Remote sensing of the earth's surface features involves the measurement of reflected solar radiation and the interpretation of the data in biophysical terms. Reflected radiation is a function of the surface properties and incident solar irradiance. The amount of radiation reflected from a surface is compared to the amount of solar radiation received at the surface as a means of comparing information from different times of day as well as for different days of the year. Thus, it is imperative to calibrate the instruments used to measure the incoming and reflected radiation

Blad, Blaine L.

Hays, Cynthia J.

Mesarch, Mark A.

Walter-Shea, Elizabeth A.

English

NASA Technical Reports Server

NASA-CR-192198
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CALIBRATION OF FIELD REFERENCE PANEL AND RADIOMETERS f--- /J
USED IN FIFE 1989
Semi-annual report for period
February i - September 30, 1990
NASA Grant NAG5-894
by
Blaine L. Blad, Elizabeth A. Walter-Shea, Cynthia J. Hays and
Mark A. Mesarch
Department of Agricultural Meteorology
Institute of Agriculture and Natural Resources
University of Nebraska-Lincoln
Lincoln, Nebraska 68583-0728
AgMet Progress Report 90-3
October 1990
(NASA-CR-192198) CALIBRATION OF
FIELD REFERENCE PANEL AND
RADIOMETERS USED IN FIFE 1989
Serni_,nnu,_l R_;poFt, 1 Feb. - 30 Sep.
1990 (kebraska Univ.) 15 p
N93-20045
Unclas
G3/92 0145800
https://ntrs.nasa.gov/search.jsp?R=19930010856 2020-03-17T08:22:10+00:00Z
INTRODUCTION
Remotesensing of the earth's surface features involves the measurement of
reflected solar radiation and the interpretation of the data in biophysical
terms. Reflected radiation is a function of the surface properties and incident
solar irradiance. The amount of radiation reflected from a surface is compared
to the amount of solar radiation received at the surface as a means of comparing
information from different times of day as well as for different days of the
year. Thus, it is imperative to calibrate the instruments used to measure the
incoming and reflected radiation.
CALIBRATION OF FIELD REFERENCE PANEL
A good estimate of the amount of incoming radiation is important in order
to calculate the reflective properties of a surface. Highly reflective reference
surfaces are used to estimate the incoming radiation. These surfaces will have
reflective properties dependent on the solar zenith angle (regardless of their
construction). It is important to periodically characterize reflective reference
surface properties as a function of solar zenith angle since dust, use and
surface deterioration will change the reflective property. Once calibrated,
reference surfaces can be used in the field to estimate the amount of incoming
radiation with the assurance of valid reflectance calculations.
Field facilities were developed for the calibration of remote sensing field
reference surfaces to be used in reflectance calculations. A goniometer was
constructed based on an instrument designed and used by Ray D. Jackson (USDA
Water Conservation Laboratory, Phoenix, Arizona) to develop a field panel
calibration technique as outlined in Jackson et al. (1987). Instruments used to
measure surface reflected radiation are used to calibrate the reference surface.
An advantage to the use of these facilities is that the irradiance and instrument
characteristics are the same as those under field situations. A disadvantage is
that clear sky conditions are essential.
A 2'x 2' halon panel (Labsphere, Inc.) was used as a standard. Spectral
directional-hemispherical reflectance factors, p_(8°,2_), of the panel were
characterized by the manufacturer and compared to a halon powder panel pressed
according to NBS standards by R.D. Jackson in April, 1989. The field reference
surface used in FIFE is a 4'x 4' halon panel (Lapsphere, Inc.). The calibration
procedure uses the fact that the bidirectional reflectance factors p_(Ss;Sv) are
related to directional-hemispherical reflectance factors (a known quantity in the
case of the pressed halon panel manufactured according to NBS standards and for
the 2'x2' halon standard measured by the manufacturer). Since the instrument
voltage response for a given waveband is proportional to the bidirectional
reflectance factor we can say that the ratio of the instrument voltage responses
measured over the field reference panel (Vf,k) and the standard surface (V,0k) is
equal to the ratio of the actual bidirectional reflectance factors measured over
the field reference (Pz,k) and the standard surface (P,,k) for each waveband, k,
and at nadir for a particular solar zenith angle (0s). That is,
Vf,k(8) Pf,k (0)
Vs,k(8) Ps,k (8)
The goniometer permits the incident radiation to strike the panels at solar
incidence angles ranging from 15-75 ° at approximately i0 ° intervals in a period
