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Capabilities of Earth Observation Satellites
Earth Observation Plans: by Measurement
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Catalogue of Satellite Missions
Catalogue of Satellite Instruments

Albedo and Reflectance

Essential Climate Variables: Albedo

Albedo is the fraction of solar energy that is diffusely reflected back from Earth to space. Measurements of albedo are essential for climate research studies and investigations of Earth’s energy budget.

Different parts of Earth have different albedos. For example, ocean surfaces and rain forests have low albedos, which means that they reflect only a small portion of the Sun’s energy. Deserts, ice and clouds, however, have high albedos; they reflect a large portion of the incoming solar energy. The high albedo of ice helps to insulate the polar oceans from solar radiation. Over the whole surface of Earth, about 30% of incoming solar energy is reflected back to space. Because a cloud usually has a higher albedo than the surface beneath it, clouds reflect more shortwave radiation back to space than the surface would in the absence of the cloud, thus leaving less solar energy available to heat the surface and atmosphere. Hence, this ‘cloud albedo forcing’, taken by itself, tends to cause a cooling or ‘negative forcing’ of Earth’s climate.

Surface albedo can be estimated from shortwave, broadband or multi-spectral radiometer measurements with good horizontal resolution. Current measurements of albedo and reflectance are obtained primarily using multi-spectral imagers such as AVHRR and MODIS, Vegetation and instruments on some geostationary satellites (such as MSG).

Clouds, aerosols and atmospheric gases affect the achievable accuracy, which is currently marginal to acceptable, but should improve as progress is made in interpreting data from high-resolution, multi-spectral instruments. Surface conditions (moisture, surface vegetation, snow cover etc.) strongly affect albedo and high-quality ground truth data are necessary in support of satellite measurements. Better understanding of the directional reflectance properties of different surfaces and more accurate aerosol data (to correct atmospheric effects) are needed to improve surface reflectance measurements.

spacer As aerosol concentration increases within a cloud, more cloud droplets form. Since the total amount of condensed water in a cloud does not change much, the average droplet becomes smaller. This has two consequences: clouds with smaller droplets reflect more sunlight and such clouds last longer. Both effects increase the amount of sunlight that is reflected to space without reaching the surface.

The Terra satellite has yielded greater knowledge of such cloud/aerosol effects, with MODIS and MISR providing data on cloud features, and ASTER providing complementary, high spatial resolution measurements. Terra’s data provide new insights into how clouds modulate the atmosphere and surface temperature. Further multi-directional and polarimetric instruments (e.g. POLDER) also provided measurements leading to better estimates of albedo.

Other sensors, such as GERB and SEVIRI on the MSG missions (starting with Meteosat-8) have provided improved capabilities for measuring surface albedo. Improved sounder performance will yield more information on the infrared surface emissivity spectrum. Multi-spectral imaging sensors such as AVHRR/3 on NOAA and Eumetsat polar-orbiting satellites, IVISSR on the Chinese FY-2 series and AWIFS on the Indian Resourcesat provide global visible, near-infrared and infrared imagery of clouds, ocean and land surfaces.

CEOS has undertaken to improve the continuity of terrestrial climate monitoring through enhancements to the moderate-resolution historical record. AVHRR data reprocessing will be undertaken to ensure a consistent dataset to contribute to historical albedo. CEOS will also work to enhance the quality of the Fundamental Climate Data Records generated from the AVHRR record.

Click to view the Albedo and Reflectance mission timeline.


Landscape Topography

Essential Climate Variables: Glaciers and Ice Caps, Ice Sheets, Lake Areas and Levels

Many modelling activities in Earth and environmental sciences, telecommunications and civil engineering increasingly require accurate, high-resolution and comprehensive topographical databases with indication of changes over time, where relevant. The information is also used by, amongst others, land use planners for civil planning and development, and by hydrologists to predict the drainage of water and likelihood of floods, especially in coastal areas. In its Fourth Assessment Report in 2007, the IPCC predicted that (by conservative estimation techniques underestimating the melting of ice sheets) global mean sea level may rise as much as 28–43 cm by the end of the 21st century. Potentially, sea-level rise will cause severe flooding, with disastrous impacts on large, densely populated, low-lying coastal cities and deltaic areas, such as Bangladesh.

