Rain Attenuation

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Rain Attenuation on Satellite Transmissions

THE FACTS - Satellite transmissions are carried on one of two frequencies: C-band or Ku-band. When operating at the higher frequency Ku-band, the strength of the satellite signal may be temporarily reduced under severe rain conditions., To compensate for these potential effects, earth stations located in heavy rain areas are designed with more transmit power. C-band transmissions are virtually immune to adverse weather conditions.

Signal attenuation due to rain is a characteristic of both microwave and satellite transmissions. It is the interference caused by raindrops on electromagnetic signals travelling through the atmosphere. When this phenomenon occurs, the transmission is weakened by absorption and scattering of the signal by raindrops.

The level of attenuation is the product of a number of variables, and to minimize its effect, Telesat includes a rain fade margin when designing its services and equipment. The rain fade margin is the amount of extra power Telesat adds to the signal strength to compensate for the possibility of rain attenuation. In most cases the reduction in signal strength due to rain does not surpass the rain fade margin and does not have any noticeable effect on transmission.

MOISTURE - Because satellite signals travel through the atmosphere, a rain cell anywhere in the signal path will cause some reduction in the strength of a transmission. Telesat's satellites are located in the a geostationary orbit 35,800 km above the earth, and since rain only forms in the troposphere which extends seven miles above the earth, a signal travelling through a rain cell will experience attenuation during only a small portion of its transmission path.

Terrestrial microwave transmissions are more susceptible to the effects of rain attenuation because their signal paths are entirely in the troposphere, and the signal may pass through an entire rain cell.

FREQUENCY - Generally, rain attenuation increases as the signal frequency increases. Therefore, transmissions at 6/4 GHz will experience insignificant attenuation, while transmissions at 14/12 GHz will experience greater attenuation. For 6/4 GHz signals to be affected would require rain storms approaching hurricane conditions. Signals at higher frequencies can be affected by less severe storms.

This is due to the wavelength of each frequency and the size of the raindrop through which the signal has to pass. Transmissions at 6/4 GHz have a longer wavelength than transmissions at 14/12 GHz, and are less susceptible to rain attenuation. For example, a 6/4 GHz frequency has a wave-length of approximately 7 cm, and a 14/12 GHz frequency has a wavelength of approximately 2 cm. Any raindrop in the path of either signal which approached half the wavelength in diameter, will cause attenuation.

TIME - How long a transmission will be affected by rain attenuation and how deep the attenuation will be is determined by the amount of rainfall. Generally, signal strength can be affected for two to three minutes during an average rainfall, and up to 15 minutes for extremely heavy rain periods. However, attenuation periods of up to 15 minutes are extremely rare, and although signal strength may be affected, there will be no noticeable effect on transmission as long as the attenuation does not exceed the allocated rain fade margin.

WEATHER PATTERNS AND ELEVATION ANGLES - Weather patterns and elevation angle must also be considered. The various regions covered by a satellite footprint experience different weather patterns, and the antennas in each region are pointed at different elevation angles.

For example, there is considerable difference between the weather patterns in Vancouver and Toronto. Although it rains very often in Vancouver, the rainfall is not as heavy as in Toronto. In Toronto, it does not rain as often but rain periods are heavier. Consequently, transmissions from Vancouver will experience minor attenuation which will probably go unnoticed, and there will be very few deep attenuation periods. Transmissions from Toronto, on the other hand, will experience deeper attenuation but less frequent fading.

The elevation angle at which the antenna is pointed toward the satellite is also a factor. This angle is dependent on the latitude and longitude of the earth station . The lower the latitude of the earth station the higher the elevation angle, and the less atmosphere through which the signal must travel. The higher the latitude, the lower the angle, and, therefore, the more atmosphere through which the signal must travel, the greater the probability of it having to travel through rain.

THE SOLUTION - To offset the effects of external forces on satellite transmissions. Telesat builds a link margin into its calculations when designing your service. This margin is the amount of extra transmission power of signal strength Telesat provides so your service is not affected by rain attenuation during normal rainfalls.

The rain fade margin is a component of the link margin, and is a calculation of expected rain attenuation over one year. It is based on rainfall data, elevation angle, and weather patterns. This margin gives each customer more power than is needed at any given time, so that when rain attenuation occurs, it rarely affects your service.

Based on the link margin and the built-in rain attenuation margin, each customer should typically meet or exceed space segment performance specifications 99.9 per cent of the time over one year for 14/12 GHz service, and 99.95 per cent of the time over one year for 6/4 GHz service. This reduces the possibility of rain attenuation affecting your service, and confines the effects to very heavy and very infrequent rain periods.