scanning I0 minutes for each panel. Using the relationship between the ratios
we can solve for the field reflectance panel bidirectional reflectance factors
at nadir as a function of solar zenith angle for each waveband k, pf,k(Si;0°).
The field reference surface was calibrated in September (DOY 261) 1989 using MMR
(S/N 114) and in March (DOY 261) 1990 using a SE590 (S/N 1571).
Reflectance factors as functions of solar incidence angle and wavelength
as measuredwith the MMRare given in Table i and as measuredwith the SE590are
given in Figure i.
Table i
4'x4' moldedhalon field reference panel
reflectance factors as functions of solar incidence angle and wavelength
as measured_ith the MMR
Solar
Zenith
Angle Bandl Band2 Band3 Band4 Band5 Band6 Band7
15 1.061 1.062 1.061 1.061 1.052 1.036 1.023
20 1.058 1.057 1.057 1.057 1.049 1.034 1.022
25 1.052 1.051 1.051 1.051 1.044 1.032 1.019
30 1.045 1.043 1.044 1.043 1.038 1.027 1.013
35 1.035 1.033 1.034 1.033 1.030 1.022 1.006
40 1.023 1.022 1.023 1.021 1.020 1.014 0.997
45 1.010 1.009 1.010 1.008 1.008 1.005 0.986
50 0.995 0.995 0.995 0.992 0.994 0.994 0.973
55 0.978 0.979 0.978 0.976 0.979 0.981 0.957
60 0.961 0.961 0.960 0.957 0.962 0.967 0.941
65 0.941 0.941 0.940 0.937 0.943 0.950 0.922
70 0.921 0.919 0.919 0.916 0.922 0.931 0.902
75 0.899 0.896 0.895 0.894 0.900 0.910 0.880
CALIBRATION OF THE RADIOMETERS
Temperature Sensitivity of the MMR and the SE590. The output of the MMR
viewing a constant source can vary depending on the temperature of the instrument
(Jackson and Robinson, 1985; Markham et al., 1988). The lead sulfide detectors,
used in bands 5-7, are particularly vulnerable. The output variation as a
function of ambient temperature of the SE590 is unknown. The sensitivities of
the MMR and SE590 were quantified by measuring the instrument output when viewing
a constant source as the ambient temperature was varied.
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Output was monitored during FIFE'89 using the Kansas State University
Evapotranspiration Laboratory environmental chamber at temperature settings
ranging from 16 to 43.5°C at 5 to 7°C increments. The MMR and SE590 were placed
in the chamber for several hours at each temperature setting so that the
instruments could equilibrate to the ambient temperature before viewing the
constant light source. A 30 cm integrating sphere illuminated with one and two
lamp settings was used as a source. The different lamp settings were used to
check the response at different radiant energy levels. The MMR response varied
considerably in wavebands 5, 6 and 7 (Figure 2). Temperature sensitivity
coefficients have been calculated by B. Markham according to the procedures
outlined by Markham et al (1988). Error without correcting (given the range of
temperatures in the calibration study) could be as great as 60 Wm -2 in band 5.
Correction yields fairly consistent radiance measurements regardless of ambient
temperature. (NOTE: these results are not an independent study of the accuracy
of the correction, i.e., the correction was performed on the data from which
correction coefficients were derived). The SE590 response varied beyond 0.850
_m (Figure 3). No other SE590 units were tested so that it is uncertain if the
response is instrument specific or whether all units are temperature sensitive.
Coefficients for the SE590 have not been computed, but results indicate that
using measurements at wavelengths of 1.0 _m may result in discrepancies of
approximately 50 W m °2 if the instrument temperature varies from 16 to 43.5°C.
Estimation of Reflectance Measurement Errors. The possible errors
associated in using a different instrument for the target than was used for the
estimation of incoming radiation were quantified. The procedure involved i)
calculating canopy reflectance factors in which the reflected radiation from the
reference panel and target were measured with the same instrument (MMR S/N 114),
2) calculating canopy reflectance factors as the ratio of reflected radiation
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Figure 3. Output from a 30 cm integrating sphere as measured with a Spectron
Engineering SE590 Spectroradiometer (S/N 1571) with low and high radiance
settings as a function of ambient temperature: a) uncorrected low radiance
output, and b) uncorrected high radiance output.
from the canopy (MMRS/N 114) to the reflected radiation from the samereference
panel as in #i measured by a different instrument (MMRS/N iii), and 3)
calculating the root meansquare error (RMSE)using measurement#i as the "true"
value. This was possible with the measurementprocedure used in 1989. MMRS/N
iii wasmountedover the reference panel and collected data every minute during
the helicopter flights. MMRS/N 114, mounted on a portable mast, collected
canopyreflected radiation data andperiodically took measurementsover the panel
