🛰️ What is Remote Sensing?
Remote sensing is the process of gathering information about Earth from a distance, typically using satellites, aircraft, or drones. Instead of physically measuring a tree or a river, we measure the electromagnetic energy it reflects or emits.
Is Remote Sensing a Science?
We build models. We take a continuous, complex reality (the Earth), abstract it into pixels, filter it through atmospheric noise, and apply algorithms written by humans with biases.
"Don't trick yourself to think when you're doing remote sensing that you're being a rational person... You're actually not being rational, you're being an artist and you don't realize it."
Like a painter, you choose your "paint" (the bands), your "brush" (the algorithm), and you interpret the result. Two scientists can map Amazon deforestation and get two different numbers. The "Truth" is not in the pixel; it is in the interpretation.
☀️ Passive vs. 🦇 Active Sensors
Passive Sensors
Like a camera or the human eye, passive sensors detect natural energy (sunlight) reflected off the Earth. They require daylight and clear skies to work.
Examples: Landsat, Sentinel-2, Photography.
Active Sensors
Think of a Camera with a Flash. It doesn't wait for the sun; it brings its own light.
Like a bat using echolocation, active sensors emit a pulse of energy and measure the return echo. They can "see" at night and through clouds.
Examples: LiDAR (Laser), RADAR (Radio waves).
🔍 The Four Resolutions
To evaluate any satellite sensor, we must look at its four types of resolution:
- Spatial Resolution: The size of the smallest object visible (e.g., 30m vs 1m pixels).
- Spectral Resolution: The number and width of spectral bands the sensor captures.
- Temporal Resolution: How often the satellite revisits the same spot (e.g., every 16 days).
- Radiometric Resolution: The sensitivity of the sensor to small differences in energy (often measured in "bits").
We often celebrate higher spatial resolution, but is there a limit? At 30m (Landsat), we see forests. At 1m (commercial), we see cars. At 30cm (military grade), we can potentially identify people. As sensors improve, the line between Earth observation and surveillance blurs. Who owns your "spectral signature" when you are in your own backyard?
🔄 The Physics of Orbits
A satellite's path determines what it can see and how often. There are three main orbital types designed for different goals:
Parks 36,000 km above the equator, matching Earth's rotation speed. It stares at the same spot continuously.
Best for: Weather monitoring (GOES).
Loops North-to-South while the Earth spins underneath. Covers the entire globe every 14 days or so.
Best for: Global mapping (Landsat).
A special polar orbit that crosses the equator at the same local time (e.g., 10:30 AM) every pass. Ensures consistent shadow lengths.
Best for: Vegetation analysis.
Major Satellite Programs
| Program | Spatial Res | Revisit Time | Application |
|---|---|---|---|
| Landsat 8/9 | 30m | 16 Days | Agriculture / History |
| Sentinel-2 | 10m | 5 Days | Vegetation health |
| MODIS | 250m-1km | Daily | Global climate |
| WorldView-3 | 0.3m | On-demand | Military / Commercial |
Explore these satellite layers interactively in your browser.
Living Off the Land: The Logic of Regolith
In traditional geography, we map resources to extract and move them. In space exploration, the cost of moving things is too high. This forces a shift to In-Situ Resource Utilization (ISRU).
The Problem: The Supply Chain Breaks
You cannot email a screwdriver to Mars. As one student group noted: "We can't keep telling Earth, 'Please send us a resupply ship.' We need to produce on the fly."
The Solution: 3D Printing with Regolith (loose lunar/Martian soil). Instead of shipping concrete from Earth, robots scan the local terrain, identify suitable sediment, and "print" habitats or tools. This changes the map from a "Treasure Map" (where is the gold?) to a "Utility Map" (where is the printable dirt?).
Geography Concept:
Friction of Distance
When the "cost" of transport becomes infinite (space travel), local geography becomes the only geography.
Summary of Big Ideas
- Active Sensors (like LiDAR) provide their own energy source; Passive Sensors (like Landsat) rely on the Sun.
- Sun-Synchronous Orbits ensure the satellite passes over at the same time each day for consistent lighting.
- Composite Imagery combines multiple bands to reveal features (e.g., False Color Infrared).
Chapter 09 Checkpoint
1. A satellite that revisits the same location every 16 days is said to have a 16-day:
2. Which satellite program has the longest continuous record of Earth's surface (starting in 1972)?