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Satellite remote sensing has gradually developed into one of the most effective tools for measuring the earth at the local, regional and global spatial scales.

Radar and laser are amazing properties of science fiction movies, which are no longer what they were a few decades ago. Since science has taken a big step forward, these innovations have been widely used as sensor types in remote sensing.

With the passage of time, remote sensing technology has been applied in all fields of human life. Farmers benefit from all kinds of remote sensing satellites every day. Many important decisions are based on data such as RADARSAT,TerraSAR-X,SRTM,EOS,ERS,Sentinel,LANDSAT.

01. What is active remote sensing? In contrast to "passive remote sensing", active remote sensing is called active remote sensing with the help of artificial energy such as electric bulb lamp. It usually takes place at dark or at night.

In order to become active remote sensing, they must provide their own energy to illuminate the projects they are working on.

The remote active sensor emits radiation in the direction of the research target. The sensor then detects and measures reflected or backscattered radiation from the target.

Active remote sensing is also used for a number of security purposes, including monitoring in the oceans and the Arctic. Double-bounce diffusion provides important information for search and rescue missions.

Remote sensing sensor has the advantage of obtaining data at any time of the day or season, which is called active remote sensing.

There are two main types of remote sensing that are classified according to the signal sources they use to explore objects, active and passive. Active remote sensing instruments use their own emission sources or light sources, while passive remote sensing instruments rely on reflection sources or light sources. Radiation also varies with different wavelengths, which can be divided into short wavelengths (visible light, near infrared light, near infrared light) and long wavelengths (microwave).

Active remote sensing working schematic radar and lidar are epic examples of active remote sensing.

Each active sensor in active remote sensing directs its signal to the object and then checks the number of responses-received. Most devices use microwaves because they are relatively unaffected by weather conditions. Active remote sensing technologies differ in what they transmit (light or waves) and what they determine (for example, distance, altitude, atmospheric conditions, etc.).

Radar is a sensor that helps to measure radio signals. Its specific feature is that the antenna transmits pulses. When the energy flow in radar active remote sensing encounters obstacles, it will scatter back to the sensor to a certain extent. According to its number and driving time, the distance of the target can be estimated.

Lidar uses light to determine the distance. Lidar active remote sensing means transmitting light pulses and checking the number of retrieved. Understand the target position and distance by multiplying time by the speed of light.

The laser altimeter uses lidar to measure height.

Ranging instruments use one or two identical devices on different platforms to send signals to each other to estimate the range.

The silencer studies the weather vertically by emitting pulses to prevent it from falling into the category of activity.

A scatterometer is a specific device for measuring bounce (backscatter) radiation.

Applications and advantages in addition to various implementations, active remote sensors basically have no restrictions on the research conditions. Active remote sensing systems play a full role at any time of the day because they do not need sunlight and are relatively unaffected by atmospheric scattering.

Various types of remote sensing technology are used in the scientific sector and in more practical industries. The space shuttle radar topographic mapping mission collects earth elevation data, and the active remote sensing of light radar in the sky helps to make a digital model of the surface of our planet.

The data obtained by remote sensing instruments serve farmers and foresters. They are crucial in places that are difficult to reach in marine science and rescue missions.

The sounder helps to develop weather forecasts with vertical profiles where humidity, precipitation, temperature and clouds do not exist / exist.

Active remote sensing can provide two main benefits

1. Be able to collect images at all times of day and night.

two。 Clouds and bad weather don't seem to disturb it.

02. What is passive remote sensing? The sun is the most convenient energy source for remote sensing. The energy of the sun is either reflected or absorbed. We can see anything through the reflected energy at the wavelength of visible light.

Remote sensing systems can obtain naturally available energy through sensors. These systems are called passive sensors, and remote sensing through naturally available energy sources such as the sun is called passive remote sensing. But the sensor can only measure the sun's energy when it illuminates the earth during the day. The sensor can detect the energy emitted by the earth day and night for 24 hours.

Passive remote sensing is the study of the interaction between light sources and various earth surface features, each of which has its own spectral response.

Radiation is provided from two sources. The sun is the source of visible to shortwave infrared light collected by remote sensing systems.

Some of the radiation received by the sensor is reflected on the earth's surface, while the rest is scattered by the atmosphere and never reaches the earth.

