IAPRS & SIS, Vol.34, Part 7, "Resource and Environmental Monitoring", Hyderabad, India,2002
DESIGN AND DEVELOPMENT OF A RATIO RADIOMETER FOR SNOW AND
GLACIAL STUDIES
S. S. Manjul, A. V. Kulkarni, and J. Srinivasulu
Space Applications Centre
Ahmedabad 380 015.
KEYWORDS: Radiometer, Design, Development, Spectral, Reflectance, Measurement, Snowlce, Indices.
ABSTRACT:
A portable, battery operated, field instrument, Ratio Radiometer, has been indigenously designed and developed for the
measurement of spectral reflectances of terrain surfaces for snow and glacial studies. The instrument has flexibility in incorporating
any two spectral bands in the range from 400 nm to 1800 nm. Hence, the instrument has capability to tune it for any other remote
sensing application also. The instrument simultaneously produces two outputs that can be stored in a data logger. Its software
computes the outputs corrected for offsets and temperatures. The software further computes ratio index and normalized difference
index. In present model, two spectral bands with central wavelengths 550 nm and 1625 have been incorporated on the basis of
standard available spectral reflectance data. At these wavelengths, the spectral reflectances are unique and different, which would
differentiate the snow packs with respect to other terrain surfaces. The instrument is being used extensively. It has been found that
the instrument produces useful results and it works satisfactorily under the typical environmental conditions of Himalayas.
1. INTRODUCTION
Optical remote sensing is concerned with measurement and
analysis of reflected/emitted spectral radiance from the earth
surfaces. Measurements of spectral reflectance under "in-situ"
conditions are useful for classification, identification of terrain
surfaces. Measurements are also useful for correlation and
analysis of spacebourne and air borne data for various remote
sensing applications.
For most of the natural surfaces, laboratory and field measured
spectral reflectance data is readily available. This data, after
careful study, is useful to derive the basic knowledge about the
potentially useful spectral bands and to eliminate the rest of the
redundant spectral information for the specific remote sensing
application. Subsequently, there would not be any need to
acquire voluminous ground truth data periodically. Further, the
simple techniques, such as ratio index, normalized difference
spectral index are applied to transform spectral information into
indicators. Thus, the use of limited number of potentially
important spectral bands saves time, money and efforts.
Design and developmental activity of a portable, battery
operated, field instrument, Ratio Radiometer, was undertaken
for the measurement of spectral reflectances of terrain surfaces
and to derive the indices for snow and glacial studies. The
instrument has flexibility in incorporating any two spectral
bands in the range from 400 nm to 1800 nm. Hence, the
instrument has capability to tune it for any other remote sensing
application also. The instrument is being used extensively for
field measurement for snow monitoring/ mapping under the
typical environmental conditions of Himalayas.
This paper presents some of the design and developmental
aspects of Ratio Radiometer (RR) and the results based on
measured field data with this instrument.
2. GENERAL BACKGROUND AND SPECTRAL
COMPONENT ANALYSIS RATIONALE
A spectral reflectance factor (R3)' is defined as the ratio of the
radiant flux or luminous flux reflected in the directions
delimited by the given cone by the target to that reflected into
the same geometry by a perfectly reflecting diffuser
(Lambertian) identically irradiated or illuminated (Bowker,
David L., et. al., 1985; Ross McCluney, 1994). The reflectance
for a standard diffuser is represented in following equation (1).
V target
Ra m reme NUR A, standard reference (1)
V standard reference
Where
R; is spectral reflectance factor.
R 3. standard reference 1$ reflectance factor of standard reference.
Viarget 1S instrument response when viewing target surface.
V standard reference 1S instrument response when viewing standard
reference surface.
For the measurement of reflectance, it is essential that
directions and the solid angles should be specified. Based on
the geometry, reflectance factor could be measured with nine
possible combination of solid angles as follows:
1. Bi-directional
Bi-hemispherical
Bi-conical
Hemispherical - conical
Hemispherical - directional
Conical - hemispherical
Conical - directional
Directional-conical
Directional-hemispherical
Y: 96-9 ta RSS
° At a specified wavelength, at a surface element, for the part of
the reflected radiation contained in a given cone with the apex
at the surface, and for incident radiation of a given composition
of spectral, polarization, and geometrical distribution.
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