Cloud Attenuation

One of the largest challenges to communicating with a lander on Venus is transmitting a signal through the thick acid clouds that blanket the surface. These clouds will greatly attenuate our transmitted signal. We can quantify this attenuation approximately using the equation below[1]. 

Figure 1: Cloud Attenuation Across Many Freq

This figure assumes pressure is 1 atm. The normalized attenuation factor is obtained according to the frequency of our communication system. Higher frequencies make much more signal attenuation than lower frequency. Now we consider comparatively lower frequency range which will be used to communicate with the relay orbiter around Venus.

Figure 2: Cloud Attenuation in Lower Freq

Among four cases the worst case is that frequency is 2 GHz and temperature is 100 degrees Celsius. The normalized attenuation value is approximately 1.1. Usually, SO2 density is 20%. So mixing ratio SO2 to CO2 is 0.2. This increases attenuation to 0.22. Also, the height of the distribution of sulfuric cloud ranges from 50 km to 60 km. The difference of height is 10km. So, attenuation is 2.2dB. However, this is the ideal case when it is predictable. Also, cloud data is limited in the specific region. So, we assume that pressure is 5atm. This means that height is from 40km to 50km. The result is as following.

Figure 3: Cloud Attenuation at 5 Atm

In the worst case, the normalized attenuation value is approximately 7.5, making attenuation be 1.5 dB/km. The layer of sulfuric acid clouds is approximately 10 km thick. This makes the total attenuation to be 15dB. A safety margin of 10 dB has been added, giving us a final projected signal attenuation of 25 dB.

[1] Steffes, P. G., and Eshleman, V. R., "Laboratory Measurements of the Microwave Opacity of Sulfur Dioxide and Other Cloud-Related Gases under Simulated Conditions for the Middle Atmosphere of Venus," Icarus 48, pp180-187, 1981.