SAR (Synthetic Aperture Radar)
Active sensor — transmits its own signal. Works day or night, through cloud and smoke. Ideal for floods, subsidence, ice monitoring.
Optical / Thermal IR
Passive sensors — collect reflected or emitted energy. Optical blocked by cloud; thermal blocked by thick cloud. High resolution, vivid imagery.
Hyperspectral
Hundreds of narrow spectral bands. Can detect specific gases (CH₄, CO₂, SO₂) by their chemical fingerprint. Partial cloud penetration.
GEO vs LEO
GEO satellites sit at 36,000 km — fixed position, continuous coverage, near-instant tasking. LEO orbits at 400–2000 km — higher resolution but must wait for the next overpass (minutes to hours).
Revisit Time
How often a satellite can image the same point. LEO revisit depends on orbit inclination and swath width. Constellation design trades revisit against resolution.
L1 Lagrange Point
Gravitational balance point ~1.5M km sunward. A satellite here stays between Earth and Sun perpetually — ideal for solar wind monitoring and early space weather warning.