Satellite techniques offer a unique, cost-effective and comprehensive source of landscape topography data. At present, most information is obtained primarily from multi-band optical imagers and synthetic aperture radar instruments with stereo image capabilities. The pointing capability of some optical instruments allows the production of stereo images from data gathered on a single orbit (e.g. by ASTER on Terra or HRS on SPOT-5) or multiple orbits (e.g. by SPOT and Pleiades series). These are then used to create digital elevation maps, which give a more accurate depiction of terrain.

Since SARs can also be used in interferometric mode to detect very small changes in topography, they have important applications in monitoring of volcanoes, landslides, earthquake displacements and urban subsidence. Current missions include Envisat (operations concluded in April 2012), RADARSAT-2, the Cosmo-SkyMed constellation and TerraSAR-X. From 2014, ESA’s Sentinel-1 mission, and JAXA's ALOS-2 will also contribute to such information.

Radar altimeters can also provide coarse topographic mapping over land. They have been supplemented by a new generation of laser altimeters, such as GLAS (on ICESat, ended 2010) which can provide landscape topography products with height accuracies of order 50–100 cm, depending on slope.

The role of these satellites and their importance in mitigating geo-hazards, such as earthquakes, landslides and volcanic eruptions, is the focus of the GEO Supersites initiative.

GCOS notes that measurements of lake area and lake level give an indication of the volume of the lake, an integrator variable that reflects both atmospheric (precipitation, evaporation-energy) and hydrological (surface water recharge, discharge and ground water tables) conditions. GCOS threshold requirements for these variables are currently met by existing missions.

Click to view the Landscape Topography mission timeline.

Soil Moisture

Essential Climate Variables: Soil Moisture

Soil moisture plays a key role in the hydrological cycle. Evaporation rates, surface runoff, infiltration and percolation are all affected by the level of moisture in the soil. Changes in soil moisture have a serious impact on agricultural productivity, forestry and ecosystem health. Monitoring soil moisture is critical for managing these resources and understanding long-term changes, such as desertification, and should be developed in proper coordination with other land surface variables. There is a pressing need for measurements of soil moisture for applications such as crop yield predictions, identification of potential famine areas, irrigation management and monitoring of areas subject to erosion and desertification, as well as for the initialisation of NWP models.

Direct measurement of soil moisture from space is difficult. Most of the active and passive microwave instruments provide some soil moisture information for regions of limited vegetation cover. However, under many conditions remote sensing data are inadequate and information regarding moisture depth remains elusive. While recent studies have successfully demonstrated the use of infrared, passive microwave and non-SAR sensors to obtain soil moisture information, the potential of active microwave remote sensing based on SAR instruments remains largely unrealised. The main advantage of radar is that it provides observations at a high spatial resolution of tens of metres compared to tens of kilometres for passive satellite instruments, such as radiometers, or non-SAR active instruments, such as scatterometers (e.g. QuikSCAT, ERS, Envisat and MetOp). The main difficulty with SAR imagery is that soil moisture, surface roughness and vegetation cover all have an important and nearly equal effect on radar backscatter. These interactions make retrieval of soil moisture possible only under particular conditions, such as bare soil or surfaces with low vegetation, or through complex modelling to ‘subtract’ the contributions/effects of vegetation.

