Guest Editors Elhadi Adam, Craig Coburn, and Anthony Campbell introduce the Special Section on the 50th Anniversary of Landsat—Current Achievement and Future Directions.
The Radiometric Calibration Test Site (RadCaTS) is a suite of commercial and custom instruments used to make measurements of the surface reflectance and atmosphere throughout the day at Railroad Valley, Nevada. It was developed in response to the need for daily radiometric calibration data for the vast array of Earth-observing sensors on orbit, which is continuously increasing as more nations and private companies launch individual environmental satellites as well as large constellations. The current suite of instruments at RadCaTS includes five ground-viewing radiometers (GVRs), four of which view the surface in a nadir-viewing configuration. Many sensors such as those on Landsat-7 and Landsat-8 view Railroad Valley within 3° of nadir, while others such as those on Sentinel-2A and -2B, RapidEye, Aqua, Suomi NPP, and Terra can view Railroad Valley at off-nadir angles. Past efforts have shown that the surface bidirectional reflectance distribution function (BRDF) has minimal impact on vicarious calibration uncertainties for views <10°, but the desire to use larger view angles has prompted the effort to develop a BRDF correction for data from RadCaTS. The current work investigates the application of Railroad Valley BRDF data derived from a BRF camera developed at the University of Arizona in the 1990s (but is no longer in use) to the current RadCaTS surface reflectance measurements. Also investigated are early results from directional reflectance studies using a mobile spectro-goniometer system during a round-robin field campaign in 2018. This work describes the preliminary results, the effects on current measurements, and the approach for future measurements.
Attempts to use pseudoinvariant calibration sites (PICS) for establishing absolute radiometric calibration of Earth observation (EO) satellites requires high-quality information about the nature of the bidirectional reflectance distribution function (BRDF) of the surfaces used for these calibrations. Past studies have shown that the PICS method is useful for evaluating the trend of sensors over time or for the intercalibration of sensors. The PICS method was not considered until recently for deriving absolute radiometric calibration. This paper presents BRDF data collected by a high-performance portable goniometer system to develop a temporal BRDF model for the Algodones Dunes in California. By sampling the BRDF of the sand surface at similar solar zenith angles to those normally encountered by EO satellites, additional information on the changing nature of the surface can improve models used to provide absolute radiometric correction. The results demonstrated that the BRDF of a reasonably simple sand surface was complex with changes in anisotropy taking place in response to changing solar zenith angles. For the majority of observation and illumination angles, the spectral reflectance anisotropy observed varied between 1% and 5% in patterns that repeat around solar noon.
Several sites from around the world are being used operationally and are suitable for vicarious calibration of space-borne imaging platforms. However, due to the proximity of these sites (e.g., Libya 4), a rigorous characterization of the landscape is not feasible, limiting their utility for sensor intercalibration efforts. Due to its accessibility and similarities to Libya 4, the Algodones Sand Dunes System in California, USA, was identified as a potentially attractive intercalibration site for space-borne, reflective instruments such as Landsat. In March 2015, a 4-day field campaign was conducted to develop an initial characterization of Algodones with a primary goal of assessing its intercalibration potential. Five organizations from the US and Canada collaborated to collect both active and passive airborne image data, spatial and temporal measurements of spectral bidirectional reflectance distribution function, and in-situ sand samples from several locations across the Algodones system. The collection activities conducted to support the campaign goal is summarized, including a summary of all instrumentation used, the data collected, and the experiments performed in an effort to characterize the Algodones site.
The use of Pseudo Invariant Calibration Sites (PICS) for establishing the radiometric trending of optical remote sensing systems has a long history of successful implementation. Past studies have shown that the PICS method is useful for evaluating the trend of sensors over time or cross-calibration of sensors but was not considered until recently for deriving absolute calibration. Current interest in using this approach to establish absolute radiometric calibration stems from recent research that indicates that with empirically derived models of the surface properties and careful atmospheric characterisation Top of Atmosphere (TOA) reflectance values can be predicted and used for absolute sensor radiometric calibration. Critical to the continued development of this approach is the accurate characterization of the Bidirectional Reflectance Distribution Function (BRDF) of PICS sites. This paper presents BRDF data collected by a high-performance portable goniometer system in order to develop a temporal BRDF model for the Algodones Dunes in California. The results demonstrated that the BRDF of a reasonably simple sand surface was complex with changes in anisotropy taking place in response to changing solar zenith angles. The nature of these complex interactions would present challenges to future model development.
