In free-space laser communications, an optical signal propagating through the atmosphere experiences power fluctuations and fading due to pointing inaccuracies and changes in the refractive index of the atmosphere. Determining how a receiver will detect the distorted signal over time is advantageous for the design of robust optical terminals, for the evaluation of mitigation techniques, and for the development of Automatic Repeat request (ARQ) and Forward Error Correction (FEC) protocols. In this work, the impact of atmospheric effects is considered to generate numerical time series of received power for a ground-to-satellite link (uplink) scenario. The generation procedure of the numerical series is described, and the generated series and their statistics are presented and compared with existing theory. As channel characteristics may vary rapidly during links with satellites at lower orbits, an uplink to a Low-Earth-Orbit (LEO) satellite is selected as scenario to illustrate the change in channel characteristics with satellite elevation and slew rate. However, this work is applicable to any uplink scenario. The results and analysis presented in this work can serve for link-budget design, for dimensioning interleavers, for delay analysis in ARQ protocols, for development of FEC schemes, for standardization, and for evaluation of power-fading mitigation techniques.
The demand for satellite links with ever higher bandwidths continues to grow with the market of Earth observation services. Free bands in the microwave spectrum are already a highly limited resource. Thus, optical bands are by now operationally utilized for satellite-to-satellite communications. In parallel, free-space optical communications increase the link security and relieve complex licensing procedures. The same technology holds an immense potential for Direct-To-Earth (DTE) downlinks. The optical ground station LaBoT (Laser-Bodenstation Trauen) is built by the RSC³ for Laser-based communication with satellites in Low-Earth Orbit (LEO). When the station turns fully operational, it will connect to terminals that follow the CCSDS standard for Optical On-Off-Keying (O3K). We lay out the station design, show its current implementation state and explain the next steps in commissioning the system. After successful commissioning, the station will extend the actual DLR ground station network by contributing to the overall link availability. Moreover, the station’s design enables its relocation to different locations and, thus, allows to investigate differences in the impact of regional atmospheric conditions.
Optical free-space data downlinks from LEO satellites benefit considerably from reduced effort on the space segment, when a dedicated pointing mechanism and active tracking of a ground beacon can be avoided. Instead, the attitude of the satellite is dynamically determined from its star cameras and other sensors. Initial calibration for this technique requires recording of the spatial and temporal beam distribution on the Earth’s surface. We describe the measurement of the beam intensity on ground by the power detectors of three ground stations in parallel, exemplarily for one specific downlink. From this data we derive the instantaneous center of gravity of the beam spot, and its dynamic movement during the downlink. By comparison with the satellite’s own recorded attitude data and its error, the dynamic offset to be corrected on the satellite can be calculated, resulting in optimized pointing-control for future operational open-loop downlinks.
KEYWORDS: Satellites, Beam divergence, Signal intensity, Error analysis, Receivers, Astronomical imaging, Free space optics, Signal attenuation, Satellite communications, Data communications
Different pointing errors from different sources cause an angular deviation in the uplink beam transmitted from an optical ground station (OGS) to a satellite. In optical link-budget calculations, the beam intensity loss due to pointing errors, “pointing loss”, is usually given a constant value regardless of the satellite elevation. In this paper, elevation-dependent intensity losses are calculated, considering a transmitted uplink beam with a Gaussian profile. The elevation of the satellite and the divergence of the uplink beam are considered to assess the impact of the following sources of tracking and pointing errors: OGS static pointing misalignment, uncorrected or fixed-corrected point-ahead angle (PAA), satellite orbital data uncertainties (specifically the along-track error), and mechanical jitter at the OGS. Each source of error is first evaluated separately and then the combination of their effects on the intensity loss in the LEO uplink is determined. It is demonstrated that the elevation-dependent pointing errors analyzed in this work have a greater impact on the intensity loss for satellites at lower altitudes and higher elevations. Therefore, not considering that the value of the “pointing loss” varies with elevation -especially for LEO satellites-, would result in lower link performance. Values of intensity loss are provided for LEO satellites at different altitudes and elevations, and uplink beam divergences. The results provided can be used for link-budget calculations in the optical LEO uplink in the presence of elevation-dependent pointing errors, and for system improvements in the design of future ground and space optical terminals.
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