  An appropriate instrument for measurements of soil moisture would appear to be the passive microwave radiometer, although some success has been achieved by radar, despite the complications of analysing the signals reflected from the ground. Microwave radiation emitted at the ground can be monitored to infer estimates of soil moisture. Passive microwave sensors can be used to do this, based on detection of surface microwave emissions, although the signal is very small and frequently polluted by radio-frequency interference from illegal sources. Reliable data (high signal-to-noise ratio) need to be taken over a large area, which introduces the problem of understanding how to interpret the satellite signal, since it consists of radiation from many different soil types.

SAR data currently provide the main source of information on near-surface (10–15 cm) soil moisture. ASCAT (an improvement of the ERS-1/2 scatterometer) on EUMETSAT’s MetOp series also provides data from which soil moisture information can be inferred.

AMSR-E on Aqua (ended late 2011) and now AMSR-2 on GCOM-W (since mid 2012) provide a variety of information on water content by measuring weak radiation from Earth’s surface..

Launched in late 2009, the first mission to satisfy requirements for observing soil moisture from space for the primary applications of hydrologic and meteorological modelling is ESA’s Soil Moisture and Ocean Salinity mission, carrying the MIRAS (Microwave Imaging Radiometer using Aperture Synthesis) passive L-band 2D interferometer. The new capabilities provided by SMOS will help to reduce process uncertainties and improve climate models. NASA's SMAP mission, planned for launch in late 2014, is also aimed at providing soil moisture monitoring capabilities.

Click to view the Soil Moisture mission timeline.


Essential Climate Variables: Land Cover, Fire Disturbance (Burnt Area), Leaf Area Index (LAI), Fraction of Absorbed Photosynthetically Active Radiation (fAPAR), Above-ground Biomass

Changes in land cover are important aspects of global environmental change, with implications for ecosystems, biogeochemical fluxes and global climate. Land cover change affects climate through a range of factors from albedo to emissions of greenhouse gases from the burning of biomass.

Deforestation inter alia increases the amount of carbon dioxide (CO2) and other trace gases in the atmosphere. When a forest is cut and burned to establish cropland and pastures, the stored carbon joins with oxygen and is released into the atmosphere as CO2. The IPCC Third Assessment Report (2001) noted that about three-quarters of the anthropogenic emissions of CO2 to the atmosphere during the past 20 years was due to fossil fuel burning. The rest was predominantly due to land use change, especially deforestation. The IPCC Fourth Assessment Report (2007) confirmed this statement with improved confidence levels.

In 2005, a number of developing countries proposed to incorporate deforestation prevention into the Kyoto Protocol, in part through an emissions trading system. The initiative, known as REDD, (Reducing Emissions from Deforestation in Developing countries) would allow developing countries to sell emissions savings from forest conservation. Developed countries would buy the savings to credit against their own emissions targets. In 2009 CEOS agencies began actively supporting efforts within GEO for the development of a global forest carbon tracking framework, providing satellite data acquisitions and related expertise.

IGOS set up an Integrated Global Carbon Observation Theme to develop a flexible, robust strategy for international global carbon observations over the next decade. A key component of IGCO is terrestrial carbon observations aimed at the determination of terrestrial carbon sources and sinks with increasing accuracy and spatial resolution. The IPCC has highlighted an improved understanding of carbon dynamics as vital in tackling one of the biggest environmental problems facing humanity. The IGCO report (2004), further developed as a GEO Carbon Strategy (2010), is providing an essential input to the implementation of the United Nations Framework Convention on Climate Change, particularly on the role of natural sinks in meeting targets under the UNFCCC Kyoto Protocol.

  Satellite observations allow scientists to map land cover and the dynamics of fire disturbance, and track two key elements of Earth’s vegetation – the ‘Leaf Area Index’ (LAI) and the ‘Fraction of absorbed Photo-synthetically Active Radiation’ (fAPAR). LAI is defined as the one-sided green leaf area per unit ground area in broadleaf canopies, or as the projected needle leaf area per ground unit in needle canopies. fAPAR is the fraction of photosynthetically active radiation absorbed by vegetation canopies. Both LAI and fAPAR data are necessary for understanding how sunlight interacts with Earth’s vegetated surfaces.