The thermal radiation directly released by matter on the earth interacts with the self-emitted thermal radiation in the atmosphere because it propagates upward in the thermal infrared.

Film photography, infrared, charge-coupled devices and radiometers are examples of passive remote sensors.

Unlike active sensors, passive sensors in remote sensing do not stream their own energy to the object or surface under study. Passive remote sensing depends on the natural energy (solar light) reflected by the target. Therefore, it can only be used in proper sunlight, otherwise there will be nothing to reflect.

Passive remote sensing uses multi-spectral or hyperspectral sensors to measure the number of collected data through the combination of multiple bands. These combinations vary depending on the number of channels (two or more wavelengths). The range of bands includes the spectra inside and outside of human vision (visible light, infrared, near infrared, TIR, microwave).

The working principle diagram of passive remote sensing the most popular examples of passive remote sensing devices are various types of radiometers or spectrometers.

The names of the instruments clearly identify what they measure:

The spectrometer distinguishes and analyzes the spectral bands.

The radiometer determines the radiation power emitted by an object in a specific range of bands (visible light, infrared, microwave).

The spectral radiometer detects the power of radiation in several band ranges.

Hyperspectral radiometers use the most accurate passive sensors used in remote sensing. Because of its high resolution, it can distinguish hundreds of final narrow spectral bands in visible, near infrared and MIR regions.

An imaging radiometer scans an object or surface to reproduce an image.

The sounder senses atmospheric conditions vertically.

The accelerometer detects a change in speed per unit of time (for example, linear or rotation).

The application and advantages of passive remote sensing in the example of passive sensor in remote sensing, Landsat is definitely the longest Earth observation task. It monitors our planet and records the data obtained, enabling us to analyze how it has changed over the past 40 years. The greatest advantage of this task is that the public can access information and apply interpretation in geology, surveying and mapping, ecology, forestry and agriculture, marine science, meteorology and so on.

In agriculture, remote sensing uses the reflection characteristics of vegetation to measure vegetation, and vegetation index is used to evaluate crop health. This is possible because specific values of the vegetation index are related to certain species at a particular growth stage. It can help global agribusinesses manage their daily tasks, as well as check the state of the fields and maintain healthy vegetation.

The disadvantage of passive remote sensing is that

It is difficult to record data at night

Due to energy shortage, some sensors may fail.

The atmosphere has a great influence.

03. What is microwave remote sensing? The classification of microwave sensors includes active and passive types, and is based on the principle of sending and receiving signals or receiving only signals. The difference is the wavelength. In this special case, it ranges from 1 centimeter to 1 meter. Unlike shorter wavelengths, they can break through almost any atmospheric condition except heavy rain. Their sensitivity to aerosols can be monitored in almost any weather and at any time.

Passive microwave remote sensing passive microwave sensor to check the microwave emission of the target. A passive sensor such as a radiometer or scanner distinguishes natural energy and records it, except that its antenna specifically detects microwaves rather than other shorter waves. This method enables experts to understand the temperature and humidity of the target through some correlation with the amount of radiation. The energy detected by this type of sensor is emitted, transmitted or reflected.

This method provides data for many branches, such as meteorology, hydrology, agriculture, ecology, oceanography and so on. In particular, it enables scientists to check soil moisture, atmospheric water and ozone concentrations, distinguish oil spills and solve water pollution problems.

Active microwave remote sensing active microwave sensor radiates its own signal to the target and checks the amount of rebound. The different backscattering characteristics and travel time of different targets make it possible to outline their contours, find out the distance, and know how long it will take for the signal to return. The quantity also depends on the lighting angle and surface uniformity / roughness.

The most typical example of such equipment is radar (using microwave operation). The two basic remote sensing types in this category are:

Imaging (two-dimensional, such as radar)

Non-imaging (linear, such as altimeter or scatterometer).

The technology is particularly useful for the aircraft / spacecraft industry, marine science and meteorology.

Earth observation: reliable data remote sensing satellites for multiple uses revolve around our planet at established intervals, providing almost real-time data and recording it. The information obtained can not only analyze the current situation of things, but also analyze the historical background.

Technological discoveries nourish scientific research and promote people's daily activities in many branches of practice and theory. There are a variety of applications and benefits to enjoy, but there is still a lot to discover.

This article comes from the official account of Wechat: new Research (ID:chuxinyanjiu), author: Tang Shi

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