Direct-To-Earth vs Relay trade-off study
Our analysis showed that a radioisotope thermoelectric generator would have to be used on the lander in order to meet the mission's goals. Using such a device is not a light decision, as it greatly increases costs and environmental risks. However, we show here that using a relay satellite orbiting around Venus would not result in lower power requirements.
If we used an orbiter, it would need to have an antenna with a large enough HPBW to cover the entire surface of the planet. As a result, the gain of such an antenna would be relatively low, unless the distance from Venus to the satellite is large enough. But this increases path losses. Figure 1 illustrate the product of antenna gain and path losses, with respect to the orbiter's altitude. The code for generating this figure can be found here.
The red line represents the value of this parameter that results in a transmitted power of 200W, which is the power required for a direct-to-Earth link. To compute this value, we used the following assumption for the link budget:
- 87kHz bandwidth (because the orbiter has to be above 20° elevation for communication, and thus higher data rate during visibility time)
- 730K system noise temperature, inferred from a 650 K antenna noise [11]
- The ther parameters are the same as in the DTE link.
Using a relay satellite introduces one more component in the mission. Although this doesn't necessarily mean more weight or hardware (we can reuse the carrier spacecraft as an orbiter), this would increase the risks of the mission, because we add another possible point of failure during the 90 days period. Thus, we expect the advantages of that configuration to be high enough to compensate. Indeed, to have a power reduction of 10 dB (which could allow the use of batteries on the lander) would mean orbiting the relay at an altitude less than 350 km from the surface. This would require a large quantity of propellant for orbit insertion and thus would result in a higher cost. Thus, this solution appears to our eyes as not being justified. This is why we decided to use a Direct-To-Earth architecture.