Satellite Orbits and Applications

This note covers what orbiting sensors can measure and why. India's launch vehicles that place satellites into these orbits are covered in Launch Vehicles and Propulsion.

Orbits, matched to the job

Where a satellite is placed is chosen deliberately for what it needs to do, and three orbit types recur constantly in Prelims material.

A Sun-Synchronous Polar Orbit (SSPO), a specific type of low-Earth, near-polar orbit, is timed so the satellite passes over any given point on Earth at the same local solar time on every pass, giving consistent lighting conditions for comparison across repeat images. This is the standard orbit for remote sensing satellites imaging the Earth's surface, since consistent lighting is essential to comparing images taken on different days.

A Geostationary Transfer Orbit (GTO), an intermediate orbit a launch vehicle places a satellite into on its way to a final geostationary position roughly 36,000 km above the equator, is used for communication and weather satellites that need to sit apparently fixed relative to a point on Earth's surface.

Remote sensing: measuring indirectly, through light and heat

Remote sensing means gathering information about the Earth's surface or atmosphere without physical contact, almost always by measuring how different materials reflect or emit electromagnetic radiation across different wavelengths. This indirect chain, from a physical or biological property to a measurable radiation signature, is exactly what a past question tested by listing several claimed remote-sensing capabilities and asking which are genuinely achievable.

Genuinely achievable: chlorophyll content of standing vegetation, since healthy chlorophyll-rich vegetation reflects near-infrared light distinctively, a signature satellites are specifically built to detect; and estimating greenhouse gas concentrations in the atmosphere, since specific gases absorb and re-emit radiation at characteristic wavelengths that dedicated sensors can measure from orbit.

The underlying principle worth generalising: if a real physical or biological process changes how a surface or the atmosphere interacts with light or heat, in a way that is measurably distinct from its surroundings, then remote sensing can detect it. If a proposed measurement has no such physical pathway connecting it to a radiation signature, no sensor, however advanced, can detect it from orbit, and an option describing that kind of claim is the invented one in a "which of these is real" question.

Quick revision points

• Sun-Synchronous Polar Orbit (SSPO): consistent local solar time on every pass, giving comparable lighting for repeat imaging. Standard orbit for remote sensing satellites.
• Geostationary Transfer Orbit (GTO): intermediate orbit en route to a final geostationary position (~36,000 km, apparently fixed over one point), used for communication and weather satellites.
• Remote sensing works entirely through measuring reflected or emitted radiation, never through direct physical contact. Chlorophyll content (distinctive near-infrared reflectance) and greenhouse gas concentration (characteristic absorption/emission wavelengths) are both genuinely measurable this way.
• The generalisable test for "is this really measurable by satellite": does the claimed property change how a surface or the atmosphere interacts with light or heat in a distinctive way? If there is no such radiation pathway, it cannot be sensed remotely, however plausible the claim sounds.