| | |  | An artist's impression of Spectra | |
About Spectra
The scientific objective of the SPECTRA (Surface Processes and Ecosystem Changes Through Response Analysis) mission is to describe, understand and model the role of terrestrial vegetation in the global carbon cycle and its response to climate variability under the increasing pressure of human activity.
The most evident characteristic of terrestrial biomes is heterogeneity over a range of spatial scales. The full complexity of these biomes is not well represented in global models of the Earth system, which are used to understand the overall functioning of the planet and to forecast its evolution. The key scientific issues in this respect include not only the description of the heterogeneity of representative landscapes, but also the analysis and prediction of how the underlying processes interact non-linearly across
many scales to couple events at a local scale to changes at the global scale.
To incorporate these processes and interactions in global models, they must be parameterised. This can only be achieved by documenting them at appropriate spatial and temporal resolutions, over an ensemble of regions representing all terrestrial biomes, including their functioning and heterogeneity. These regions will be selected before or during the mission, following scientific and programmatic criteria that will be defined by the scientific community, such as the presence of long-term monitoring equipment or the conduct of experimental campaigns.
Remote sensing
Remote sensing from space can complement these field investigations by offering unique capabilities to address such scaling issues, provided these observations are acquired at the appropriate spatial, temporal, spectral and directional resolutions. These measurements will provide a suitable basis to assess the role of local heterogeneity in determining processes and impacts at the regional scale albeit through parameterisations, in global biosphere and climate models. Models exist today to assimilate remote sensing measurements from low-resolution sensors.
In the near future, Earth System Models (ESMs) will include explicit dynamic representations of the biosphere and carbon cycle. They will require much better parameterisations of surface processes at scales of tens of kilometres and need to be generated for all relevant biomes. This can only be achieved through a better understanding of how physical, chemical and biological processes interact non-linearly over heterogeneous land surfaces. The availability of detailed observations such as
those provided by SPECTRA will thus present new opportunities to derive simple yet sufficiently accurate representations of the terrestrial biosphere for use in global ESMs.
Observations
The SPECTRA mission will provide detailed observations of the amount and condition of vegetation over an ensemble of regions distributed globally. These observations are needed to derive the biome-specific parameterisations of biosphere processes needed by Dynamic Global Vegetation Models (DGVMs).
These parameterisations, together with a suitable biome distribution, will enable the derivation of global estimates of carbon fluxes, using existing and future sensors offering a lower spatial resolution, but frequent global coverage.
The SPECTRA mission will be able to capitalise on the achievements of various earlier Earth-observation missions. The state and evolution of the Earth system (climate and biosphere), and in particular of the terrestrial biomes, have been monitored from space over the last twenty years or so with the help of various sensors onboard Earth-observation platforms. These missions (notably NOAA/AVHRR, Meteosat, SeaWiFS, VEGETATION, POLDER) provide essential global observations on a daily basis, in a very limited number of spectral bands and at a coarse spatial resolution.
The current generation of coarse resolution global instruments (e.g. MODIS, MISR, MERIS, AATSR, SEVIRI) offers enhanced spectral and directional sampling capacities along with high radiometric performances, but still at medium spatial resolution. These observations provide broad estimates of vegetation and soil variables. The continued availability of remote-sensing data at spatial resolutions from hundreds of metres to a few kilometres, on a global and daily basis, can be expected for the foreseeable future, because of the large number of applications that are relying on them.
Measurements
To meet the scientific objective presented earlier, the SPECTRA mission will acquire detailed spectral and directional measurements of the reflectance and emittance of the heterogeneous land and atmosphere system, at a high spatial and radiometric resolution, and with a revisit frequency sufficient to document the often rapid evolution of terrestrial vegetation.
No current or planned mission will provide the detailed measurements required to address the scientific objective of the SPECTRA mission, with the requisite spatial, temporal, spectral, directional and radiometric resolutions described in this Report. SPECTRA benefits from the extensive scientific know-how accumulated over the last decade, both in terms of algorithms and applications.
Main elements
The SPECTRA mission consists of three elements: The space segment comprises a platform in a near-polar orbit to allow repeated data acquisitions and global accessibility, and a sensor providing the required radiometric, spatial and directional performances in a wide spectral domain encompassing the visible, near-infrared, middle-infrared and thermal-infrared. The ground segment involves facilities for the management of the satellite, the acquisition of raw data and the production of basic geophysical products, addressing such generic issues as calibration, navigation, co-registration,
etc. It is anticipated that the dedicated data processing for specific applications and the archiving of intermediate and final products will take place in institutions of high scientific calibre. The field segment of SPECTRA will guarantee effective integration of in-situ measurements with SPECTRA data products over an ensemble of regions, globally distributed to sample all relevant biomes.
In summary, the SPECTRA mission addresses scientific objectives of great relevance, proposes a combined spaceborne and field observational strategy, offers a technical solution that meets the observational requirements and capitalises on a vast array of existing experimental and theoretical know-how. This mission will prove useful to a wide diversity of users and poses no major technical difficulty.
Last update: 22 April 2004 | |
