What impact does weather have on satellite TV signals?
1. The impact of rain and fog on the satellite signals required for satellite TV installation
Satellite signals are greatly affected by rain, fog, clouds, snow, and frost. This is because of electromagnetic waves. When propagating in space, part of the energy is absorbed or scattered by rain, fog, clouds, snow, and frost, causing loss. The size of the loss is related to factors such as the frequency of the radio wave, the path it passes through, the size of the rain and snow, and the concentration of clouds and fog.
The attenuation of signals propagated in rain caused by the absorption and scattering of raindrops is referred to as rain attenuation. In the frequency band above 3GHz, as the frequency increases, rain attenuation increases. Below the 10GHz frequency band, the impact of weather above moderate rain (rainfall of 4mm/h) must be considered; in the millimeter wave band, the attenuation of rainfall above moderate rain is quite serious. In the case of moderate rain, for every 10 km of the path length of the radio wave passing through the rain area, the attenuation of the c-band downlink signal is about 0.4dB; in the case of heavy rain (rainfall 100mm/h), although the loss intensity per kilometer is relatively large, the rain The area is generally less than 2km. For C-band downlink signals, the attenuation per kilometer is 0.2dB, and the total attenuation value is about 0.4dB. According to information provided by the International Telecommunication Union (ITU), Ku-band signals are attenuated by 1~10dB per kilometer during heavy rain or rainstorms.
Rainfall will also produce rainfall noise, which is equivalent to antenna thermal noise when converted to the input end of the receiving antenna. Rainfall noise has a great impact on the carrier-to-noise ratio of the received signal, and the degree of impact is related to the attenuation and the antenna structure. According to calculations, for every 0.1dB attenuation, the noise temperature increases by approximately 6.7k. Generally speaking, the higher the elevation angle of the antenna, the smaller the impact of rainfall noise. This is because the electromagnetic wave passes through the shorter path of rainfall and the attenuation is smaller.
2. The impact of star eclipses and solar transits on the signals of satellites installed in satellite TV
In addition to orbiting the earth, geostationary satellites also orbit the sun with the earth. Every year during the 23 days before and after the vernal equinox (March 21) and the autumnal equinox (September 23), when the sub-satellite point of the satellite (referring to the intersection between the satellite and the center of the earth and the earth's surface) enters around midnight local time every day
, the satellite, the earth, and the sun are in the same straight line. At this time, the earth blocks the sunlight and the satellite is in the shadow area of the earth. This astronomical phenomenon is called a star eclipse, as shown in Figure 1-12.
Each star eclipse occurs continuously for 45 days, totaling 90 days, and on the two days of the vernal and autumnal equinoxes, the eclipse lasts the longest, 72 minutes. During a star eclipse, the solar cells on the satellite cannot work properly. The energy required for the entire satellite TV installation is supplied by the onboard battery. In order to reduce the load on the battery, the satellite can be designed at a fixed position in the orbit to allow the star eclipse to occur. During the time when the communication traffic volume in the service area is the lowest.
The solar transit interruption occurs for several consecutive days before and after the vernal and autumnal equinoxes every year. During the period before and after the subsatellite point of the geostationary satellite enters the local noon, the satellite is between the sun and the earth. On the same straight line, this astronomical phenomenon is called solar transit. When a solar transit occurs, the ground station antenna may be aligned with the satellite and the sun at the same time, causing a large amount of solar noise to enter the ground receiving equipment. In severe cases, it will cause signal interruption. This phenomenon is called solar transit interruption. The main impact of solar radiation is to increase the noise temperature of the downlink transmission line, thus reducing the quality factor of the ground station TV signal (expressed by the ratio of the receiving antenna gain G and the system noise temperature T, in dB/k), and even causing the satellite signal to Interrupt. This phenomenon lasts for several days each year during the spring and autumn equinoxes, lasting about a few minutes each day. The duration of solar transit is related to factors such as the latitude of the ground station and the diameter of the antenna. The impact of solar transit interruptions is generally unavoidable, unless two satellites that do not experience solar transit interruptions at the same time are used, and the ground receiving station antenna is switched to receive the signal of another satellite before the solar transit interruption occurs.