The Earth Observing System (EOS)

The Earth Observing System is a US National Aeronautics and Space Administration (NASA) program consisting of a science segment, a data system, and a space segment made up of a series of polar-orbiting and mid-inclination satellites for long-term monitoring of the Earth as an integrated system, including observations of the land surface, biosphere, atmosphere, cryosphere, and oceans. Initially conceived in the mid-late 1980s, it was implemented as a series of large "flagship" missions and smaller focused satellites, often in partnership with instruments and sometimes spacecraft from other nations.

In 1984 NASA formed several instrument panels to develop concepts for “facility instruments” that would provide comprehensive
support to the scientific and applications’ communities, each with a supporting science team competitively selected via
peer-review. An Announcement of Opportunity was released in 1988 and proposals were solicited for three types of proposals:
(i) instrument investigations that would include the provision of instrumentation for flight on a polar platform, including
non-Earth science payloads requiring polar orbits (PI instruments), (ii) research facility instrument team member and team leader
investigations for facility instruments, and (iii) interdisciplinary science investigations to provide data analysis and modeling,
preparing to use data acquired from the space platforms. The initial selections made in 1989 included support for 24 instrument
investigations, 6 research facility instrument investigations, and 28 interdisciplinary investigations.
Although considerable changes were made over the years in the structure and orbital configuration of the various instruments
and platforms, a total of 10 different missions were eventually flown, as shown in Fig. 1. These included three multiinstrument
flagship platforms known as Terra (launched in 1999), Aqua (launched in 2002), and Aura (launched in 2004), where the Terra
mission emphasis was primarily on land processes, Aqua on the hydrological cycle, and Aura on atmospheric chemistry.
In addition to these flagship missions the EOS missions included Landsat 7 (land cover classification and change), QuikScat
(wind speed and direction over the oceans), Jason-1 (ocean surface topography and sea level), ACRIMSAT (total solar irradiance
(TSI)), SAGE III (stratospheric aerosol and ozone levels), ICESat (ice, cloud, and land elevation), and SORCE (Solar Radiation and
Climate Experiment; total and spectral solar irradiance). In addition EOS supported CERES (an Earth radiation budget sensor) and
LIS (a lightning sensor), that flew as part of the NASA/JAXA Tropical Rainfall Measuring Mission (1997), and SeaWinds (wind
speed and direction), that flew as part of the short-lived ADEOS II (Midori II, 2002) mission of JAXA (Japan Aerospace Exploration
Agency).
The EOS contributions are too numerous to present in detail in this article, but can be viewed for convenience as a means of
obtaining global measurements (with appropriate characteristics) of the 24 variables and variable groups listed in Table 1. This
table was created by the EOS community in the late 1990s as a way to succinctly indicate which instruments on the various EOS
platforms were expected to contribute to each of the specified variables in primary (bold), secondary (bold italics), or contributing
(roman) roles.
All of the science data products produced by these space missions are available without charge throughout the world, and are
thus highly valued for their many contributions to applications such as air quality, fires, land cover change, and the state of the
oceans, as well as for science and monitoring of the state of the Earth system and its changes over time. These and other
international satellites have played a crucial role in understanding and documenting global change, including (i) global surface
temperature and atmospheric temperature and water vapor profiles, (ii) glacial retreat, (iii) sea ice extent and change, (iv) solar
radiation into and out of the Earth-atmosphere system, (v) atmospheric aerosol and cloud properties, (vi) sources and sinks of