To be successful, a satellite service must be competitive with a terrestrial alternative if one is available. The level of service must equal or exceed the terrestrial service at a comparable cost if the satellite solution is to gain acceptance among potential customers.
When there are two alternatives, the choice of a satellite system will involve considerations of cost, the amount of user equipment, ease of use and reliability. For example, a potential broadband customer would have an expectation of an affordable subscription rate, convenient installation, superior performance and high availability. One of the factors that affects availability in a satellite communication link is rain.
The presence of rain can have a significant detrimental effect on the propagation of an electromagnetic signal. The rain degradation increases as the frequency increases. At C- band (6/4 GHz), rain has little effect, except for brief periods of unusually heavy rain, but at Ku-band (14/12 GHz) it can be important. Nevertheless, typical rain margins on the order of 7 dB can be allocated to accommodate rain loss at Ku-band in most geographic regions to ensure availability comparable to that of C-band. At higher frequencies in Ka-band (30/20 GHz), rain can have a very large effect that simply cannot be overcome at the usual levels of availability. The result is that the availability will be reduced. This fact must be taken into consideration when designing a Ka-band satellite system to provide a service that will be widely used and accepted.
The principal effect of rain is attenuation. A passing electromagnetic wave gives up energy to the liquid in a raindrop. The loosely bound molecules of water absorb energy from the electromagnetic wave, which in effect slightly heats the raindrop. Water in solid form, such as snow or ice, produces no attenuation, as the molecules of the crystals are tightly bound together and do not resonate with the passing wave.
The amount of attenuation increases as the density of the rain increases and as the wavelength of the electromagnetic wave approaches the size of a typical raindrop. The wavelength is equal to the speed of light (3 x 108 m/s) divided by the frequency, while the average diameter of a raindrop is about 1.5 mm. At the downlink C-band frequency of 4 GHz, the wavelength is 75 mm. Thus the wavelength is 50 times the average diameter of a raindrop and the waves pass through the rain virtually without interaction.
At the Ku-band downlink frequency of 12 GHz, the wavelength is 25 mm. Here the ratio of wavelength to raindrop diameter is still fairly large, but yet is one-third the value at C-band. Consequently, there is an interaction that can result in a noticeable absorption of energy. The received carrier power will decrease and the bit error rate of a digital signal will increase. Direct broadcast television customers in Florida are familiar with the degradation or loss of picture during a heavy rainstorm, which can be detected on the television long before the storm actually arrives.
At the Ka-band downlink frequency of 20 GHz, the wavelength is 15 mm. At this frequency, the wavelength is only 10 times the size of a raindrop. Therefore, there is a significant exchange of energy between the passing electromagnetic waves and the rain and a corresponding significant attenuation of the satellite signal. As the wavelength approaches 1.5 mm, the attenuation continues to increase and theoretically becomes maximum at a frequency of 200 GHz.
Figure 1 on page 26 shows how the specific attenuation, or attenuation per kilometer, varies with rain rate and frequency. The rain rate is a measure of the number of raindrops per unit volume. A rain rate of 25 mm/h would correspond approximately to an availability of 99.95 percent in Washington, DC. For this rain rate, the specific attenuation at 4 GHz is only 0.02 dB/km. Thus assuming a typical effective path length of 5 km through the rain layer, the attenuation is 0.1 dB, which is negligible. However, for the same rain rate at 12 GHz the specific attenuation is 1.0 dB and the total attenuation is roughly 5 dB. Consequently, the received carrier power would be less than one-third what it would be with a clear sky. The link budget would have to accommodate a rain loss of this magnitude.
At 20 GHz, the specific attenuation is somewhat less than 3 dB/km and the total attenuation is roughly 12 dB, or more than a factor of 15. A margin of this magnitude would be impractical. If a more reasonable margin of about 7 dB were allocated, then according to the graph, the maximum allowable rain rate at 20 GHz would be reduced to around 15 mm/h. This implies that the received signal strength would be maintained at the necessary level for a smaller percentage of rainfall situations.