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Observation by sensor | Type of sensor | Resolution | Swath

Type of sensor

We have learned that sensors play an important role to receive information from the ground. Here, we introduce the various types of sensors. Sensor type is roughly divided into two: Optical sensor and Microwave sensor.

(1) Optical sensor

Optical sensors observe visible lights and infrared rays (near infrared, intermediate infrared, thermal infrared). There are two kinds of observation methods using optical sensors: visible/near infrared remote sensing and thermal infrared remote sensing.

a) Visible/Near Infrared Remote Sensing
The observation method to acquire visible light and near infrared rays of sunlight reflected by objects on the ground. By examining the strength of reflection, we can understand a conditions of land surface, e.g., distribution of plants, forests and farm fields, rivers, lakes, urban areas.
During period of darkness, This method can not be observe. Also, clouds block the reflected sunlight, so this method can not be observed areas under clouds.

Satellite images observed by visible/near infrared remote sensing of optical sensor
visible lights and infrared rays - 1 visible lights and infrared rays - 2
Satellite/sensor : ADEOS/AVNIR
Location : Awaji Island and surrounding area

Gray depicts urban area, dark green is vegetation and blue is sea water.
Satellite/sensor : JERS-1/OPS
Location : Kyoto

Green depicts vegetation and raspberry is urban and residential areas.

b) Thermal Infrared Remote Sensing
The observation method to acquire thermal infrared rays, which is radiated from land surface heated by sunlight. Also it can observe the high temperature areas, such as volcanic activities and fires. By examining the strength of radiation, we can understand surface temperatures of land and sea, and status of volcanic activities and forest fires. This method can observe at night when there is no cloud.

Satellite images observed by thermal infrared remote sensing of optical sensor

thermal infrared - 1
Satellite/sensor : ADEOS/OCTS
Location : global ocean

Sea surface temperature of global ocean. The data is average value acquired from March 30 to April 5, 1997.

thermal infrared - 2
Satellite/sensor : MOS-1b/VTIR
Location : around Miyako Island

Temperature distribution of typhoon. Reddish color shows higher temperature, and bluish lower temperature.

(2) Microwave sensor

Microwave sensors receive microwaves, which is longer wavelength than visible light and infrared rays, and observation is not affected by day, night or weather. There are two types of observation methods using microwave sensor: active and passive.

a) Active type
The sensor aboard earth observation satellite emits microwaves and observes microwaves reflected by land surface. It is suitable to observe mountains and valleys.

b) Passive type
This type observes microwaves naturally radiated from land surface. It is suitable to observe sea surface temperature, snow accumulation, thickness of ice.

Satellite image observed by microwave sensor
microwave sensor

Satellite/sensor : JERS-1/SAR
Location : Mt. Fuji and vicinity

Not being affected by clouds or moisture in air, this sensor can observe the detail of land surface. Rough landform can be clearly seen.

The range of wavelength a sensor can observe depends on the type of sensor. This is because each sensor has a specific observation purpose and wavelength range to operate at to achieve it.
You can see specifications of sensors aboard earth observation satellites at this page.
Fundamentals of Remote Sensing about Earth Observation

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