1. Definition of Remote Sensing
According to Lillesand and Kiefer (2004) in Purwadhi et al. (2015), remote sensing or sensing is the science and art of getting information from an object, area, or phenomenon (geophysical) through data analysis, where in obtaining this data it is not in direct contact with the object, area, or phenomenon being studied.
The data obtained is usually in the form of satellite images which are then processed in accordance with needs until finally the desired information is created.
2. The Principle of Sensory Work
Remote sensing starts during the process of recording objects on the surface of the earth. The power used in remote sensing is the connecting power that carries data about the object to the sensor in the form of sound, magnetic force, gravity, or electromagnetism. However, in the senses only energy or energy in the form of electromagnetism can be used.
Electromagnetic energy in passive systems is sunlight. Sunlight on objects on the surface of the earth is then partially absorbed and partly reemitted by the object so that the sensor can capture electromagnetic waves coming from objects that are on the surface of the earth.
Sensors used to capture electromagnetic waves can be mounted on satellites or on airplanes (usually using drone aircraft). After the sensor captures electromagnetic waves then the sensor converts it into digital signals which are finally stored in the sensor’s storage room.
2.1 Satellite Orbit Direction
Satellite is a vehicle that is often used to get an image. The satellite contains sensory sensors that have unique orbital directions and are adjusted to the needs of the data to be examined. Based on the direction of its orbit, satellite orbits are divided into two types, namely polar orbits and stationary orbits.
Polar orbit orbits vertically and almost approaches the north-south plane. The inclination angle formed from the direction of this orbit is only about 8 – 9 degrees. Usually the height of this satellite ranges between 600-1000 km. The direction of this orbit is usually arranged so that it cuts across the equator at a fixed time, this is also often called the sun synchronous orbit (sun synchronous orbit).
The other type of orbit is a geostationary orbit. This orbit is also called the earth’s synchronous orbit (geo-synchronous orbit). These orbital satellites are usually at an altitude of 36 000 km and are commonly used for the purposes of remote sensing of the environment, weather and communication.
Satellites with this type of orbit also have the same speed as the motion of the earth’s rotation so as if it is somewhere and does not change position. This type of satellite is able to catch waves in the same place at different times (temporal resolution).
3. Remote Sensing Components
In remote sensing various components are known so that this technology can be used. Components in the senses include power sources, atmosphere, interactions between energy and objects on the surface of the earth, sensors, data processing systems, and various data uses.
3.1 Energy Resources
In the mechanism of recording objects that are on earth, sensors require energy to be able to recognize these objects. Power sources in remote sensing can be divided into two types, namely natural and artificial power sources. The source of natural energy on earth is the sun, this source of natural energy is used in the passive system. Artificial power sources are used in active remote sensing systems, such as radar and lidar.
The main natural energy source on earth is sunlight, but not all electromagnetic radiation from the sun can be used in remote sensing. Solar radiation used in recording remote sensing data depends on the electromagnetic wavelength.
Some of the sun’s radiation falling to the earth will be absorbed and some will be reflected. Remote sensing sensors found on satellite vehicles can only receive electromagnetic waves resulting from the reflection of solar radiation by objects on earth so the remote sensing system is called a passive system.
Even solar radiation does not reach all of its wave spectrums, so the reflected electromagnetic waves are uneven.
Part of the spectrum of reflected solar radiation used is visible waves and their expansion (ultraviolet, blue, green, red and near infrared) which have wavelengths of 0.3 – 1.3 micrometers, thermal infrared wavelengths (3-5 micrometers and 8-14 micrometers) , and microwaves (0.1 – 100 cm).
Visible wavelengths and their expansion are the result of reflectance of earth’s surface objects. This wavelength can be used to visually analyze the state of the earth’s surface.
Recording using thermal wavelengths can be used to find out the difference in temperature of the earth’s surface objects.
Data recording using microwaves can use two recording systems, namely by using a passive system and an active system.
