SAM 400 Series
(Sun and Aureole Measurement)
Calibration Procedures
White Paper
With Exemplary Data from the SAM 300 Series
Dennis Villanucci (villa@visidyne.com)
Andrew LePage (lepage@visidyne.com)
John DeVore (devore@visidyne.com)
A.T. Stair (ats@visidyne.com)
Visidyne, Inc.
99 South Bedford St., Suite #107
Burlington, MA 01803
(781) 273-2820
(781) 272-1068 (fax)
April 2010
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1.0
Executive Summary
The fourth generation of the SAM sensors (the 400 series) is undergoing test and evaluation. The
functional design is the same as the previous (300) series;
however, there are some major changes:
1) The 400 series is weatherized, semi-autonomous and
portable, if desired. The sensor, shown in Figure 1, stands
about 1 m high and weighs about 18 kg. Not shown is the
small, weatherized container with data acquisition and data
reduction/processing computers and uninterruptable power
supply.
2) Another significant improvement is the incorporation of a
weather-resistant tracker that is more capable and improved
solar disk tracking software. More accurate tracking allows
the use of a smaller diameter solar beam dump in the
aureolegraph. This, in turn, allows a reduction in the data
gap between the limited aureole measured by the solar disk
camera and that measured by the aureolegraph, which
occurs for thin clouds in the small-angle portion of aureole
measurements.
Figure 1. Weatherized, portable, SAM
This paper describes the calibration procedures for the 400 400 series sensor, shown without
series using data examples from 300 series sensors (the ones container for data processing computers
that use a weather protection dome) Two of which are and uninterruptible power supply.
currently operating daily, one at the Goddard Space Flight
Center in Maryland under a NASA grant and the second at the DOE/ARM SGP site in Oklahoma
with both NASA and DOE/ASR support.
2.0
Calibration Overview
The following list and Table 1 provide an overview of SAM calibration:




Solar disk radiance and aureole (CSR) profiles
o At wavelength 670 +/- 5 nm (selectable)
Laboratory calibration uses a NIST-traceable source
Field verification of radiometric accuracy uses AERONET as the secondary standard
o Solar disk radiance:
 Error <1% for values from the maximum brightness (clear sky) down to
about 1/2 max (up to ODs <~0.6)
 Error ~2 to 3% for values from OD 0.6 to ~1.5 (DNI >~250 W/m2)
 Error increases to ~10% at higher ODs
o Aureole radiance ~5%
Aureole profile precision (gradient):
o Pixel-to-pixel standard deviation <0.5%
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o Consistently uniform response over 18 months operation
o Reproduced with a second SAM sensor, over 5 months operation
Table 1. Parameters measured for calibration (standard procedures)
Parameter
Solar Disk Camera
Aureole Camera
Radiometric accuracy
Measured end-to-end at 40 Responsivity measured
(NIST traceable sphere +/- 2%) ms (no ND filter)
end-to-end near image
(See above for accuracies)
Referenced to AERONET center at longest exposure
results (§3)
and multiple temperatures
Six exposures: 0.16 to 990 ms
Measured for all exposures
(CSR needs only the first two) 2 to 3.4% uncertainty relative to 40 ms
Linearity of responsivity
Measured and corrections applied (see §4)
Flat-field
Corrected
Corrected
See §5
Lab source (uniformity 0.5%)
See §5
See
§5,
10,
11,
12
Field variability
See §5, 10, 11, 12
Temperature range
Responsivity corrected from -6ºC to 44ºC (selectable)
Instrument internal scattering
Limits aureole angle data to Correction
based
on
(low OD, e.g. high disk radiance values > ~1/8 of the external solar occultation
radiance issue)
solar disk values
measurements
Neutral
density
characterization
Pixel scale (deg/pixel)
Digitization noise
3.0
filter Vendor specification to 1%
No ND used
Laboratory image measurement: 0.017 deg per pixel;
~3% uncertainty
~0.5%
SAM/AERONET Radiance Statistics
SAM sensors are calibrated for disk radiance
errors using nearby AERONET sun photometers.
Such comparisons are performed using
measurements of aerosols, which have very little
forward scattering, so that the difference in pixel
field-of-view (AERONET 1.2o versus SAM
0.017o) is not important. As illustrated in Figure
2, thousands of data points are used to achieve an
error of less than 0.7% relative to AERONET,
which because of the high standards to which
these instruments are calibrated represents a
Figure 2. Example of a SAM-AERONET data set
reliable secondary standard. These cross used for cross checking SAM calibration.
calibrations are generally applicable only for
very thin conditions (ODs < ~0.6) covering the DNI condition from the maximum to
approximately one half maximum.
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4.0E-04
are made at other exposure durations, and
for each of the cameras used
in the SAM sensor.
