Remote Sensing Basics: Electromagnetic Spectrum & Satellite Imagery
**Remote Sensing** is the science of acquiring information about the Earth's surface **without physically touching it** — typically using sensors mounted on satellites or aircraft. These sensors detect energy reflected or emitted from the ground and convert it into images we can analyze. ---
📋 Prerequisites
- No prior GIS software required.
- Curiosity about spatial data and maps.
Step-by-Step Instructions
Understand the Electromagnetic Spectrum
All remote sensing is based on **electromagnetic (EM) energy** — radiation that travels in waves, ranging from very short wavelengths (gamma rays) to very long wavelengths (radio waves/microwaves). Satellites are designed to capture specific portions ("bands") of this spectrum: - **Visible light** (0.4–0.7 µm) — what our eyes see (Red, Green, Blue) - **Near Infrared (NIR)** — reflected strongly by healthy vegetation - **Shortwave Infrared (SWIR)** — useful for detecting moisture and minerals - **Thermal Infrared** — measures surface temperature - **Microwave** — used by radar systems (can penetrate clouds) ---
Passive vs Active Remote Sensing
There are two fundamental types of remote sensing systems: | Type | How It Works | Examples | |---|---|---| | **Passive** | Uses reflected sunlight | Landsat, Sentinel-2, MODIS | | **Active** | Sends its own energy and measures the return | SAR (Sentinel-1), LiDAR | **Key point:** Active sensors like SAR can work at night and through clouds, since they don't depend on sunlight. ---
Learn Key Remote Sensing Terms
- **Spatial Resolution:** Size of one pixel on the ground (e.g., 30 m for Landsat, 10 m for Sentinel-2) - **Spectral Resolution:** Number and width of spectral bands a sensor can capture - **Temporal Resolution:** How often the satellite revisits the same location (e.g., every 5 or 16 days) - **Radiometric Resolution:** Sensitivity of the sensor to differences in energy (measured in bits, e.g., 8-bit, 16-bit) ---
How a Satellite Image Is Formed
1. Energy (sunlight or radar pulse) interacts with the Earth's surface 2. Some energy is absorbed, some reflected/emitted 3. The sensor onboard the satellite records the reflected/emitted energy 4. This data is transmitted to ground stations and processed into an image 5. Each pixel in the final image stores a numeric value representing the recorded energy ---
Common Satellite Missions to Know
| Satellite | Operator | Resolution | Use Case | |---|---|---|---| | Landsat 8/9 | NASA/USGS | 30 m | Land cover, long-term monitoring | | Sentinel-2 | ESA | 10 m | Agriculture, vegetation | | Sentinel-1 | ESA | 10 m (SAR) | Flood mapping, all-weather monitoring | | MODIS | NASA | 250 m–1 km | Daily global monitoring | ---
🧠 Quick Recap & Practice Questions
## Quick Recap
- Remote sensing captures Earth data using reflected/emitted EM energy
- Passive sensors need sunlight; active sensors emit their own signal
- Key resolutions to remember: Spatial, Spectral, Temporal, Radiometric
- Landsat and Sentinel are the most widely used free satellite data sources
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## Practice Questions
1. What's the difference between passive and active remote sensing?
2. Why can SAR satellites capture images through clouds?
3. Name two satellites and their approximate spatial resolution.
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*Next Tutorial: Coordinate Systems, Map Projections & Datums*