2010 CEOS Field Reflectance Intercomparisons

1. 2010 CEOS Field Reflectance Intercomparisons K. Thome1, N. Fox2 1NASA/GSFC , 2National Physical Laboratory

2. Outline 10 countries and 13 organizations Simulated calibration of sensors with varying spatial resolution Talk covers Motivation Measurement overview Recommendations and lessons learned Summarize lessons learned during joint field campaigns to TuzGolu, Turkey

3. Background Vicarious calibration is a critical part of the calibration of earth imagers Each sensor team has own vicarious calibration plans Essential to ensure different groups obtain consistent results to prevent biases Need to ensure accurate results with SI-traceability Permit evaluation of the repeatability and accuracy of vicarious calibration Emphasis on surface reflectance Compare techniques and instrumentation for vicarious calibration of optical imagers

4. Importance of Reflectance Reflectance uncertainty dominates for sites with reflectance>0.2 Errors based on Monte Carlo simulations of typical input uncertainties Nearly all error is due to reflectance uncertainty at longer wavelengths Dots indicate MODIS spectral bands

5. CEOS Key Comparison Objectives Committee of Earth Observations Satellites (CEOS) goal is to inform about uncertainties Determine biases between field instrumentation using laboratory and in situ cross-comparisons Estimate reflectance uncertainties Evaluate differences in sampling methods Document “best practices” used by the participants • Not intended to force identical data collection and processing approaches

6. Data Protocols Comparing results from separate groups is complicated by differences in data formats Standardised format includes appropriate documentation of errors and uncertainties Type A is the uncertainty resulting from the statistical analysis of the data Type B standard uncertainty quantified by means other than statistical analysis of data Use of both Type A and B permits evaluation of equipment verses methodology uncertainties Recommendation: Establish a standardised format for reflectance-based calibration measurements to enable easier comparisons of data from site characterisations

7. Field spectrometer Absolute calibration supplied by the instrument manufacturer for all but one participant All groups used Analytic Spectral Devices FieldSpec FR Field spectrometers were used by all but one group

8. White reference White reference calibration relied on the calibration supplied by the manufacturer One group characterized their own reference in their own laboratory One group relied on a third party to characterize their reference All manufacturer-based calibrations were in terms of a hemispheric-directional characterization All groups used a PTFE-based white reference

9. Measurement approaches Single, specific approach for characterization not feasible due to differences in vicarious methods Methodology based on Number of personnel available Length of time to collect Slowest method is stop and stare Fastest is continuous sampling Equipment carrying varies by group Interference between user and measurement Logistics typically determines how a group collects data

10. Measurement protocols Goal of measurement protocols is to improve methods so sampling dominates differences Protocols must use methods usable by all groups Sensitivity studies and defensible and traceable error budgets provide the basis for improvements Surface properties and uniformity should dominate Reduce impact from instrument and other error sources

11. Measurement protocols Recommendation:Use of an invariant standard before and after site characterizations is needed to evaluate instrument performance Recommendation: A standardised radiometer should be developed that can act as transfer standard to link test-sites traceability Limited bands with limited field of view Likely not portable – not suitable for characterizing the test site Provide means to ensure calibration of white reference Monitor field radiometer behavior across multiple groups Travelling standard allows a few groups to shoulder the costs of developing and operating radiometer

12. Processing Protocols Processing methodology currently plays a limited role in surface reflectance differences Sun angle effects White reference calibration Largest Type B error attributed to using hemispheric-directional reflectance Recommendation: Reflectance factor of white reference panel and test site should be based on a bi-directional (Gonio) characterisation at appropriate angle(s)

13. Processing methodology A bi-directional characterization creates far lower Type B errors, especially at longer wavelengths Offers the opportunity for a correction of diffuse-light effects at shorter wavelengths. Recommendation: Look-up table of panel BRF for range of incident angles should be developed as a first order correction

14. Measurement Protocol Provides a Type A uncertainty assessment Describes effects such as measurement repeatability and the variability of the site Recommendation: Individual site "point measurements" should consist of statistically significant number Recommendation: Perform "repeatability measurement" before and during site characterisation based on ratio of repeated panel views to repeated views of a single surface location

15. Way forward to TOA radiance Next step is to include the radiative transfer codes and atmospheric characterization Use TuzGolu campaign data to create a standard input data set Compare the top-of-atmosphere radiances TuzGolu comparisons lead to path forward for best practices for atmospheric correction

16. TOA radiance comparison Code comparisons show favorable results when given identical inputs Input parameters to codes can help define atmospheric characterization approaches Sensitivity analyses for understanding key inputs specific to selected sites New philosophy is development of a data set for radiative transfer code intercomparison

17. Lessons Learned from 2009 and 2010 campaigns Collaborative efforts between NMIs and vicarious calibration laboratories are essential Future comparisons must include a greater diversity of field instrumentation Knowledge of type B errors and uncertainties is inadequate Data collected are insufficient to determine Type A uncertainties Clearer understanding of systematic and random biases/errors is necessary Ensure SI traceability Development of proper error budgets 2-3% absolute uncertainty for reflectance-based calibration requires well developed error budgets

18. Summary Terrestrial imagers operated by multiple countries create challenges to develop climate-quality data Different reference standards Independent routes of “traceability” Education process needed to ensure of SI-traceable, error budgets Uncertainties <2% requires more rigorous collection approaches similar to laboratory practices Such approaches are needed to reach the level of climate-quality data sets Full results of this comparison are available on the GEO/CEOS Cal/Val portal The CEOS-led campaigns to TuzGolu provide a unique opportunity to evaluate vicarious calibration