At the end of August 2010, scientists from eleven countries on four continents each measured the reflectance of a few square kilometers of salt. These measurements, taken over a two-week period, will have a major impact on the future of satellite-based Earth observation.
Policymakers are increasingly relying upon Earth monitoring from space to make critical decisions about the planet's future. The number of satellites is growing rapidly, funded by government agencies and commercial organizations. It is thus imperative that all stakeholders be able to rely upon the products and results from all of these missions.
The Group on Earth Observations (GEO) Global Earth Observation System of Systems (GEOSS) aims to deliver comprehensive knowledge-information products in a timely manner to satisfy its nine Societal Benefit Areas, of which the most demanding (in terms of accuracy) is climate. Accomplishing this vision using today's disparate systems requires establishing an internationally coordinated operational framework to facilitate interoperability and harmonization. The Committee on Earth Observation Satellites (CEOS), the space arm of GEO, has led the development of a Quality Assurance Framework for Earth Observation (QA4EO)1 based on the adoption of two key principles. First, all Earth observation data products have associated with them an unequivocal quality indicator. Second, this indicator should be based on documented evidence of its degree of consistency with an internationally agreed reference standard, ideally International System of Units (SI) traceability.
While significant effort is being expended to design and calibrate sensors prior to launch, they rarely can be relied on post-launch, at least for the optical domain. An established best practice is to use well-characterized terrestrial targets, chosen for spatial uniformity and stability in reflectance, to validate sensor performance and facilitate cross-comparisons for evaluating sensor biases.
Although there are many potential sites around the globe, many of which have recently been cataloged,2 CEOS has endorsed a subset to serve as reference standard test sites for international efforts. They endorsed eight instrumented sites, known as Landnet, that are regularly characterized by ground teams to determine satellite radiometric gain and five pseudo-invariant desert sites, along with the moon, for long-term stability and cross-comparison.3 Each site, its instruments and methodologies, must be consistently traceable to SI units.
Tuz Gölü in southern Turkey is one of the Landnet sites. During summer months, instead of a lake it becomes a largely dry salt bed that is very bright and spatially uniform (see Figure 1). In August 2010, representatives from the international field-spectroscopy community, experts in characterizing such test sites, compared instrumentation and sampling methodologies using this site as a common reference. The calibration of all instrumentation was made SI traceable (see Figures 2, 3) through participation of the National Physical Laboratory (NPL), and the UK national standards laboratory, which is also organizing and analyzing the comparison, supported by the European Space Agency (ESA) and the UK government.
Figure 1. Tuz Gölü, southern Turkey as viewed by the UK-Disaster Monitoring Constellation satellite.
Figure 2. Field spectroradiometers calibrated against a National Physical Laboratory (NPL) standard radiance source to ensure International System of Units (SI) traceability.
Figure 3. Participant reference reflectance panels calibrated to the NPL standard using the sun as the source.
In addition to the ground activities, many satellites also were involved in a parallel comparison using this same site. The timing of the ground measurements matched the cross-overs of various satellites. The results will be propagated to the top-of-atmosphere to enable radiometric gains of individual sensors to be evaluated against those of satellite-to-satellite measurements.
A pilot exercise was carried out in 2009 with a small group of European teams. The results are available through the Calibration and Validation (Cal/Val) of Earth Observation portal.3 An example of the surface reflectance measured by two teams is shown in Figure 4, with Figure 5 showing the angular variability of the surface reflectance, an essential component as many sensors do not always view completely at nadir. This comparison is one of an ongoing sequence organized by CEOS, and in the last two years has included sensor-to-sensor comparisons over the snow fields of Dome-C in Antarctica, sea-surface temperature, and ocean color. The results of all these will be available through the Cal/Val portal.3 There also will be a workshop in October 2010 in Ispra, Italy, organized by CEOS, to review the results of these comparisons and develop future strategies.4 The activities of the 2010 comparison can be followed on a daily blog.3
Figure 4. Surface reflectance factor (left axis) measured by two teams. Right axis shows the percentage difference between lab A and B.
Figure 5. Angular surface reflectance measured by the NPL Gonio Radiometric Spectrometer System (right). UTC: coordinated universal time.
The long-term goal of these activities is to establish a means to ensure that all Earth observation sensors and associated products are suited for their intended purpose and use. Test sites and comparisons are an essential component of this process and the only way to assure international harmonization and interoperability. However, to make such a system truly operational we need to make reference measurements from space using a sensor that can claim and demonstrate SI traceability of sufficient accuracy in its own right to be considered a benchmark. Two complementary mission proposals currently seek to provide this from slightly different perspectives, making the ideal solution an international calibration constellation of the two. The Climate Absolute Radiance and Refractory Observatory5 aims to provide reference calibrations largely through near-simultaneously-matched observations with a focus on climate sensors, while the Traceable Radiometry for Terrestrial and Helio Studies mission6 has a higher spatial resolution and will, in addition to the same climate objectives and sensors, calibrate test sites. Both missions aim to provide SI-traceable spectrally resolved uncertainties in a radiance of <0.3%.
In summary, the international community, under the auspices of CEOS and GEO, is very active in trying to harmonize measurements of the Earth from space. It has embraced the need for improved quality assurance and has embarked on a series of international comparisons to evaluate and ideally remove biases between different measurement instrumentation and methodologies, and in so, doing establish and document best practices.
National Physical Laboratory
Nigel Fox, head of Earth observation at the UK National Standards Laboratory, has a 30-year career that has involved the development of primary optical radiation measurement standards and techniques, the last 15 of which focused on Earth observation. He chairs the IR, Visible and Optical Sensors sub-group of the Committee on Earth Observation Satellites-Working Group on Calibration and Validation.