Editing Earth observation satellite (section)

=== Environmental monitoring ===
[[File:Blue Marble 2002.png|thumb|upright=1.40|left|Composite satellite image of the Earth, showing its entire surface in [[equirectangular projection]]]]

Other environmental satellites can assist [[environmental monitoring]] by detecting changes in the Earth's vegetation, atmospheric trace gas content, sea state, ocean color, and ice fields. By monitoring vegetation changes over time, droughts can be monitored by comparing the current vegetation state to its long term average.<ref>NASA, [http://earthobservatory.nasa.gov/Drought/ Drought.] {{webarchive |url=https://web.archive.org/web/20080819121047/http://earthobservatory.nasa.gov/Drought/|date=19 August 2008}} Retrieved on 4 July 2008 {{PD-notice}}</ref> For example, the 2002 oil spill off the northwest coast of [[Spain]] was watched carefully by the European [[Envisat|ENVISAT]], which, though not a weather satellite, flies an instrument (ASAR) which can see changes in the sea surface. Anthropogenic emissions can be monitored by evaluating data of tropospheric NO<sub>2</sub> and SO<sub>2</sub>.{{cn|date=March 2024}}

These types of satellites are almost always in [[Sun-synchronous orbit|Sun-synchronous]] and [[Frozen orbit|"frozen"]] orbits. A Sun-synchronous orbit passes over each spot on the ground at the same time of day, so that observations from each pass can be more easily compared, since the Sun is in the same spot in each observation. A [[Frozen orbit|"frozen"]] orbit is the closest possible orbit to a circular orbit that is undisturbed by the [[Geopotential model|oblateness of the Earth]], gravitational attraction from the Sun and Moon, [[solar radiation pressure]], and [[air drag]].{{cn|date=March 2024}}