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Gonio Radiometric Spectrometer System (GRASS)

The instrument has been designed to be easily and quickly assembled in remote situations, be robust and able to be transported by “estate car”. 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.
Six of these arms have five collecting optics (referred to as a “camera”), and the seventh will have six, to be able to capture the nadir measurement, as well. This results in 36 cameras / measurement angles, within the half of the hemisphere, which measure an area approximately 25 cm in diameter. Each camera consists of a collimating lens and an optical fiber. The fibers from the entrance optics feed to a series of multiplexers to give one optical output that can be coupled to a spectrometer. To be able to take measurements at all chosen geometries, the arms of the Goniometer have been designed so that they can rotate on the circular base of the structure allowing the forward  and backward scattered radiation to be measured.

The positions of the entrance optics on the arms are also designed to be moveable so that effectively any five-zenith angles (up to 60°) can be chosen to be captured during each measurement sequence. Another design feature of GRASS is that the lenses on the end of each of the fibers can be removed, and replaced with a cosine diffuser, and the orientation of the viewing optic rotated such that the entrance optic can then measure the down-welling irradiance. This means that the instrument can measure both the radiance and irradiance at concurrent angles. An ASD FieldSpec Pro was used during the campaign for acquiring the spectral data from GRASS.

Measurement aims: To conduct a series of multi-angular spectral measurements of the surface, perform measurements at different solar zenith angles, perform measurements over the same surface on sequential days to determine the temporal stability, and perform diffuse irradiance measurements.

Resolution: see ASD radiometer Set-up time: about 2 hours to set-up, and similarly 2 hours to dismantle at the end of the day. The instrument was left assembled on site overnight to reduce construction time.

Transportation: The instrument breaks down into three boxes.

  • 2 x (640 x 1150 x 780) mm (WxLxH) Weight - of the order of 90kg each
  • 1 x (425 x 500 x 1235) mm (WxLxH) Weight - of the order of 55kg

Power Requirements: To run the instrument requires some power – this was achieved through the use of the Petrol Generator.

References

[1] H. M. Pegrum a, *, N. P. Fox a, E. J. Miltonb, M. Chapmana. DEVELOPMENT OF THE GONIO RADIOMETRIC SPECTROMETER SYSTEM TO CONDUCT MULTI-ANGULAR MEASUREMENTS OF TERRESTRIAL SURFACES - PDF
 

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