3.2 Sensors and Rides
The sensor is a device used to record data from electromagnetic waves reflected from the surface of the earth. This sensor can be mounted on a variety of vehicles, such as aircraft, satellites, rockets, stratospheric balloons, or captive balloons.
Sensors and vehicles have different characteristics from sensors and other vehicles. The characteristics of sensors and vehicles are divided into spatial characteristics, sweep width, spectral characteristics, temporal resolution, and radiometric resolution.
In this article we will not discuss more about the characteristics of sensors and vehicles, to get this information you can access the article “Digital Image Resolution”.
Along with the development of remote sensing technology, various sensor resolutions are increasing so that the data obtained is more accurate and more detailed. Nevertheless, the data generated by the sensor will be useful information if the user can understand the sensor’s working principles, understand how to interpret images correctly, and understand how to use them appropriately.
3.3 Interaction of Power and Objects
The interaction of power and objects (earth’s surface) needs to be studied so that the information interpreted is not biased or even wrong. The study of interactions between power and objects is important because of the role of the atmosphere which often limits the portion of the electromagnetic spectrum that can be used in the senses.
Obstacles that occur in the atmosphere are scattering in the spectrum of visible light and absorption in the infrared ray spectrum. Scattering itself is the distribution of solar radiation by particles in the atmosphere.
Scattering in the atmosphere and can interfere with remote sensing systems are rayleigh scattering, noodle scattering, and non-selective scattering.
Rayleigh scattering occurs during sunny weather and short wavelengths will be more strongly affected by this Rayleigh scattering because solar energy interacts with small particles whose size is much smaller than the wavelength traversed. Due to this scattering problem, digital images must be corrected first.
Noodle scattering affects long wavelengths. The scattering of noodles is caused by particles larger than the wavelength emitted by the sun. Examples of particles that cause scattering of noodles are water vapor and dust. The scattering of noodles itself is very influential when the weather conditions are rather dark.
Non-selective scattering occurs when a wave hits a particle whose diameter is much larger than the wavelength itself. This scattering affects all wavelength spectrums, visible rays and their expansion. This is precisely what causes clouds and fog to appear white at all wavelengths.
Atmospheric uptake is the opposite of scattering events. Atmospheric uptake causes a loss of energy in certain light spectrums. This absorption of solar radiation is most efficiently absorbed by water vapor, carbon dioxide, and ozone.
The magnitude of the effect of particles on the absorption of waves depends on the function of electromagnetic wavelengths according to the spectral range contained in the atmospheric window. Therefore, in making sensory sensors must pay attention to the following things:
1. Spectral sensitivity of the sensor
2. Spectral presence in the atmosphere window
3. The power source or the amount of power available in the spectral range in the atmosphere window
Based on this, the selection of the spectral range on the sensor must be based on the way power interacts with the sensed surface of the earth.
An object when receiving a wave will be reflected, absorbed, and continued with different energy, depending on the object subject to the wave. Based on this, an object which cannot be seen in certain spectral can be seen in the other spectral range.
The atmosphere is part of the earth which contains air. Chemical compounds contained in this atmosphere include oxygen, carbon dioxide, nitrogen, hydrogen, and helium. Like the discussion in the power and object interaction section, these compounds have a role in receiving, absorbing, transmitting, and reflecting waves.
In the atmosphere also known as the atmospheric window, this window is part of the atmosphere that can pass electromagnetic waves at certain spectral.
3.5 Data Acquisition
Data obtained in conducting remote sensing can be either manual data or numeric data. Manual data is the result of image interpretation (usually aerial photographs) using a stereoscope (a tool to see three-dimensional impressions on aerial photographs). Numerical (digital) data is obtained through the use of special software for the senses. Commonly used software include Erdas Imagine and Envi.
3.6 Data Users
The user is an important final component in the sensory system. These sensory users can be individuals or institutions. If there are no users, the information obtained is of no use.
Here are some institutions that are often users of remote sensing systems:
1. Forestry sector
2. Military sector
3. Population sector
4. Field mapping
5. Fields of meteorology and climatology