from which the incident radiation was interpolated following the method of Ranson
et al., 1985.
Data collected on DOY220 1989 was used. All instruments performed well
with the highest "error" being approximately 1.5% (absolute) in wavebands5 and
6 (Table 2).
Table 2
Root MeanSquare Error from Comparisonof Reflectance Factors
Computedby Ratioing CanopyReflectance output (MMRS/N 114) with panel output
from two different MMR's(S/N 114 and S/N iii)
Waveband RMSE(%)
i 0.06
2 0.04
3 0.02
4 0.46
5 1.27
6 1.48
7 0.38
Errors Associated with Leaf Optical Property Measurements. The
transmittance of neutral density filters (Merle Groltt, Inc.) of known
transmittance were measured with the NMLRmounted to a LI-COR Integrating Sphere.
Neutral density filter transmittance values ranged from i to 56.5%, a value
range typically encountered in leaf optical measurements. The RMSE was used to
8
quantify the departure of the measured transmittance from the "true"
transmittance value of the neutral density filters. (The filters were
characterized using optics facilities at NASAGoddard in the fall of 1989). The
NMLRhas both the LI-COR standard light source as well as a specially adapted
lamp which has a narrower beam. The beamwas narrowed for use on the narrower
grass leaves. Typically the leaves do not fill the entire sample port of the
integrating sphere so that with the standard light source, an area larger than
the leaf blade would be illuminated whereaswith the narrowed beamof the altered
light source only a portion of the leaf is illuminated.
Errors associated with leaf optical properties are approximately 0.5%
within wavebandsi through 4 while the errors are generally higher in the lead
sulfide detector bands (bands 5-7), in particular, band 7 (Table 3). This error
could be due to the temperature sensitivity of the detector (as in the MMR).
However, the leaf radiometer is not equipped with a thermistor to indicate the
temperature of the instrument which would permit correction of the response as
is done with the MMR.
Table 3
Comparison of Measured Neutral Density Filter Transmittances with
Calibrated Neutral Density Filter Values
RMSE
Standard
Waveband Light Source
(%)
I 0.5
2 0.4
3 0.4
4 0.5
5 1.0
6 0.6
7 2.9
Narrow-beam
Li_t Source
(%)
08
05
05
05
25
14
68
Thermal Instrumentation Calibration. The thermal channel of MMRs S/N iii,
114 and 117 were calibrated at the USDA-ARS Water Conservation Laboratory in
Phoenix, Arizona in early July 1989 against a variable blackbody source at a
range of ambient temperatures. Correction coefficients were calculated by using
the procedure as outlined in Markham et al. (1988). Temperatures calculated
using the data from the calibration dataset and the derived coefficients at all
ambient temperatures (i.e., not an independent dataset) show that there is
scatter on the average of 0.6°C around the i:i line (Figure 4 a,b). Comparison
of the results between MMR S/N 117, mounted on the helicopter, and S/N 114,
mounted on the portable mast, indicate an average error of approximately 0.8°C
when comparing the target temperature measured by the instruments of the same
target (Figure 4c).
Everest Transducers were calibrated following the same procedures as with
the MMR. A linear regression was applied to the data so that once corrected the
measured values differed from the source value at most approximately 2°C at any
particular ambient condition (Figure 5). Overall, instruments measured within
I°C of the source output after correction (Figure 6). Without the linear
regression correction, measured values can differ from the source by
approximately 4°C at any ambient temperature and overall by 2°C from the source
output.
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Figure 4. Barnes Modular Multiband Radiometer (MMR)
thermal channel (band 8) response for a range of target
temperatures under a range of ambient temperatures for
a) S/N 114, b) S/N 117, and c) a comparison of response
between S/N 114 and S/N 117.
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LITERATURE CITED
Jackson, R.D., Moran, M.S., Slater, P.N. and Biggar, S.F. 1987. Field
calibration of reference reflectance panels. Remote Sens. Environ., 22: 145-158.
Markham, B.L., Wood, F.M. and Ahmad, S.P. 1988. Radiometric calibration of the
reflective bands of NS001 Thematic Mapper Simulator (TMS) and Modular
Multispectral Radiometers (MMR). SPIE Recent Advances in Sensors, Radiometers
and Data Processing for Remote Sensing, 924:96-108.
Ranson, K.J., Daughtry, C.S.T., Biehl, L.L. and Bauer, M.E. 1985. Sun-view
angle effects on reflectance factors of corn canopies. Remote Sens. Environ.,
18:147-161.
14


Calibration of field reference panel and radiometers used in FIFE 1989

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