Introduction to Remote Sensing
Key concepts, components, and applications
1. What Is Remote Sensing?
- Definition: The acquisition of information about an object or area from a distance, typically using aircraft or satellites equipped with sensors.
- Core Idea: Detect and record electromagnetic energy (or other types of energy) reflected, emitted, or scattered by the Earth’s surface and atmosphere without physical contact.
2. Basic Principles
| Principle |
Description |
| Energy Source (Illumination) |
Sunlight (most common) or artificial sources (radar, lidar). |
| Interaction with the Target |
Energy is reflected, absorbed, transmitted, or scattered depending on surface properties (e.g., moisture, texture, material). |
| Recording the Signal |
Sensors convert the incoming energy into electronic signals (digital numbers). |
| Data Transmission & Storage |
Signals are transmitted to ground stations, then archived for processing. |
| Interpretation |
Processed data are analyzed to extract physical, chemical, or biological information. |
3. Types of Remote Sensing Systems
| Category |
Sub‑type |
Typical Platform |
Main Characteristics |
| Passive |
Optical (visible, near‑infrared, short‑wave infrared) |
Satellites (Landsat, Sentinel‑2), aircraft |
Requires external illumination (sunlight). |
|
Thermal Infrared |
Satellites (MODIS, ASTER) |
Detects emitted heat; useful for temperature and moisture studies. |
| Active |
Radar (microwave) |
Satellites (Sentinel‑1, RADARSAT) |
Emits its own microwave pulse; works day/night & through clouds. |
|
Lidar (laser) |
Aircraft, UAVs, spaceborne (ICESat‑2) |
Measures distance by timing laser returns; high‑resolution elevation data. |
4. Sensor Characteristics
| Attribute |
Explanation |
| Spatial Resolution |
Size of the smallest object that can be resolved (e.g., 10 m, 30 m, 1 km). |
| Spectral Resolution |
Number and width of wavelength bands captured (multispectral vs. hyperspectral). |
| Radiometric Resolution |
Number of digital values per pixel (e.g., 8‑bit = 256 levels, 12‑bit = 4096 levels). |
| Temporal Resolution (Revisit Time) |
Frequency with which a sensor observes the same location (daily, weekly, monthly). |
| Swath Width |
Width of the ground area captured in a single pass. |
5. Data Acquisition Process
- Mission Planning – Define objectives, select sensor, schedule observations.
- Sensor Calibration – Convert raw digital numbers to physical units (radiance, reflectance).
- Image Capture – Sensor records energy as digital images or point clouds.
- Georeferencing & Orthorectification – Align data to a map coordinate system, correct terrain distortions.
- Pre‑processing – Radiometric correction, atmospheric correction, noise removal.
- Data Storage – Formats: GeoTIFF, HDF, NetCDF, ENVI, etc.
6. Data Processing Levels
| Level |
Description |
| Level‑0 |
Raw, unprocessed sensor output (binary). |
| Level‑1 |
Radiometrically corrected; still sensor‑specific geometry. |