Abstract:
Above-water RRS measurements were taken using an Analytical Spectral Device (ASD) FieldSpec Spectroradiometer during October 2015. In situ measurements of remote sensing reflectance (RRS) provide a direct comparison tool for the calibration and validation of satellite radiometric sensors. In addition, the spectral shape of the reflectance spectrum can be used to quantitatively describe the bio-optical properties of the water, including estimates of chlorophyll-a, absorption by phytoplankton/detritus/CDOM, and backscattering.
Suggested Citation:
Ryan Vandermeulen, Robert Arnone. 2017. Above water radiometry using the Analytical Spectral Device (ASD) FieldSpec Spectroradiometer to characterize ocean color, October 2015. Distributed by: GRIIDC, Harte Research Institute, Texas A&M University–Corpus Christi. doi:10.7266/N7B56H2M
Purpose:
In situ measurements of remote sensing reflectance (RRS) provide a direct comparison tool for the calibration and validation of satellite radiometric sensors. In addition, the spectral shape of the reflectance spectrum can be used to quantitatively describe the bio-optical properties of the water, including estimates of chlorophyll-a, absorption by phytoplankton/detritus/CDOM, and backscattering.
Data Parameters and Units:
Station name, Date (MM/DD/YYYY), Time (GMT), Latitude (decimal degrees), Longitude (decimal degrees), Remote sensing reflectance (Rrs, units: 1/sr, Wavelengths: 325-999 nm)
Methods:
Above-water remote sensing reflectance measurements are taken using multiple Analytical Spectral Device (ASD) FieldSpec(TM) Spectroradiometer. These instruments enable the derivation of above-water Rrs using un-calibrated radiance relative to reflectance plaque measurements. The reflectance plaque is a 10% grey card with a known bi-directional reflectance function (BRDF), and is assumed to be a semi-Lambertian surface. The field collection protocols are described as follows. Using a 10 degrees fore-optic attachment, five consecutive radiometric spectrum (S) measurements are taken of each of the following targets: Grey card (Sg), water (Ssfc), and sky (Ssky). Prior to measuring each individual target, the ASD instrument is manually re-optimized (i.e. integration time of sensor was changed based on relative brightness of the target AND new dark counts are taken to correct for instrument noise). Integration times typically range from 68 - 4352 milliseconds. Most measurements are taken from the stern of the ship. The exact location of sampling (Port v. Starboard) is dependent on the orientation of the ship relative to the sun, and determined in order to eliminate the influence of shadowing from the vessel. Regardless of location, the optical sensor zenith angles (\theta) for the water (\theta sfc), grey card (\theta g), and sky (\theta sky) measurements are 135 degrees, 135 degrees, and 45 degrees, respectively. The relative azimuth angle of the sensor to the sun (\phi) is 90 degrees, but may be adjusted up to 135 degrees, depending on the location and projection of sea foam derived from the ship. The processing protocols for deriving Rrs from above water radiometry follow method 2 of chapter 3, Above-Water Radiance and Remote Sensing Reflectance Measurement and Analysis Protocols from NASA/TM-2003-21621/Rev-Vol III, Ocean Optics Protocols For Satellite Ocean Color Sensor Validation, Revision 4, Volume III: Radiometric Measurements and Data Analysis Protocols. To compute the reflectance, the sensor response signal, S, is obtained from n readings from each target and normalized to the same consistent integration time (1 sec). The computed Rrs should be "black" about 750 nm. If not zero, then it assumed that the reflected skylight term (Ssky) was not estimated correctly. Following the "quick and easy" algorithm of Carder and Steward (1985), it is further assumed that any error in the skylight reflection term is white (not wavelength dependent) and one may simply subtract the computed Rrs (750) from the entire spectrum. In practice, this may lead to negative radiance values Rrs near 750 nm are actually lower then Rrs (750). Therefore, the processing subtracts the smallest Rrs in the range from 700 nm to 825 nm.
Instruments:
References: Carder, K. L. and R. G. Steward. (1985) "A remote-sensing reflectance model of a red-tide dinoflagellate off West Florida", Limnol. Oceanogr., Vol. 30, pp 286-298. Lee, Zhongping, Kendell L. Carder, Robert A. Arnone. (2002) "Deriving inherent optical properties from water color: a multiband quasi-analytical algorithm for optically deep waters", Applied Optics, Vol. 41, No. 27, pp 5755-5772. Mueller, James L., Giulietta S. Fargion and Charles R. McClain, Editors, "Ocean Optics Protocols For Satellite Ocean Color Sensor Validation, Revision 4, Volume III: Radiometric Measurements and Data Analysis Protocols", NASA Technical Memorandum, NASA/TM-2003-21621/Rev-VolIII.