was error-free thanks to the coding in the Laser Ethernet Transceiver (LET). The LET converts the user data to a proprietary format. The systems could recover successfully outages up to 10 milliseconds. The coding and synchronization schemes have been optimized for overcoming the inherent spurious effects of the free-space optical communication channel.
Coherent, free-space optical communication technology offers near-quantum-limited receiver sensitivity and high spectral efficiency compared to conventional direct detection systems. In this paper, we will present the initial results from a bidirectional air-to-ground demonstration of a coherent optical link.
High speed optical backbone links between a fleet of UAVs is an integral part of the Facebook connectivity architecture. To support the architecture, the optical terminals need to provide high throughput rates (in excess of tens of Gbps) while achieving low weight and power consumption. The initial effort is to develop and demonstrate an optical terminal capable of meeting the data rate requirements and demonstrate its functions for both air-air and air-ground engagements. This paper is a summary of the effort to date.
For bi-directional links between high-altitude-platforms (HAPs) and ground, and air-to-air communication between such platforms, a hemispherical +30°C field-of-regard and low-drag low-mass two-axis gimbal was designed and prototyped. The gimbal comprises two servo controlled non-orthogonal elevation over azimuth axis, and inner fast steering mirrors for fine field-of-regard adjustment. The design encompasses a 7.5cm diameter aperture refractive telescope in its elevation stage, folded between two flat mirrors with an exit lens leading to a two mirrors miniature Coude-path fixed to the azimuth stage. Multiple gimbal configurations were traded prior to finalizing a selection that met the requirements. The selected design was manifested onboard a carbon fiber and magnesium composite structure, motorized by custom-built servo motors, and commutated by optical encoders. The azimuth stage is electrically connected to the stationary base via slip ring while the elevation stage made of passive optics. Both axes are aligned by custom-built ceramic-on-steel angular contact duplex bearings, and controlled by embedded electronics featuring a rigid-flex PCB architecture. FEA analysis showed that the design is mechanically robust over a temperature range of +60°C to -80°C, and with first mode of natural frequencies above 400Hz. The total mass of the prototyped gimbal is 3.5kg, including the inner optical bench, which contains fast steering mirrors (FSMs) and tracking sensors. Future version of this gimbal, in prototyping stage, shall weigh less than 3.0kg.