Charles Bachmann, Andrei Abelev, Marcos Montes, William Philpot, Deric Gray, Katarina Doctor, Robert Fusina, Gordon Mattis, Wei Chen, Scott Noble, Craig Coburn, Tom Corl, Lawrence Slomer, C. Reid Nichols, Elena van Roggen, Roy Hughes, Stephen Carr, Sergey Kharabash, Andrew Brady, Michael Vermillion
This paper describes a portable hyperspectral goniometer system for measurement of hemispherical conical reflectance factor (HCRF) data for terrestrial applications, especially in the coastal zone. This system, the Goniometer for Portable Hyperspectral Earth Reflectance (GOPHER), consists of a computer-controlled Spectra Vista Corporation HR-1024 full-range spectrometer mounted on a rotating arc and track assembly, allowing complete coverage in zenith and azimuth of a full hemisphere for recording HCRF. The control software allows customized scan patterns to be quickly modified in the field, providing for flexibility in recording HCRF and the opposition effect with varying grid sizes and scan ranges in both azimuth and zenith directions. The spectrometer track can be raised and lowered on a mast to accommodate variations in terrain and land cover. To minimize the effect of variations in illumination during GOPHER scan cycles, a dual-spectrometer approach has been adapted to link records of irradiance recorded by a second spectrometer during the GOPHER HCRF scan cycle. Examples of field data illustrate the utility of the instrument for coastal studies.
Ground reference data are important for understanding and characterizing angular effects on the images acquired by
satellite sensors with off-nadir capability. However, very few studies have considered image-based soil reference data for
that purpose. Compared to non-imaging instruments, imaging spectrometers can provide detailed information to
investigate the influence of spatial components on the bidirectional reflectance distribution function (BRDF) of a mixed
target. This research reported in this paper investigated soil spectral reflectance changes as a function of surface
roughness, scene components and viewing geometries, as well as wavelength. Soil spectral reflectance is of particular
interest because it is an essential factor in interpreting the angular effects on images of vegetation canopies. BRDF data
of both rough and smooth soil surfaces were acquired in the laboratory at 30° illumination angle using a Specim V10E
imaging spectrometer mounted on the University of Lethbridge Goniometer System version 2.5 (ULGS-2.5).
The BRDF results showed that the BRDF of the smooth soil surface was dominated by illuminated pixels, whereas the
shaded pixels were a larger component of the BRDF of the rough surface. In the blue, green, red, and near-infrared
(NIR), greater BRDF variation was observed for the rough than for the smooth soil surface. For both soil surface
roughness categories, the BRDF exhibited a greater range of values in the NIR than in the blue, green, or red. The
imaging approach allows the characterization of the impact of spatial components on soil BRDF and leads to an
improved understanding of soil reflectance compared to non-imaging BRDF approaches. The imaging spectrometer is an
important sensor for BRDF investigations where the effects of individual spatial components need to be identified.
In most forestry remote sensing applications in steep terrain, simple photometric and empirical corrections are confounded as a result of variable stand and species structure with terrain and the anisotropic reflective properties of vegetated surfaces. To address these problems, we test two new topographic correction approaches based on Sun-Canopy-Sensor (SCS) geometry. SCS is more appropriate than strictly terrain-based corrections in forested areas since it preserves the geotropic nature of trees (vertical growth with respect to the geoid) regardless of terrain, view and illumination angles. The first SCS approach accounts for diffuse atmospheric irradiance based on the C-correction (SCS+C). Secondly, a new multiple forward mode (MFM) canopy reflectance model based correction (MFM-TOPO-COR) is introduced which normalizes topographically induced signal variance as a function of forest stand structure and sub-pixel scale components, while also maintaining proper SCS geometry. These two new techniques are compared to existing correction methods (cosine, c correction, Minnaert, statistical-empirical, SCS, and b correction) in a Rocky Mountain forest setting in western Canada. The ability of these eight correction methods are tested and compared for removing topographically induced variance and for improving the classification accuracy of a SPOT image over this sub-alpine and alpine forest area. The new MFM-TOPO-COR canopy reflectance model correction method is shown to provide the greatest improvement in classification accuracy within a species and stand density based class structure. The potential and limitations of this new approach are critically discussed.
Access to the requested content is limited to institutions that have purchased or subscribe to SPIE eBooks.
You are receiving this notice because your organization may not have SPIE eBooks access.*
*Shibboleth/Open Athens users─please
sign in
to access your institution's subscriptions.
To obtain this item, you may purchase the complete book in print or electronic format on
SPIE.org.
INSTITUTIONAL Select your institution to access the SPIE Digital Library.
PERSONAL Sign in with your SPIE account to access your personal subscriptions or to use specific features such as save to my library, sign up for alerts, save searches, etc.