Multiple types of satellite observations are used in agricultural applications. Space imagery provides information which can be used to monitor quotas and to examine and assess crop characteristics and planting practice. Information on crop condition, for example, may also be used for irrigation management. In addition, data may be used to generate yield forecasts, which in turn may be used to optimise the planning of storage, transport and processing facilities. Classification and seasonal monitoring of vegetation types on a global basis allow the modelling of primary production – the growth of vegetation that is the base of the food chain – which is of great value in monitoring global food security.

A number of radiometers provide measurements of vegetation cover, including the ATSR series, AVHRR/3, MODIS, MERIS (ended 2012), SEVIRI and Vegetation. These instruments are helping production of global maps of surface vegetation for modelling of the exchange of trace gases, water and energy between vegetation and the atmosphere. Multi-directional and polarimetric instruments (such as MISR and POLDER) provide more insights into corrections of land surface images for atmospheric scattering and absorption, as well as Sun-sensor geometry, allowing better calculation of vegetation properties.

Synthetic aperture radars are used extensively to monitor deforestation and surface hydrological states and processes. The ability of SARs to penetrate cloud cover and dense plant canopies makes them particularly valuable in rainforest and high-latitude boreal forest studies.

Instruments such as ASAR (Envisat - ended 2012), SAR (RADARSAT-2) and, in the near future, PALSAR (ALOS-2, launch early 2014) provide data for applications such as agriculture, forestry, land cover classification, hydrology and cartography.

CEOS and GCOS have concluded that many of the Essential Climate Variables related to vegetation and supported from space will require reprocessing of the moderate resolution historical record
(in particular AVHRR) to be of greater value for climate purposes, and appropriate actions have been defined.

Click to view the Vegetation mission timeline.

Surface Temperature (Land)

Essential Climate Variables: Fire Disturbance (Active Fires), Land Surface Temperature

Land surface temperature varies widely with solar radiation. It is of help in interpreting vegetation and its water stress when the ranges of temperatures between day and night and from clear sky to cloud cover are compared.

Estimates of greenhouse gas emissions due to fire are essential for realistic modelling of climate and its critical component, the global carbon cycle. Fires caused deliberately for land clearance (agriculture and ranching) or accidentally (lightning strikes, human error) are a major factor in land cover changes, affecting fluxes of energy and water to the atmosphere.

On a local scale, surface temperature imagery may be used to refine techniques for predicting ground frost and to determine the warming effect of urban areas (urban heat islands) on night-time temperatures. In agriculture, temperature information may be used, together with models, to optimise planting times and provide timely warnings of frost. Measurements of surface temperature patterns may also be used in studies of volcanic and geothermal areas and resource exploration.

Land surface temperature measurements are made using the thermal infrared channel of
medium/high-resolution multi-spectral imagers in low-Earth orbit. In addition, visible/infrared imagers on geostationary satellites also provide useful information, with the advantage of very high temporal resolution.

  However, difficulties remain in converting the apparent temperatures as measured by these instruments into actual surface temperatures – variations due to atmospheric effects and vegetation cover, for example, require compensation using additional imagery/information.

A number of capable sensors designed to provide land surface temperature data are currently operating or planned. These include advanced sounders (IASI, HIRS/4) on operational meteorological platforms. On the Suomi NPP satellite (and future JPSS missions), VIIRS combines the radiometric accuracy of AVHRR with the high spatial resolution of the DMSP’s OLS instrument.

The Hot Spot Recognition Sensor (HSRS) on BIRD (launched 2001) demonstrated its value as a purpose-built fire detection instrument until its partial failure in 2004, while MODIS provides regular sampling of active fires, SEVIRI observes the diurnal cycle of fire occurrence in Africa and the (A)ATSR series, despite not being designed for active fire observations, has produced the longest record of hot spot detection (at night). ESA offered a monthly world fire atlas product available online at until Envisat concluded operations in 2012.

