Satellite Communication

Atmospheric Absorption (Specific Attenuation)

Electromagnetic waves are absorbed in the atmosphere according to wavelength. Two compounds are responsible for the majority of signal absorption: oxygen (O₂) and water vapor (H₂O).

The first peak occurs at 22 GHz due to water, and the second at 63 GHz due to oxygen. The actual amount of water vapor and oxygen in the atmosphere normally declines with an increase in altitude because of the decrease in pressure, so these graphs apply from sea level to around 1 km altitude.

Total attenuation through the atmosphere at any frequency through unobstructed atmosphere is the sum of free space path loss. attenuation caused by oxygen absorption and attenuation caused by water vapor absorption. Rain attenuation, when present adds an additional element. For a chart of sunlight power density in the visible light and infrared / ultraviolet region of the electromagnetic spectrum.

So, Atten_Total = Atten_{FreeSpacePathLoss} + Atten_Oxygen + Atten_WaterVapor + Atten_Rain

Why Uplink frequency is different from Downlink frequency in satellite communication?

The uplink frequency is the frequency which is used for transmission of signals from earth station transmitter to the satellite.The downlink frequency is the frequency which is used for transmission of signals from the satellite to the earth station receiver. Uplink frequency is different from Downlink frequency for following reason:

• If both Uplink and Downlink shared the same frequency the satellite transmitter generates a signal that would jam its own receiver.

• Trying to receive and transmit an amplified version of the same uplink waveform at same satellite will cause unwanted feedback or ring around from the downlink antenna back into the receiver.

• Frequency band separation allows the same antenna to be used for both receiving and transmitting, simplifying the satellite hardware.

• To overcome the above-mention difficulties satellite repeaters must involve some form of frequency translation before power amplification. So, Uplink frequency is different from downlink frequency

In satcomm, the signals have to cross the atmosphere which presents a great deal of attenuation. The higher the frequency, the more is the signal loss and more power is needed for reliable transmission.[Sound waves in atmosphere, water, or any other material are pressure or longitudinal waves. The reason high frequency waves have greater attenuation than low frequency waves is due to viscosity. The high pressure portion of the wave compresses the medium creating heating. This radiates energy and reduces the amplitude of the wave. Low frequency waves have longer wavelengths and their peaks have lower pressure than high frequency waves. Consequently they lose less energy with distance.] So now you would say why use higher frequencies if signal loss is more and you need more power? It's because lower frequencies get reflected by atmospheric bands and cannot penetrate to get through to the satellite.

Now, a satellite is a light-weight device which cannot support high-power transmitters on it. So, it transmits at a lower frequency (higher the frequency, higher is the transmitter power to accommodate losses) as compared to the stationary earth station which can afford to use very high-power transmitters. This is compensated by using highly sensitive receiver circuits on the earth station which is in the line-of-sight (LOS) of the satellite.

Uplink frequency is kept higher than Downlink frequency for the following reasons:

• attenuation have direct relation with frequency. The more the freq. the more the attenuation. Because of that we need more powerful devices to send the signal.Because on the earth we are able to equip the facilities easier and there is no weight limitation, we are able to send stronger signal and compensate the attenuation loss.On the other hand, in the down-link, because we have some space and weigh limitations on the satellite we should diminish the attenuation. So, we should reduce the freq. to make a trade-off in that case.At higher frequency attenuation is more hence more power will be required for signal transmission to ensure that it reaches the destination with the required minimum power. Higher power requirements involve the use of high power amplifiers with high ratings and heat sinks. This will increase the weight and power supply ratings will not make any difference. However for the satellite this will result in higher power consumption, which results in avoidable inefficiency.The earth station antenna must direct the beam towards the satellite with as little spill over as possible i.e. the antenna must have high directivity. Beam width (measure if directional property) of the antenna is directly proportional to wavelength. Higher frequency means lesser wavelength and hence lower beam width, which implies greater directivity.