Satellite Data Sensor
Passive Sensors Vs Active Sensors
Have you ever wondered why Landsat 8 satellite images only photograph the earth’s surface during the day? Or are there satellite images that photograph the earth’s surface at night?
That is one of the basic questions to determine the type of satellite imagery that suits your needs. It all depends on the type of sensor used on the satellite.
What is a Sensor?
When we use remote sensing technology, we recognize a tool that serves to capture information called a sensor.
The sensor is a device that receives signal waves and converts the signal into information data [1]. The sensor can measure physical input from its environment and converts it into data that can be interpreted by either a human or a machine.
Sensor is an Important Part in Remote Sensing
Remote sensing consists of 3 important components, namely the object being sensed, sensors to record objects, and electronic waves that are reflected or emitted by the earth’s surface. If one of these components is not present, then there will be no data that can be obtained in the remote sensing process.
The simple stage of remote sensing is that energy is emitted to an object, then the energy will be reflected by the object or that is called reflectance. The sensor has an important role as a reflectance catcher returned by the object and then processes and converts it into a value or data. In satellite imagery, these values and data are grouped into pixels that make up an image.
Passive Sensors vs. Active Sensors
In general, sensors are divided into two types, namely passive sensors and active sensors.
Passive Sensors
A passive sensor is a microwave instrument designed to receive and to measure natural emissions produced by constituents of the Earth’s surface and its atmosphere.
Another meaning of passive sensor is a sensor that receives reflectance from objects that are given natural energy such as from the sun. The sun provides a very convenient source of energy for remote sensing. The sun’s energy is either reflected, as it is for visible wavelengths, or absorbed and then re-emitted.
Passive sensors only can detect the reflectance of natural energy. This means that the reflectance received by the passive sensor will only be received when the sun shines on the earth. If there is no energy from the sun, the passive sensor will not be able to shoot satellite images.
Most passive systems used by remote sensing applications operate in the Visible, Infrared, Thermal Infrared, and Microwave Portions of the Electromagnetic Spectrum. These sensors measure land and sea surface temperature, vegetation properties, cloud and aerosol properties, and other physical properties.
Do you remember this opening question,
“Have you ever wondered why Landsat 8 satellite images only photograph the earth’s surface during the day?”
The answer is because Landsat 8 satellite imagery uses an infrared type sensor which is a passive sensor so that Landsat 8 recording requires sunlight.
Active sensors
An active sensor is a sensing device that requires an external source of power to operate. active sensors contrast with passive sensors, which simply detect and respond to some type of input from the physical environment.
In the context of remote sensing, an active sensor is a device with a transmitter that sends out a signal, light wavelength or electrons to be bounced off a target, with data gathered by the sensor upon their reflection.
In other definition of active sensor is a radar instrument used for measuring signals transmitted by the sensor that were reflected, refracted or scattered by the Earth’s surface or its atmosphere.
Unlike the case with passive sensors, active sensors receive object reflectance that comes from the sensor’s own energy beam. This means that the active sensor does not depend on the presence or absence of sunlight shining on the object. The energy reflected to the object comes from the sensor itself so that satellite imagery can be taken at any time.
Active sensors include different types of Radio Detection and Ranging (Radar) sensors, Altimeters, and Scatterometers. The majority of active sensors operate in the microwave band of the electromagnetic spectrum, which gives them the ability to penetrate the atmosphere under most conditions.
These types of sensors are useful for measuring the vertical profiles of aerosols, forest structure, precipitation and winds, sea surface topography, and ice, among others.
Did You Know?
“…say ‘Cheese’!…” when we take a photo.
It shows that a camera provides an excellent example of both passive and active sensors. During a bright sunny day, enough sunlight is illuminating the targets and then reflecting toward the camera lens, that the camera simply records the radiation provided (passive mode).
On a cloudy day or inside a room, there is often not enough sunlight for the camera to record the targets adequately. Instead, it uses its own energy source — a flash — to illuminate the targets and record the radiation reflected from them (active mode).
Which One is Best to Use?
To determine that, let’s look at the advantages and disadvantages of each sensor type.
Advantages of Passive Sensors
1. Produces a clearer and more realistic image because it uses direct lighting from the sun
2. Generally the data size is smaller than the active sensor (approximately 2 Gb per image)
3. Easy to process data by users
Advantages of Active Sensors
1. Image capture does not depend on day or night
2. Objects on the earth’s surface can still be photographed even though they are covered by clouds
3. Generally produces better resolution than passive sensors
Disadvantages of Passive Sensors
1. Limited by the presence or absence of sunlight
2. Strongly influenced by weather conditions such as clouds.
3. Smaller resolution than active sensors
Disadvantages of Active Sensors
1. It is very difficult to process the data
2. Data size is bigger than passive sensor
3. There are still errors when transmitting and capturing energy
Finally
From this article we can determine what type of sensor image is suitable for use in our research. Choose the type of sensor that suits your needs and understand how to use it
Let’s learn Remote Sensing!
References
[1] Patel BC, Sinha GR, Goel N. 2020. Introduction to Sensors. Bangaloe (IN): IOP Publishing Ltd.
