The impact of conditioned attenuation statistics on the performance of VCM techniques for Earth Observation satellites in LEO orbit

In the frame of the “Advanced Techniques for High Data Rate Links for Earth Exploration Satellites" study, ESA Contract n. 22455/09/NL/JK, a set of SW [1] was developed to allow the system designer computing the end-to-end performance of the data downlink architecture of Earth Observation satellite in LEO orbit when using Variable Coding Modulation (VCM) techniques and comparing it against the achievable figure when Constant Coding Modulation (CCM) is used. In particular, the SW implements modulation and coding described in the CCSDS standard [2], pre-distortion techniques, and on-ground algorithms for mitigation of channel contribution to dual-polarization transmission. Moreover, the SW allows the user selecting the main mission parameters (e.g. orbit, ground station, link availability, non-linear amplifier, on-board RF filters, EIRP etc.) according to its customized scenario. VCM demonstrates the improvement of downlink data rate of Earth Observation satellite in LEO orbit system performance, by adapting case-by-case the coding/modulation scheme, especially for systems characterized by a ground station network based on different climate areas (e.g. Mediterranean, Near Polar, Desert Ground station) [3]. For a K-Band system, considering the typical availability requirement for Earth Observation satellite data downlink (i.e. 99.5%) a special care shall be paid to the atmospheric propagation; it is well known that rain attenuation is the most important impairments for probability lower than about 1%. The total attenuation exceeded for 0.1% of the average year in Ka band can be several dBs larger than attenuation exceeded for the same time percentage during non-rainy periods (about 95% of the total year for European climate). The high difference between atmospheric propagation in rain and non-rainy condition is worth to be further analysed: the disadvantage of dimensioning the link on the average year is to have a reduced throughput even during the clear-sky (i.e. non rainy) periods characterized by low values of atmospheric attenuation.