1. Ground Waves: Radio waves that propagate on the earth's surface are called ground waves. During the propagation process, radio waves are absorbed by the ground and their propagation distance is not far. The higher the frequency, the greater the absorption from the ground. Therefore, when short and ultra-short waves propagate along the ground, the distance is relatively short, generally not more than 100 kilometers.
2. Sky waves: Radio waves that propagate by reflection from the ionosphere into the atmosphere are called sky waves, also known as ionosphere reflection waves. The radio waves emitted are reflected by the ionosphere more than 70 to 80 kilometers from the ground and then reach the receiving location. The propagation distance is relatively long, generally more than 1,000 kilometers.
3. Spatial linear waves: radio waves that propagate in linene straight line from the transmitting location to the receiving location in space are called spatial linear waves, also called linear waves or line-of-sight waves. The propagation distance is the line-of-sight range, which is only tens of kilometers.
Detailed informationCharacteristics of radio waves:
1. Time dispersion and equalization
Time dispersion comes from reflection and its reflected signal comes from a distance of approximately a few meters from the receiving antenna. Objects thousands of meters away. For example, if the base station constantly sends a sequence of "1s" and "0s", if the time when the far reflected signal reaches the mobile terminal is one bit later than the direct signal, then the terminal receiving signal will detect “0” of the direct signal and also detect the “0” of the direct signal. “1” is detected in the reflected signal.
2. Featuresfading of radio waves
The fading of radio waves during the propagation process is a very important characteristic and can be described in three scales: large, medium and small. . Large scales are used to describe the median signal (regional average). It exhibits power-law propagation characteristics, that is, the median power of the signal is inversely proportional to a certain power increased by the length of the distance.
Baidu Encyclopedia - Radio Waves
Why can sonar be used to measure sea depth?
The reason sea water appears blue is due to the effect of sunlight. Sunlight is made up of seven colors: red, orange, yellow, green, cyan, indigo and violet. The longer the wavelength of light, the easier it is to transmit in sea water and is easily absorbed byr sea water or marine life. Most short wavelengths are reflected and scattered and fail to penetrate seawater.
Our eyes observe the world and recognize colors by receiving light from the outside world. The essence of light is electromagnetic waves, and different wavelengths of light correspond to different colors. When we see things with our eyes, we are actually using them to receive visible light of different wavelengths.
When we see an object with a certain color, it means that the object reflects a certain wavelength of light. For example, when we see green leaves, it is because the chlorophyll in the leaves absorbs red and purple light from visible light, causing the leaves to reflect green light. Then our eyes receive green light, which creates the sensation that the leavesare green. the leaves are green.
The color of the water will change depending on the environment around the water. Dissolved elements or suspended impurities in water can cause the water to produce different colors. The water we usually see appears colorless and transparent because the amount of water is relatively small and the water molecules in it absorb too little visible light. Ignore it. Therefore, almost all visible light penetrates through water, making it naturally transparent to our eyes.
But in fact, pure water will appear light blue. We can easily spot it when the water is particularly abundant and deep. Although it has been said before that the absorption of visible light by water molecules is minimal, there is something called a "hydrogen bond" in water molecules. Under its action, water absorbs red and green light slightly more than lighte blue, which gives blue. more visible light.
So when pure water reaches a certain depth, it appears blue. The deeper the water, the bluer it becomes, like those huge ice cubes in oceans and glaciers. As light penetrates deep seawater or ice, more and more red light is lost along the way. By the time it reaches a depth of several meters, most of the red light has been consumed, while the blue light can penetrate very far. Before it is completely absorbed, blue light is free to scatter and reflect, and some eventually returns to the surface of seawater or ice, projecting blue light into our eyes. Additionally, the surface of oceans and lakes often reflects blue skies, which can also make them look bluer.
Sonar (sonar) is the transliteration of the English abbreviation “SONAR”, and its full name in cChinese is: navigation and sound telemetry. “Sound Navigation and Ranging” is an electronic device that uses the propagation characteristics of sound waves underwater to carry out underwater detection and communication tasks through electro-acoustic conversion and information processing. ? There are two types: active and passive, and belongs to the category of acoustic positioning. ?Sonar is an electronic device that uses underwater sound waves to detect, locate and communicate underwater targets. Sonar is the most used and important equipment in the field of underwater acoustics.
Since electromagnetic wavesattenuate very quickly in water, they cannot be used as a detection signal source< /strong>. Acoustic detection of artificial underwater objects has become the most useful methodilized. ? Whether on submarines or surface ships, systems based on this technology are used to detect underwater objects or serve as a basis for navigation.
As a form of energy transmitted over long distances. “Sonar technology, a technology for detecting underwater targets, came into being. “Sonar technology is 100 years old. It was invented in 1906 by Lewis Nixon of the British Navyin 1906. “The first sonar he invented was a passive listening device, mainly used to detect icebergs. This technology was used on the battlefields of World War I to detect submarines hidden underwater.
Observe and measure in water. Only sound waves have unique conditions. ? This is because the distance of other detection methods is very short and the penetration of light into water is very limited. EvenIn the clearest water, people can only see objects within a radius of ten to ten meters. Electromagnetic waves attenuate too quickly in water. The shorter the wavelength, the greater the loss. Even if high-power low-frequency electromagnetic waves are used, they can only propagate a few. Ten meters. However, the attenuation of sound waves propagating through water is much lower. A bomb weighing a few kilos exploded in deep waters can also receive signals 20,000 kilometers away. Low-frequency sound waves can also penetrate several kilometers of strata on the sea floor and obtain information from the strata. “To date, no more effective method has been found to measure and observe water in water than sound waves. ?