An optimum design of deep-space downlinks affected by tropospheric attenuation

摘要

In the paper, we propose an optimum design of deep-space downlinks made with 2 hops, at K-a band and above, in which each hop should be designed for providing half of the total noise-to-signal power ratio. We have derived this result from maximizing the ratio between the tropospheric attenuation in the 2-hop downlink and that in the 1-hop downlink. The design of the 1st hop (free-space) of the 2-hop downlink can reduce the spacecraft power, for the same antennas physical size, by increasing the carrier frequency from K_a band (32 GHz) to W band (80 GHz). This choice is not available in 1-hop downlink design because of the huge Earth tropospheric attenuation expected in the W frequency band. To show a practical design, we have applied the theory to compare 1-hop downlink design at 32 GHz to 2-hop downlink design that adopts 32 or 80 GHz in the lst hop. The calculations refer to spacecrafts located at two astronomical units (300×10~6 km, about planet Mars) and to NASA and ESA receiving stations located in Goldstone (California), Cebreros (Madrid, Spain), Canberra and New Norcia (Australia). At 0.1% outage probability, in an average year or in the worst month, 1-hop downlinks show performance critical or close to fail, because of the large tropospheric attenuation (except at Goldstone), while 2-hop downlinks always work.