Effects of Link Availability on the Achievable Performance with Variable Coding Modulation Earth Observation Satellites

Future high performance Earth Observation satellites in Low Earth Orbit (LEO) will be equipped with high rate telemetry architectures either based on X-Band (375MHz, from 8.025 GHz up to 8.4 GHz) in dual-polarization or on KBand (25.5GHz up to 27 GHz); in particular, the migration to the K-Band is expected in case the available X-Band will not be able to sustain the mission requirements. They will be also characterized by the on-board availability of a set of coding and modulation schemes at several spectral efficiencies and different Signal-to-Noise Ratio (SNR) thresholds; these will enable the usage of Variable Coding Modulation (VCM) techniques to improve the system performance by adapting case-by-case the coding/modulation scheme to the actual local atmospheric conditions. Among the requirements that a communication system engineer shall consider in the design of the telemetry architecture, the link availability is usually not considered as a critical parameter: for X-Band is it a common approach to size the system at 99.5% of link availability at the end of coverage area i.e. at 5° of elevation angle of the ground station antenna. This paper will show that for future K-Band transmission the target link availability need to be carefully evaluated in dependence of the required data timeliness: as a general rule, a trade-off among data timeliness and overall data throughput is expected to be necessary before sizing the communication architecture.