Link budget

The link-budget calculation is based on Equation 10.2 in logarithmic form, where all values are expressed in appropriate decibel units: e.g. value 23 dB (over 1 K) corresponds to a system temperature of 200 K. Examples of link budget calculations for DTE and relay links for a Venus entry probe are shown in Table 10.4.

Assumptions for the budget calculations are: space losses are defined as (,/4ftD) ; no pointing losses for omnidirectional antenna of the probe; Venus' brightness temperature of 600 K in S-band and 500 K in the UHF; residual carrier transmission with 60° phase modulation, and greater processing losses on the relay orbiter. The required data margins are increased to 5 dB instead of the usual 3 dB to account for signal variations during entry and descent phases. In this example the relay link provides a gain of 150 times with respect to the DTE link.

Table 10.5 provides an overview of the probe links used on some planetary landers and entry probes. It is worth remarking that probes having a mission duration that is not large in relation to the two-way light time all have one-way communications; the mission profile is preprogrammed since there would be no time for intervention from the ground. All Venera, VeGa, early Soviet Mars landers, VeGa balloons and the Galileo and Huygens probes were of this type. While the Pioneer Venus Small Probes were also one-way, the Large Probe had a receiver for two-way Doppler ranging only, not for telecommand.

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