Optical and Infrared Imagers

Optical and Infrared Imagers, radiometers, LIDAR, spaced-based gravimetry.

Optical imagers

Optical imagers detect sunlight that bounces off objects on the ground. Light hits a detector, which uses lenses to focus light onto a light-sensitive sensor typically a CCD or CMOS. Light-sensitive sensors then convert the focused light into electrical signals that are processed to create an image.

What are spectral signatures?

Different materials reflect and absorb light in different ways at different wavelengths. This allows us to tell different objects apart in remote sensing images based on their unique reflectance and light-absorption patterns, called spectral signatures.

What is a limitation of optical imagers?

Optical imagers need daylight, as they are affected by clouds.

Infrared imagers

Infrared imagers, thermal cameras, have specialized detectors that are sensitive to infrared radiation. These detectors convert infrared energy into an electrical signal. This signal is then processed by the camera's electronics. The processed signal is used to create a visual image where colors/shades represent different temperature levels. 

What is an advantage to infrared imagers?

These detectors don't need visible light to operate and can see in complete darkness, and they can detect heat patterns through dust, smoke, and fog.

Radiometers

Electromagnetic radiation hits the radiometer's detector. The detector absorbs the energy and either heats up, generates a voltage, or changes resistance. The radiometer converts these changes into digital signals and calculates how much radiation there is.

In what conditions do radiometers work in?

Radiometers operate under varying conditions depending on the type of electromagnetic radiation they measure.

LIDAR

The sensor sends out a pulse of light that hits an object or the ground. Some of the light reflects off the surface and travels back to the LIDAR receiver. The system measures the time it took for the light to return. The LIDAR does this millions of times per second in many directions, creating a 3D map called a point cloud.

Space-based gravimetry

Measures earth’s gravitational field over time. As two satellites fly over something with more mass/gravity, they feel a tug, which causes the front satellite to speed up slightly and the distance between the satellites to change. They use a microwave/laser ranging system to constantly measure the distance between the satellites. The changes in distance = changes in gravity = changes in mass below Earth’s surface.

What is the role of IMU + GPS + accelerometers?

These track how the satellites are moving and help filter out non-gravity-related changes (like atmospheric drag or tiny orbital shifts)