If the sun disappeared, which sensor would miss it?
Strange question?
But it will help you understand the concept of active and passive sensors.
Passive sensors measure reflected sunlight that was emitted from the sun. Active sensors have its own source of light or illumination and its sensor measures reflected energy.
It turns out that passive sensors would miss the sun if it disappeared because active sensors generates its own source of illumination.
Passive Sensors Detect Reflected Sunlight
Every day the Earth’s surface is hit by incoming electromagnetic (EM) radiation from the sun. This EM radiation is known as incident energy (Ei).
What happens after light strikes the Earth?
There are three fundamental energy interactions with incident energy:
1) reflected energy (Er)
2) absorbed energy (Ea)
3) transmitted energy (Et)
Incident Energy Formula:
Ei = Er + Ea + Et
Passive sensors measure this natural energy at specific frequencies (v) (i.e. wavelength 
Sensors can have multiple bands (3 to 10 bands) known as “multispectral” imaging. Hundreds of finer bands is known as “hyperspectral” imaging.
Read more: Multispectral vs Hyperspectral Imagery Explained.
The Relationship Between Reflected Light and Spectral Reflectance
Reflected Energy Formula:
Er = Ei – Ea – Et
Spectral Reflectance Formula:
p = Er / Ei
Passive sensors would miss the sun if it disappeared because it measures natural energy being reflected at specific frequencies.
Passive Sensor Example Applications and Uses
The longest-running earth observation program is the Landsat missions. Over a span of over 40 years, Landsat has collected a wealth of information documenting history of our changing planet.
How does Landsat science help? What are some of the its applications and uses?
Agriculture, biodiversity, climate change, disasters, ecosystems, forests, energy, fire, human health, land use, urban growth and water are some of its applications.
The Landsat missions have been an eyewitness, keeping records of our changing planet. We have a historical barometer where we gauge change and plan our future as a planet. Landsat’s case studies. Thousands of publications have been created based on Landsat data.
Commercial satellite products from companies like DigitalGlobe (GeoEye, Worldview & QuickBird satellites) and Astrium / BlackBridge (SPOT & Pleiades satellites) are revolutionizing the way we see and think about our planet.
Every day, satellite data is being integrates in industries such as oil and gas, land management, energy, defense and military, agriculture and coastal applications.
Read more: 100 Earth Shattering Remote Sensing Applications & Uses
Cameras are passive AND active sensors
You hold your camera in your hand. Flash turned on. You take a picture.
What’s exactly happening here?
The camera sends light to the target. The light reflects off the target back to the camera lens. This is the light that your camera measures.
You can think of active remote sensing like a handheld camera with the flash turned on. But active remote sensing can be spaceborne satellites orbiting the Earth or airborne on an aerial unit.
- Cameras are active sensors when the photographer uses flash. It illuminates its target and measures the reflecting energy back to the camera.
- Cameras are passive sensors when the photographer does not use the flash. The camera is not providing the source of energy. It uses naturally emitted light from the sun or lamp.
Active sensors wouldn’t miss the sun because they create their own source of illumination and measure backscatter.
Active Sensors and the EM Spectrum
In our camera example, we used light as the source of energy. But there are different bands of the electromagnetic spectrum being used because of their properties in situations. For example:
- Radar remote sensing uses radio waves because radio waves can see through clouds and night-and-day..
- Topographic LiDAR uses near-infrared lasers to map the land.
- Bathymetry LiDAR uses green light because this portion of the EM spectrum improves the ability to penetrate water and measure the seafloor.
Active sensor examples include: The Canadian Space Agency’s RADARSAT-1 and RADARSAT-2 and Airbus Defense & Space TerraSAR-X Radar Satellite.
The Importance of Scattering with Side-Looking Radar
There are three common scattering mechanisms for active sensors:
Smooth surface reflection comes from flat terrain like roads or water. Very little energy of the transmitted pulse returns back to the sensor (similar to a mirror). See maroon pulse in image. Pixels will appear dark. Typical pixel values will be less than -20dB
Rough surface scattering such as plowed farm fields and vegetation. Scattering goes in all directions diffusely. See black pulse in image. Typical pixel values will be greater than -20dB
Double bounce such as vegetation and buildings. The reflected pulse hits one surface after the other. See orange pulse in image. Typical pixel values will appear brighter with values greater than -10dB
Radarsat-2 Example Image Interpretation
This Radarsat-2 image clearly shows all three backscatter mechanisms at work. A river flows in the west-east direction. Very little energy is reflected back to the radar sensor. This can also be seen in the south-east portion with the road/airport paved surface. This is specular reflection in play.
The bright white in the center of image can be interpreted as an urban feature. The radar is receiving double-bounce backscatter, meaning the transmitted pulses are returning back to the sensor. It’s unclear at this scale what this object is but it’s due to double-bounce returns.
Finally, the majority of the radar image is rough surface scattering. This may be from annual cropland, vegetation or grasses or other features. It is diffuse scattering because there’s not a high or low amount of backscatter in the image.
How is Active Remote Sensing Used?
Two of the key advantages of active remote sensing are being able to collect imagery night and day, as well as through clouds and various weather conditions. Some of the applications of active remote sensing are:
Topographic Mapping: With interferometric synthetic aperture radar, two passes of a radar satellite (or a single satellite with two antennas like SRTM) can acquire digital elevation models. Because surface images can be acquired over the same area, displacements from construction and oil and gas extraction can be better understood.
Land Cover Mapping: For land use information, active remote sensing has been used to show change detection over time. This includes understanding deforestation and forest change, flood monitoring and water quality applications.
Defence and Security: Active remote sensing has been used for a variety of security applications including marine and Arctic monitoring and as a reliable source of information for natural disasters. Double-bounce scattering has also provided critical details in search and rescue missions.
Advantages and Disadvantages of Active Sensors
An advantage of active sensors is that they can be used at any time of the day because it does not require natural light. Active sensors can also produce microwave energy. This is advantageous because microwaves are generally not affected by any type of cloud cover.
The noise in radar can be overbearing (see Radarsat-2 image above). Without experience in radar, to the average user it may look like a bunch of pixels. You don’t get a pretty picture used in a map. There is usually a clear purpose for radar – flood mapping (specular reflection where water is) or estimating surface elevation with the Space Shuttle Radar Topography Mission inSAR.
Active sensor examples include: The Canadian Space Agency’s RADARSAT-1 and RADARSAT-2, Airbus Defense & Space TerraSAR-X Radar Satellite and LiDAR (light detection and ranging).
Passive sensors use reflected energy from the sun while active sensors create its own source of energy to illuminate the Earth. Passive sensors are the most common type of Earth observation technique. Multispectral and hyperspectral are example passive remote sensing products. Active sensors and microwave energy gives day and night imaging and are least affected by cloud cover.
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