In the last decade, the value of Earth Observation (EO) to society has significantly grown among both the public at large and policy makers worldwide. As presented in a recent article on Earthzine (Scanning the Globe) the Group on Earth Observations (GEO) is coordinating efforts to build a Global Earth Observation System of Systems, or GEOSS. Launched in response to calls for action by the 2002 World Summit on Sustainable Development and by the G8 (Group of Eight) leading industrialized countries, GEOSS is to: enhance the coordination of observations of the Earth system, facilitate data management and distribution solutions, reach international agreements on data sharing principles, and directly encourage the development of operational applications linked to societal needs.
|The Sandmeier Field Goniometer, a measurement
tool for the Bidirectional Reflectance Distribution
Function. (Photo Courtesy NASA)
In addition to improving data access and availability, Michael Rast, a GEO Secretariat member seconded by the European Space Agency, has often underlined the need to assure the quality of such data. Quality is indeed a condicio sine qua non GEOSS can achieve its objectives. Indeed, for the “System of Systems” to be fully successful, the calibration, validation and intercalibration between instruments are tasks of primary importance. Harmonised procedures for such activities would ensure the required data quality control and the sensors interoperability in the framework of long-term and multi-mission applications.
The Committee on Earth Observation Satellites (CEOS), the space contribution to GEOSS as defined during its 19th Plenary meeting, identified the need to take a lead in tackling these issues. The CEOS Working Group on Calibration and Validation (WGCV), established consensus within the international community on a roadmap towards the establishment of Cal/Val best practices built upon the key principle of demonstrating traceability to internationally agreed references. These best practices will be issued as CEOS endorsed guidelines, under the auspices of GEO, for implementation by the member agencies of CEOS. It is important to appreciate that such guidelines could, with time, be improved upon in order to accommodate new sensors, technology and methodological improvements.
Harmonisation of Best Practices
A common and endorsed set of fully-documented best practices for calibration, validation and intercalibration processes would harmonise the sometimes disjointed procedures and directly respond to the GEOSS requirement for data quality assurance.
Through CEOS WGCV, international experts are discussing and agreeing on this set of procedures, covering all EO space-borne instruments and specialities. More specifically, the WGCV has the following subgroups: Atmospheric Composition, Infrared and Visible Optical Sensors (IVOS), Land Product Validation (LPV), Microwave Sensors (MS), Synthetic Aperture Radar (SAR), Terrain Mapping (TM). They shall address:
(1) Pre-launch activities:
(a) Full instrument cycle test (including instrument and environmental modeling) to ensure every element is traceable to SI standards where possible
(b) All calibration data and procedures should be documented and kept
(2) Onboard calibration devices and activities (when applicable):
(a) Should be concept proven and characterized
(b) Should be traceable
(c) The witness samples should be kept
(3) Post launch activities:
(a) Vicarious calibration using ground sites/scenarios
(b) Permanent reference sites that can be used for cal/val and inter-calibration of other satellite sensors via simultaneous and collocated observations
(4) Auxiliary tools and methods such as Radiative Transfer Models, as well as full end-to-end system simulation tools for all sensors must be documented, maintained and openly available.
Cal/Val Scenario Endorsement
|Launch of a stratospheric balloon for Envisat
atmospheric composition instruments validation in
Kiruna, Sweden. (Photo Courtesy ESA)
The harmonisation guidelines will also establish a set of Cal/Val scenarios that seek to serve all sectors of the Cal/Val community and that shall be endorsed by CEOS. In this context, a scenario could either be a single site or may well be a series of linked (by common protocols and facilities) ‘sites’, e.g. the Network for Detection of Atmospheric Climate Change (NDACC) series would be regarded as a ‘reference site’.
Each WGCV subgroup (representing each major thematic EO specialisation) is currently tasked to define the requirements for Cal/Val scenarios and the characteristics that they should exhibit and satisfy in relation their field of expertise and specific objectives.
CEOS has also agreed that Cal/Val data (satellite, airborne and in situ) and relevant documentation on the best practices should be made freely accessible online. Preliminary services are currently being provided through the CEOS Cal/Val Portal developed by ESA. The employment of a dedicated Cal/Val portal is ensuring that datasets are not duplicated unduly, that an efficient and secure knowledge transfer is implemented and that cooperation is encouraged through an international effort. It is important to stress that the provided data shall include complete documentation of the entire process to ensure that there is full traceability.
|Alaska Satellite Facility, Gen III corner reflector
used to calibrate SAR data products.
In the specific case of Cal/Val data, totally open access to the whole EO community could hurt the Cal/Val activity dramatically if for instance users erroneously ‘re-do’ calibrations. Therefore, it will be necessary to clearly control data access and data upload. Currently a solution would be to have internationally peer-reviewed projects that obtain accessibility to specific data and the possibility to upload their measurements, ensuring both the quality and traceability of such ‘raw’ data. Additionally, the actual data utilisation would also have to be governed by a dedicated Cal/Val data policy (code of use) to avoid misuse of preliminary data sets and publication of erroneous results.
Overall, these harmonisation guidelines are addressing data quality information. The computation and presentation of data accuracy levels, error bars, completeness and correctness of each processing step should be harmonised. All processes need to be traceable and shall contain the required quality information needed to understand the suitability of the procedures used and subsequently to progress in the chain of analysis.
Comprising 26 Members (most of which are space agencies) and 20 Associates (associated national and international organizations), CEOS is recognized as the major international forum for the coordination of EO satellite programmes and for interaction of these programmes with users of satellite data worldwide. Through its open workshops and meetings, CEOS promotes the widest possible involvement in its processes and always encourages proactive contributions by all participating bodies.