Click to view the Surface Temperature (Land) mission timeline.

Multi-purpose Imagery (Land)

Essential Climate Variables: Land Cover (Including Vegetation Type)

The spatial information that can be derived from satellite imagery is of value in a wide range of applications, particularly when combined with spectral information from multiple wavebands of a sensor. Satellite Earth observation is of particular value where conventional data collection techniques are difficult, such as in areas of inaccessible terrain, providing cost and time savings in data acquisition – particularly over large areas.

At regional and global scales, low-resolution instruments with wide coverage capability and imaging sensors on geostationary satellites are routinely exploited for their ability to provide global data on land cover and vegetation. Land cover change detection is important for understanding global environmental change and has profound implications for ecosystems, biochemical fluxes and climate. Instruments on satellites with wide and frequent coverage provide data useful for spin-off applications. AVHRR on NOAA’s polar orbiting satellite series was originally intended only as a meteorological satellite system, but it has subsequently been used in a multitude of diverse applications, while the Envisat MERIS instrument has been used to generate global land cover imagery at 300 m resolution.

On national and local scales, the spatial resolution requirements for information mean that moderate resolution imaging sensors, such as those on the SPOT, Landsat and IRS series, and imaging radars (such as those on Envisat and RADARSAT) have been most useful. Such sensors have been routinely used as practical sources of information for:

— Agriculture monitoring, farming and production forecasting;

— Resource exploration and management, e.g. forestry;

— Geological surveying for mineral exploration and identification;

— Hydrological applications such as flood monitoring;

— Civil mapping and planning, involving cartography, infrastructure and urban management;

— Coastal zone monitoring, including oil spill detection and monitoring;

— Topographic mapping, generation of DEMs.

  The Landsat 5 satellite operation was suspended in November 2011 after a 27-year long mission. Landsat 7 (launched 1999) continues to collect imagery worldwide with partially compromised quality for some applications. Landsat 8 – also known as Landsat Data Continuity Mission or LDCM – was launched in February 2013. The evolving SPOT series has been discontinued, with only SPOT 5 still in operation in 2013. The ESA Sentinel-2 series (first launch late 2014) is expected to extend Landsat-type and SPOT-type moderate resolution imagery acquisition for a minimum period of 15 years.

SAR data are particularly useful in monitoring and mapping floods because they are available even in the presence of thick cloud cover. Instruments on RADARSAT-2 and TerraSAR-X continue to provide improved capabilities in this field. Such multi-incidence, high-resolution SAR systems will also be useful for landslide inventory maps and earthquake prediction. Moreover, InSAR techniques can be used to document deformation and topographic changes preceding, and caused by, volcanic eruptions. Volcanic features also have distinctive thermal characteristics which can be detected by thermal imagery, such as that provided by the ASTER radiometer flying on Terra. The IGOS Geo-hazards Theme report provides a comprehensive guide as to the value of satellite Earth observations for such applications. Future SAR instruments will continue to be important for land imagery because of their all-weather, day and night observing capability and high spatial resolution (1–3 m), as provided by RADARSAT-2 and SAR-2000 on the
Cosmo-SkyMed satellites.

Innovative instruments, such as AVNIR-2 and PRISM on ALOS (completed April 2011), provided enhanced land observing technology and improved data products. In general, future sensors will benefit from a greater number of sampling channels. NOAA’s VIIRS instrument, for instance, has multi-channel imaging capabilities and combines the radiometric accuracy of AVHRR with the high spatial resolution of the OLS flown on DMSP missions.

CEOS has initiated a virtual constellation study team for land surface imaging to provide the coordination framework necessary to secure continuity of moderate resolution imagery used for many land surface applications, including their relation to climate.

Click to view the Multi-purpose Imagery (Land) mission timeline.