2.5E-04
2.0E-04
1.5E-04
1000 ms
Linear
1.0E-04
Poly. (1000 ms)
5.0E-05
Polynomial: y = 6.615E-12 x3 - 1.348E-9 x2 + 1.633E-6 x1
0.0E+00
0
50
100
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200
250
Figure 3. Example of SAM camera linearity
measurements.
Before Flat
Fielding
After Flat
Fielding
NIST Source
Flat to 0.5%
Figure 4. Example of aureolegraph imaging screen
data before and after flat fielding.
Up
Aureole FPA Trace Definitions
To check for uniformity of a cloud scene that is
to be used for extracting an aureole radiance
profile, a selected set of independent pixels are
examined, and the combined data must pass a
benchmark for “smoothness” to produce a level 2
data product. Depicted in Figure 5 are five
independent groups, labeled as shown. After the
“Up”, “Down”, “Left”, “Right”, and “Average”
data are accepted; the azimuthally averaged
values at each angle from the sun are used for the
profiles. The standard deviations of the Average
values for each reported angle are also reported
in the level 1 data product.
150
Pixel Value Above Bkgnd
Flat Fielding (Aureole Camera)
The solar aureole out to about 8º from the center of
the solar disk is imaged on SAM’s aureolegraph
imaging screen. Flat fielding of the aureolegraph is
performed by illuminating the imaging screen
using a NIST traceable sphere source, which is flat
to 0.5%. The aureolegraph camera pixel responses
are measured and corrected for non-uniformities,
as illustrated in Figure 4. The corrected responses
are flat to the accuracy of the source and nonuniformities are unobservable afterwards. This
assures that the shapes of the aureole profile
(gradients) are very accurate. This is critical to
both the CSR (out to ~3º) and to the retrieval of
particle size distributions.
6.0
brightness at one exposure and
temperature. Similar measurements
3.0E-04
2
The linearity of SAM cameras is measured using a
NIST-traceable extended source (i.e., a sphere).
Camera radiance measurements are recorded with
controlled changes in the exposure time and
camera temperature. The results are very linear
over the lower range of digital values and are fitted
using a polynomial function for the entire range as
illustrated in Figure 3. The resulting function is
then used for correction.
5.0
3.5E-04
Linearity (Typical Camera)
Radiance (W/cm /sr)
4.0
S/N 300
Linearity of camera response with scene
Left
Right
Av
er
ag
e
Down
Figure 5. Illustration of directions used to describe
aureole traces.
4
Typical Uniform Cloud Data Traces
8.0
Radial Average Statistics
Figure 7 shows the same case as in Figure 6, but
plots the average aureole radiance profile using
error bars to indicate the standard deviation of
the measurements averaged to produce the
profiles.
9.0
Uniform Cloud Example, a Year Later
(GSFC), FPA Long Term Stability
Figure 8 shows another example of aureole
radiance profiles. These data, taken a year later,
demonstrate the excellent stability of the focal
plane in maintaining the flat-field corrections,
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Average
17 JAN 09 ~ 18.5308 UT
Solar Elev ~ 28.0 deg
Right
Left
OD ~ 0.83
Up
Down
2
Radiance (W/cm /sr/um)
SAM #303 - GSFC
1.E+00
1.E-01
0.5
1.0
1.5
2.0
2.5
3.0
Angle From Sun Center (deg)
Figure 6. Example of up, down, left, right, and
average aureole profile data viewed through cirrus
with an optical depth of ~0.75.
1.E+01
SAM #303 - GSFC
17 JAN 09 ~ 18.5308 UT
OD ~ 0.83
2
Radiance (W/cm /sr/um)
Average
1.E+00
1.E-01
0.5
1
1.5
2
2.5
3
Angle From Sun Center (deg)
Figure 7. The average profile from Figure 6 with
error bars showing the standard deviation (onesigma).
1.E+01
SAM #303 - GSFC
Average
21 JAN 10 ~ 18.5381 UT
OD ~ 0.44
Right
Left
Up
Down
2
Figure 6 shows an example of aureole radiance
profiles measured by SAM through cirrus of
optical depth ~0.75. These data illustrate focal
plane array (FPA) uniformity and precision
(gradient) of aureole profile measurements. To
assess cloud uniformity, one can look at uncalibrated Total Sky Imager (TSI) pictures,
which are available in some cases. However,
there is no “field standard” for cloud uniformity
at small forward scattering angles. Therefore, the
only way to separate the aureolegraph focal
plane response “flatness” from the uniformity of
the clouds is by looking at repeatability and
statistics. In 18 months of taking data at GSFC
(~every 40 seconds during the day), there are at
least a thousand measurements that pass the
uniformity test, discussed in §6.0 above. In
general, perhaps only one in twenty data sets of
an apparently uniform cloud pass this test. Any
substantial change in the focal plane’s response
in the form of a gradient (shape) change, e.g.
involving multiple, correlated pixels, would be
apparent in subsequent uniform cloud data.
