If the sun disappeared, would passive or active sensors miss it most?
If you can answer this question, it will help you understand the concept of active and passive sensors in remote sensing.
Active sensors have its own source of light or illumination. In particular, it actively sends a wave and measures that backscatter reflected back to it.
But passive sensors measure reflected sunlight emitted from the sun. When the sun shines, passive sensors measure this energy. But more on this later.
Cameras are passive AND active sensors
When you take picture with the camera flash turned on, what’s exactly happening here?
The camera sends its own source of light to the target and reflects back to the camera lens. This is the light that your camera captures.
You can think of active remote sensing like a handheld camera with the flash turned on. But active remote sensing can be space-borne 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. Because the camera is not sending the source of light, it uses naturally emitted light from the sun.
As you’re about to find out, there are hundreds of applications for passive and active remote sensing. But first, let’s dig a bit deeper in both types of remote sensing.
Examples of passive and active remote sensing
Now that we have a clear understanding of passive and active remote sensing, let’s see it in action for satellite sensors. In the schematic below, you can see how the sun emits light. For the light that passes through the atmospheric window, it reflects off Earth to a satellite sensor orbiting Earth.
On the other hand, active sensors illuminate their target. In this example, it’s a side-looking sensor that sends its own pulse to Earth’s surface. First, it bounces off the ground. Then, it bounces again off a building. Finally, it returns back to the sensor again. Actually, this type of backscatter is called double bounce backscatter. More on this later.
OK, now here’s an image using active remote sensing
If you ever have a chance to see a radar image, it will look speckled like this:
For the untrained eye, it’s just a bunch of black and white pixels. But the reality is that there’s more that meets the eye.
For example, specular reflection is where dark spots are in the image. In this case, it’s the smooth surfaces like the east-west flowing river and paved surfaces.
And the bright white in the center is double-bounce backscatter at work. As shown in the schematic above, it’s an urban feature like a building but it’s not entirely clear at this scale.
Finally, the majority of the radar image is rough surface and diffuse scattering. This may be from the growing vegetation in the agriculture areas.
Finally, here’s an image using passive remote sensing
Really, passive remote sensing can be very similar to how our eyes interpret the world. For example, here is the Rocky Mountains area in true color.
But the powerful aspect of passive remote sensing is to see light in the whole electromagnetic spectrum. For example, this multispectral image can have different band combinations like color infrared.
What’s important to realize is that how it emphasizes healthy vegetation in bright red. To say the least, there is a lot of it in this scene. While the bright white is built-up areas, the darkest shade is water. In the east, this could be a transmission line right-of-way because how it’s constantly the same width.
Finally, you can see the world much sharper using the panchromatic band. If you want to pan-sharpen an image, this is the spectral band that you use.
What are some applications for passive remote sensing?
In terms of passive remote sensing, the Landsat mission is the longest-running earth observation program. For over 40 years, Landsat has collected and documented our changing planet.
For over 40 years, the Landsat missions have been an eyewitness of our changing planet. Because of it, we have a historical barometer where we gauge change and plan our future as a planet.
What are some applications for active remote sensing?
Two of the key advantages of active remote sensing are:
- The capability to collect imagery night and day.
- It is unfazed by clouds and poor weather conditions.
The Shuttle Radar Topography Mission (SRTM) uses inSAR which measures Earth’s elevation with two antennas. In only a couple days, SRTM has collected one of the most accurate digital elevation models of Earth.
Light detection and Ranging (LiDAR) is an active sensor that measures ground height. Using light from an airplane or helicopter platform, it measures the time it takes to bounce back to the sensor. From this, you can create Digital Surface Models which is useful in forestry.
Active remote sensing has been used for a variety of security applications including marine and Arctic monitoring. As shown previously, the double-bounce scattering has provided critical information in search and rescue missions.
Remote Sensing Types
So it turns out that passive sensors would miss the sun if it disappeared because active sensors generates its own source of illumination.
This is because passive sensors use reflected energy from the sun while active sensors create its own source of energy to illuminate the Earth.
If you want to learn more about passive remote sensing, check out our guide on multispectral and hyperspectral sensors. Or if you want to learn more about active remote sensing, take a look at our guide on Light Detection and Ranging.