More then 105 different Earth observation satellite missions are estimated to be currently operating (October 2009). Many of these comprise series of missions planned to provide the continuity which is essential for many observations and applications. The principal satellite series are highlighted below.

Geostationary meteorological satellites:
There is a worldwide network of operational geostationary meteorological satellites which provide visible and infrared images of the Earth's surface and atmosphere. Countries/regions with current geostationary operational meteorological satellites are the USA (NOAA GOES series), Europe (EUMETSAT Meteosat series), Japan (JMA MTSAT series), India (IMD INSAT series), China (CMA FY series), Russia (Roshydromet GOMS/ Elektro-L series), and (from 2009) Korea (KMA COMS series).

Crustal motion and gravitational field series:
A number of small satellite missions designed to measure the Earth’s crustal motion and the Earth’s gravitational field have been launched since 1967. The space segment typically comprises corner cube laser retroreflectors and the ground segment is a global network of transportable laser sites. The design life of the space segment is many thousands of years. These missions include the Diademe and Starlette series (CNES) and the LAGEOS series (NASA and ASI). More recently, missions such as CHAMP (DLR) and GRACE (NASA/DLR) have been launched to provide high precision measurements of the Earth’s gravitational field.

DMSP series:
The long-term meteorological programme of the US Department of Defense (DoD) – with the objective of collecting and disseminating worldwide atmospheric, oceanographic, solar-geophysical and cloud cover data on a daily basis.

NOAA and EUMETSAT polar orbiters:
Until 2006, operational polar orbiting meteorological satellites were provided only by NOAA – with two satellites maintained in polar orbit at any one time, one in a ‘morning’ orbit and one in an ‘afternoon’ orbit. The series provides a wide range of data of interest, including sea surface temperature, cloud cover, data for land studies (notably the AVHRR sensor), temperature and humidity profiles, and ozone concentrations (AMSU and HIRS sensor packages). Since 2006, these have been supplemented by the first of the EUMETSAT Polar System satellites, MetOp-A, offering additional measurements such as high resolution temperature and humidity profiles, wind speed over the oceans, ozone and measurements of trace gases such as carbon dioxide, nitrous oxide and methane.

Topex/Poseidon and Jason series:
These satellites form a joint NASA/CNES precision radar altimetry mission to measure ocean topography and hence, the speed and direction of ocean currents. The follow-on OSTM (Jason-2), developed by NASA/CNES and operated by NOAA/EUMETSAT, has been in operation since June 2008, and provides a core contribution to GOOS.

ERS and Envisat series:
ERS-1 was launched by ESA in July 1991, ERS-2 in April 1995, and Envisat in March 2002. This series concentrates on global and regional environmental issues, making use of active microwave techniques that enable a range of measurements to be made of land, sea and ice surfaces, independent of cloud cover and atmospheric conditions. In addition, the ATSR/AATSR instruments on these missions provide images of the surface or cloud top and the GOME instrument on ERS-2 provides measurements of ozone levels. ERS-1 and ERS-2 operated in tandem for around 1 year in 1995 and 1996, providing data for topographic applications such as differential interferometry. Envisat features a range of new sensors for land surface and atmospheric studies.

IRS series:
The Indian IRS satellites include three thematic series addressing the areas of: land and water resources; cartography; and ocean and atmosphere (which include the RESOURCESAT, CARTOSAT and OCEANSAT missions). These are coordinated through the unique institutional framework of the National Natural Resources Management System (NNRMS). Their primary objectives are in support of agriculture, disaster management, land and water resource management, cartographic mapping, and studies of ocean and atmosphere. The latest in the series are Kalpana launched in December 2002; RESOURCESAT-1 (IRS-P6) launched in October 2003; CARTOSAT-1 launched in May 2005; CARTOSAT-2 launched in January 2007; RISAT-2 launched in April 2009; OCEANSAT-2 launched in September 2009. The Indian EO segment will be augmented with the launch of RISAT-1, MEGHA-TROPIQUES, INSAT-3D, and SARAL during 2009–10.

Meteor series:
Roshydromet maintains these missions, mainly for operational meteorological purposes. Other applications include experimental measurement of ozone and Earth’s radiation budget.

RADARSAT series:
Launched in November of 1995, RADARSAT-1 provides researchers and operational users with a range of SAR data products which are used for marine applications such as ship routing and ice forecasting, as well as land applications, such as resource management and geological mapping. RADARSAT-2 was launched in December 2007 to ensure data continuity.

SPOT and Landsat series:
The SPOT satellites (lead agency CNES) and the Landsat series (lead agency USGS) provide high resolution imagery in a range of visible and infrared bands. They are used extensively for medium resolution land studies. Data from these satellites are supplemented by availability of very high resolution imagery (up to 1 m) from various commercial satellites.

CBERS series:
A joint mission series of China and Brazil, aimed at environmental monitoring and Earth resources. The latest in the series (CBERS-2B) was launched in September 2007.

KOMPSAT (Arirang) series:
These South Korean missions are aimed at cartography, land use and planning, and ocean and disaster monitoring. The first satellite was launched in December 1999, with a second launch in July 2006.

NASA’s EOS missions:
Carrying the latest advanced sensors, each mission is dedicated to investigation of particular Earth System issues. In addition to the Terra, Aqua and Aura missions, NASA has also launched a number of missions aimed at developing understanding of the Sun’s variability and its influence on our climate, including ACRIMSAT, SORCE, and TIMED.

Cloud properties and climate links:
Since April 2006, a multiple satellite (NASA and CNES) constellation, dubbed the A-Train, has been in place (comprising CloudSat, Aqua, Aura, CALIPSO and PARASOL) flying in orbital formation to gather data needed to evaluate and improve the way clouds are represented in global models, and to develop a more complete knowledge of their poorly understood role in climate change and the cloud-climate feedback.

Polar ice cap studies:
Given the significance of information on changes in the continental ice sheets, two missions have been dedicated to their study: NASA’s ICESat (launched January 2003) and ESA’s CryoSat (lost on launch in October 2005 but rebuilt as CryoSat-2 for launch in 2010).

Gravity and magnetic field studies:
The GRACE (from 2002) and GOCE (from 2009) missions are dedicated to providing more precise measurements of the geoid, while DEMETER (currently in orbit), Kanopus-Vulkan, and Vulkan-Kompas-2 will study links between electromagnetic fields and earthquake predictability.