Since this does not appear, Q.E.D. the FPA has
remained uniform. More examples of uniform
clouds are shown in subsequent graphs.
1.E+01
Radiance (W/cm /sr/um)
7.0
1.E+00
1.E-01
0.5
1.0
1.5
2.0
2.5
3.0
Angle From Sun Center (deg)
Figure 8. Another example of up, down, left, right,
and average aureole profile data viewed through
cirrus with an optical depth of ~0.44.
5
Another SAM sensor suite (#300) was deployed
to the DOE ASR site in Oklahoma. The precision
and uniformity of the aureolegraph focal plane is
again illustrated (Figure 9). The excellent
performance is reproduced. This sensor was
calibrated and flat-fielded in the laboratory in
Sept 2008 and took similar data in Burlington,
MA from Oct 2008 to Mar 2009. It was then
deployed in early Nov 2009 and the data shown
were taken on Jan 20, 2010.
11.0
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Down
2
Radiance (W/cm /sr/um)
Left
1.E+00
0.5
1.0
1.5
2.0
2.5
3.0
Angle From Sun Center (deg)
Figure 9. Another example but from a second SAM
of up, down, left, right, and average aureole profile
data viewed through cirrus with an optical depth of
~0.70.
1.E+01
SAM #303 - GSFC
Average
17 JAN 09 ~ 18.5867 UT
OD ~ 0.96
Right
Left
Up
1.E+00
Down
1.E-01
1.E-02
1.5
2.5
3.5
4.5
5.5
6.5
7.5
Angle From Sun Center (deg)
Figure 10. Example of up, down, left, right, and
average profile data exhibiting cloud nonuniformity beyond 3º.
1.E+02
Gap: 0.44
to 0.61 deg
1.E+01
SAM #303 - GSFC
17 JAN 09 ~ 19.0753 UT
Solar Elev ~ 25.4 deg
OD ~ 2.48
Average
Right
Left
Up
Down
2
Radiance (W/cm /sr/um)
The data shown in Figure 11, taken later on the
same day as Figures 6 and 10, but when the
clouds were thicker, demonstrate the radiometric
accuracy of the solar disk camera relative to the
aureolegraph camera. The solar disk radiance is
reduced relative to the forward scattering and
allows the aureole to be measured by that camera
out to 0.44o. In this example the aureolegraph
data stops at ~0.6o whereas the 400 series will
Right
Up
0.5
12.0 Moderately Thick Cloud Example:
Radiometric Accuracy re two cameras
Average
20 JAN 10 ~ 20.1853 UT
OD ~ 0.70
1.E-01
Non-uniform Cloud Example
Cloud non-uniformity is detectable from
differences in the aureole shape in different
directions; therefore these data can be eliminated
from particle size distribution level 2 data
products. In the case shown in Figure 10
however, the CSR is approximately uniform out
to ~3o.
SAM #300 - ARM SGP
2
10.0 Uniform Cloud Aureole Example from
a Second SAM at the DOE/ARM SGP Site:
Reproducibility
1.E+01
Radiance (W/cm /sr/um)
resulting in precision profiles (shapes) as
measured by the five different groups of focal
plane pixels. To repeat, any substantial change
in the focal plane’s response in the form of
gradients (shape), e.g. multiple, correlated
pixels, would be apparent in subsequent uniform
cloud data such as this; Q.E.D the FPA has
remained uniform.
Solar
1.E+00
Disk
Camera
Aureole Camera
1.E-01
0.0
0.5
1.0
1.5
2.0
2.5
3.0
Angle From Sun Center (deg)
Figure 11. Example of up, down, left, right, and
average profile data for thicker cirrus where the data
gap has been nearly closed.
6
extend this to ~0.4o and overlap the solar disk camera measurements. The aureolegraph radiance
accuracy can be improved incrementally by comparison with AERONET measurements of the
forward scattering at angles in common where the solar scattering does not contaminate their
results.
13.0
Field Calibration Verification
Field calibration involving cross comparison with AERONET sun photometers (over the range
of the latter instruments) has been ongoing for ~18 months at GSFC and ~5 months at ARM
(OK). Absolute radiometric calibration of solar disk cameras has been verified with many
thousands of measurements next to AERONET sensors. Consistent aureole pixel-to-pixel
response, i.e., “gradient” correction, has been verified with hundreds to a thousand
measurements of uniform clouds with FPA response essentially unchanged over more than a
year.
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