Gonio Radiometric Spectrometer System (GRASS)
Field instruments, such as GRASS, provide the in-situ data to validate the satellite measurements. Due to their macro- and microphysical construction, natural targets reflect light with different intensities in different directions. Depending on the viewing angle the surface can appear brighter or darker - as seen for example on a mown lawn or a forest canopy. Every surface on Earth has a specific Bi-Directional Reflectance Distribution Function (BRDF) property that describes the relationship between the reflected radiance from the surface and the directions of irradiance and the viewing angle.
A crucial parameter required to calculate the Earth's Radiation budget is the surface albedo, which is defined as the directional integration, over all sun-view geometries, of the fraction of incident radiation that is reflected by a surface. Satellite instruments do not directly measure surface albedo. It must be inferred through a series of manipulations of the raw data. If the BRDF is not considered it can lead to large errors in the retrieved albedo. Since albedo is used in climate models, it is vital to reduce these errors by developing our understanding of the angular spectral reflectance of terrestrial surfaces. The solution is a field goniometer, such as GRASS (Figure 1), which can measure angularly resolved spectral reflectance/radiance and therefore provide data for the development and validation of models. This is essential to improve our understanding of the planet, not only for climate change but also more immediate activities such as agriculture, pollution and disaster mitigation.
The GRASS instrument will provide the structure and optics to collect the radiation, which will then connect to a variety of spectroradiometers. GRASS has been designed to measure the Earth's reflected sunlight over half a hemisphere at 30-degree intervals i.e. 0°, 30°, 60°, 90°, 120°, 150°, 180°, on a series of seven arms. GRASS has 36 cameras (Figure 2), (five on each arm and one at nadir) each consisting of an entrance optic and an optical fibre (Figure 3). The fibres feed to a set of multiplexers, which give one optical output that can be coupled to a spectrometer and thus give traceability to SI units through NPL.
GRASS has several advantageous design features. The hemispherical structure is designed with a set of legs that are adjustable, so for example a spectral measurement can be taken of the top of a vegetation canopy. The angular sampling is also programmable, meaning a predefined set of measurements can be taken in the field and a smaller sample of measurements at a few angles can be taken without moving the instrument. Another design feature is that the lenses on the fibres can be replaced with a cosine diffuser, and orientation changed, such that it gives GRASS the ability to measure both radiance and irradiance at multiple angles. Grass was first deployed in June 2006 at the NCAVEO field Experiment. In the future, the GRASS instrument will be used by UK researchers to provide data for comparison with satellite information to develop a better understanding of the anisotropy of natural targets.
For more information, please contact Nigel Fox