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Test Site Catalog

 

Radiometry Sites Resources

 

Radiometry test sites are core to any Quality Assurance/Quality Control (QA/QC) strategy. These sites can be useful for stability monitoring and are essential for vicarious absolute calibration campaigns. They provide a convenient means of obtaining information to verify sensor performance. Test sites are the only practical means of deriving knowledge on biases between sensors and they allow, at some level, a means of bridging anticipated data gaps caused by lack of measurement continuity, due to lack of co-existent in-flight sensors.

There are currently 47 Radiometric Sites and 4 Thermal Sites in the catalog. To view these sites, click on a continent below, or select a location from the Radiometric Site drop down box to the right.

• Shape File for Radiometry Sites – Zip
• Google KMZ File for Radiometry Sites – Download Google Earth KMZ File
• Excel File for Radiometry Sites – Excel

 

Radiometric Test Site Catalog Information

I. Need for a Global, Integrated Network of Calibration Sites
II. Well-Established Radiometric Test Site Selection Criteria
III. Distribution of World-Wide Radiometric Sites
IV. Review of Radiometric Calibration Methods
V. Proposed Future Plans
VI. Prime Candidate Earth Targets for the Post-Launch Radiometric calibration
VII. List of CEOS IVOS Prime Candidate Earth Targets for the Post-Launch Radiometric calibration

 

 


Need for a Global, Integrated Network of Calibration Sites

 

• User communities increasingly rely on information products from multiple satellite sensors.

• Better calibration can result from more post launch calibration, involving standardized measurement protocols, instrumentation, and processing

• Field measurements remain a resource intensive activity.

• Less expensive, complementary approaches can provide more frequent calibration updates and enable the monitoring of sensor performance trends, even without surface measurements.

• Future global monitoring systems, using increasingly complex constellations of satellites with multiple sensors, such as the Global Earth Observation System of Systems (GEOSS), will amplify the need for this initiative.

 


Well-Established Radiometric Test Site Selection Criteria

 

• The site should have high spatial uniformity, relative to the pixel size, to minimize the effects of scaling radiometric data to the size of the entire test site. This is especially important for cross-calibration between instruments because it minimizes the effects of misregistration. The site should also be centered in an area large enough to accommodate the sampling of a large number of pixels and to minimize atmospheric adjacency effects due to light scattered from outside the target region.

• The site should have a surface reflectance greater than 0.3 in order to provide higher signal-to-noise ratio (SNR) and reduce uncertainties due to the atmospheric path radiance.

• The surface of the site should have flat spectral reflectance. This is especially important if the multiple instruments involved in cross-calibration have spectral bands with different response profiles.

• The surface properties of the site (reflectance, BRDF, spectral) should be temporally invariant. Otherwise, adequate accuracy would be obtained only if these properties were measured for every calibration. This implies that the site should have little or no vegetation and minimal chance for snow.

• The surface of the site should be horizontal and have nearly Lambertian reflectance to minimize uncertainties due to differences in solar illumination and observation geometries. It should also be flat to minimize slope-aspect effects.

• The site should be located at high altitude (to minimize aerosol loading and the uncertainties due to unknown vertical distribution of aerosols), far from the ocean (to minimize the influence of atmospheric water vapor), and far from urban and industrial areas (to minimize anthropogenic aerosols).

• The site should be in an arid region to minimize the probability of cloudy weather and precipitation that could change the soil moisture and hence the surface reflectance. The low probability of cloud coverage also increases the probability of the satellite instruments imaging the test site at the time of overpass.

 

Sites per Country

Distribution of World-Wide Radiometric Sites

 

Review of Radiometric Calibration Methods

 

The following calibration methods are used: Absolute, Pseudo-Invariant, and Cross-Calibration.

Absolute Calibration (A)

An absolute calibration site is a location where in situ ground measurements of key physical parameters are acquired by calibrated ground instruments, allowing a detailed comparison to instruments of an orbiting sensor. Although many satellite sensors are launched with well-characterized calibration sources, these sources change with time after launch. Absolute calibration of an orbiting sensor requires well-characterized ground targets. This approach attempts to measure the radiance seen by the sensor in orbit by in situ measurement of an absolute calibration site combined with a radiative transfer computation to predict the effects of transmission through the atmosphere. An ideal absolute calibration site is exceptionally homogeneous over the spatial scale of an instrument pixel. Systematic surface measurements are made to characterize the site during instrument flyover. This limits calibration work to days when the satellite is collecting data over the test site. The maximum number of possible calibrations for a given test site during a given year for Landsat, which has a 16-day repeat cycle, is 22. Absolute calibrations are also dependent on favorable weather conditions, and the need for ground truth limits possible sites to those that are reasonably accessible. The difficulty of using absolute calibration sites is balanced by the relatively high accuracy of calibration that can be obtained.

Pseudo-Invariant Calibration (I)

A pseudo-invariant site is a location on the Earth’s surface that is very stable both temporally and spatially over long periods of time and over significant spatial extent. These sites are typically located in desert regions that receive little rainfall and have few surface features. The pseudo-invariant calibration sites are usually used to monitor the stability of the sensor over a long period of time. For sensors with no onboard calibrators, these sites provide an excellent source of information to study the sensor’s behavior as a function of time.

Cross-Calibration (X)

A cross-calibration site is a location on the Earth’s surface that contains large homogenous regions that are viewable by two or more satellite sensors within a relatively short time period. Due to changes in sun angle and atmospheric content and condition, it is recommended that the time separation between images be measured in minutes, not hours, for this method to be effective. Also, with orbital differences between any two given sensors, some cross-calibration sites will not work as well as others. Users are advised to use caution and discernment in choosing sites that will work best for the sensors they are considering. This cross-calibration approach involves comparison of near-simultaneous observations of the Earth’s surface based on image statistics from large common areas within imagery from each of the sensors being compared. If the calibration of one of the sensors is well understood, it can be used as a baseline against which the other sensors are assessed. For sensors that have not been well characterized, this method allows the most direct method for quantifying the differences in response between sensors, although not necessarily in terms of an absolute physical standard. Any pair of images with relatively large, spatially homogenous regions can be cross-calibrated. Many of the catalog sites will work well with this method of calibration. Table 1 provides the provisional radiometry calibration site categorizations.

 

Proposed Future Plans

• Refine the selection of recommended primary sites

• Gather complete site characterization data and information

• Define core measurements (eg. Instruments)

• Develop protocols and fund pilot projects

• Create a “calnet” or “landnet”

• Agenciest should acquire and archive imagery of all primary sites

• Develop online calibration data access infrastructure

• Create tools to identify the potential co-incident image pairs

• Extend the list to include snow fields, begetation targets and water targets

• Integrate the catalog into the CEOS EO Cal/Val portal

• Establish traceability chain for primary site data

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Page Last Modified: January 8, 2013