Why is the uplink frequency greater than download frequency in satellite communication?

There are two main reasons for the up-link frequency being grater than down-link frequency. Attenuation and Power limits.

• On the earth, the transmitting station has a large bank of power and also may need to transmit to multiple satellites. Higher frequency means the signal has higher energy (E=h\v$E=h\text{v}$), though this has the effect of being attenuated easily by the atmosphere. Hence, the signal needs to be of higher intensity to compensate. Example, blue light is absorbed easily by the atmosphere than red, hence the evenings have red skies. So why do we need higher frequencies?
• The atmosphere has the Ionosphere
  
  
   at an altitude of about 60–1000 Km. This layer contains ionized particles of gas that block out and reflect these lower frequency waves. Hence, we need to use signals with higher frequency.
• Since the ground stations have rather larger sources of power as mentioned above, they can afford the higher power consumption to get the signal to the satellite. Also, satellites only have so much power source to amplify the received signal.

The satellite faces technical problems of its own.

• Satellites are powered by solar panels and this calls for low power transmission. Low frequency signals can only be transmitted efficiently by the satellite.
• It also does not face the problem of signals getting reflected by the ionosphere. So lower frequency signals are used by satellites.
• This is not a problem to receiving stations on earth as they use sensitive circuits and can afford to have large antennas that usually operate on LOS (Line Of Sight) which ensures the signal is captured reliably.