Satellites create images by capturing data and converting it into visual representations using sensors that measure reflected or emitted energy from the Earth's surface and atmosphere.
Here's a breakdown of the process:
1. Sensors Capture Data:
- Satellites are equipped with sensors, also known as remote sensors or imaging systems. These sensors detect electromagnetic radiation (EMR), which includes visible light, infrared, and other wavelengths. Different sensors are designed to detect specific ranges within the electromagnetic spectrum.
- The sensors don't "take pictures" in the traditional sense of a camera. Instead, they measure the intensity of the EMR reflected or emitted from the Earth.
2. Pixels and Digital Values:
- The area being observed is divided into a grid of tiny squares called pixels.
- For each pixel, the sensor records the intensity of the electromagnetic radiation within a specific wavelength band (also called spectral band).
- This intensity is then converted into a digital number (DN), a numerical value representing the brightness or reflectance of that pixel in that particular band. Higher DN values typically indicate higher reflectance or emission.
- Think of it as assigning a numerical value representing the color and brightness of each tiny square on the "image."
3. Different Wavelengths/Bands Create Different Images:
- Satellites often use multiple sensors to capture data in different spectral bands (e.g., red, green, blue, infrared). Each band provides different information about the Earth's surface.
- For example, infrared bands are useful for detecting vegetation health or measuring temperature.
- A single image is created for each band, consisting of the DN values for all pixels in that band.
4. Image Processing and Interpretation:
- The raw data (the collection of DN values for each pixel in each band) is then processed to correct for distortions, atmospheric effects, and other factors.
- These DN values are then used to create a visual representation, which is what we see as a satellite image. This process involves assigning colors to the different digital numbers.
- For example, in a natural color image, the digital values from the red, green, and blue bands are assigned to the corresponding red, green, and blue colors on a display.
- By combining data from different bands, we can create false-color images that highlight specific features or characteristics of the Earth's surface. For example, healthy vegetation often appears bright red in infrared composite images.
5. Examples:
- Landsat satellites: These satellites have been used for decades to monitor land use, vegetation health, and other environmental factors. Landsat satellites capture data in multiple spectral bands, allowing for detailed analysis of the Earth's surface.
- MODIS (Moderate Resolution Imaging Spectroradiometer): Used for global monitoring of various Earth processes, including cloud properties, ocean color, and land surface characteristics.
In summary, satellites make images by measuring the intensity of electromagnetic radiation reflected or emitted from the Earth's surface and atmosphere, assigning a digital value to each pixel based on that intensity, and then processing and displaying these values as a visual representation.
