Autonomous Visible to SWIR ground-based vicarious Cal/Val will be an essential Cal/Val component with such a large number of systems. Radiometrically calibrated spectroradiometers can improve confidence in current ground truth data through validation of radiometric modeling and validation or replacement of traditional sun photometer measurement. They also should enable significant reduction in deployed equipment such as equipment used in traditional sun photometer approaches. Simple, field-portable, white-light LED calibration source shows promise for visible range (420-750 nm). Prototype demonstrated <0.5% drift over 10-40 C temperature range. Additional complexity (more LEDs) will be necessary for extending spectral range into the NIR and SWIR. LED long lifetimes should produce at least several hundreds of hours or more of stability, minimizing the need for expensive calibrations and supporting long-duration field campaigns

Frisbie, Troy

Harrington, Gary

Holekamp, Kara

Pagnutti, Mary

Russell, Jeffrey

Ryan, Robert E.

Stanley, Thomas

NASA Technical Reports Server

Participation in this work by Science Systems and Applications, Inc., and by Computer Sciences Corporation was supported by 
NASA at the John C. Stennis Space Center, Mississippi, under Task Order NNS04AB54T.
Emerging Techniques for Vicarious Calibration of Visible 
Through Short Wave Infrared Remote Sensing Systems
Authors:  Robert E. Ryan,1 Gary Harrington,1 Kara Holekamp,1 Mary Pagnutti,1 Jeffrey Russell,2 Troy Frisbie,3 Thomas Stanley3
1Science Systems and Applications Inc., SSC       2Computer Sciences Corporation, SSC
3Applied Research & Technology Project Office, NASA, SSC
POC: Robert E. Ryan, rryan@ssc.nasa.gov;  Mary Pagnutti, mpagnutt@ssc.nasa.gov
Summary
• Autonomous Visible to SWIR ground-based vicarious Cal/Val will be an essential Cal/Val 
component with such a large number of systems
• Radiometrically calibrated spectroradiometers can improve confidence in current ground 
truth data
– Validation of radiometric modeling 
– Validation or replacement of traditional sun photometer measurements
– Should enable significant reduction in deployed equipment such as equipment used 
in traditional sun photometer approaches
• Simple, field-portable, white light LED calibration source shows promise for visible range 
(420–750 nm)
– Prototype demonstrated <0.5% drift over 10–40 °C temperature range
– Additional complexity (more LEDs) will be necessary for extending spectral range 
into the NIR and SWIR
– LED long lifetimes should produce at least several hundreds of hours or more of 
stability, minimizing need for expensive calibrations and supporting long-duration field 
campaigns
– Enabling technology for developing autonomous sites
CHECK AND 
REVIEW INPUT 
PARAMETERS
COLLECT 
GROUND 
TRUTH DATA
RADIOSONDE, TOMS, 
SENSOR-VIEWING & 
SOLAR GEOMETRY
(P, T, H2O, O3, θV, θS)
SUN PHOTOMETER 
(AEROSOL PROPERTIES)
SPECTRORADIOMETER, REFERENCE 
AND TARGET BRDF, BACKGROUND 
ALBEDO, SENSOR SPECTRAL 
RESPONSE
ATMOSPHERIC 
GASEOUS 
PROFILE
AEROSOL 
ASYMMETRY 
AND VISIBILITY 
PROPERTIES
TARGET AND 
BACKGROUND 
REFLECTANCE
MODTRAN 
INPUT
MODTRAN VERIFICATION
GROUND RADIANCE ESTIMATE FOR 
REFERENCE PANEL COMPARED TO 
CALIBRATED ASD RADIANCE
NO RADIANCE 
ESTIMATE AGREES 
WITH GROUND 
MEASUREMENTS?
MODTRAN 
SENSOR 
SPECTRAL 
RESPONSE
MODTRAN
AT-SENSOR 
RADIANCE 
ESTIMATION
Radiative 
Transfer Input 
Parameter 
Setup 
Software 
Suite
YES
Typical Radiometric Vicarious Calibration
Radiative Transfer Validation
• Verify parameters used to generate MODTRAN at-sensor radiance estimate
– Measure the radiance off a Spectralon® panel with a well-calibrated spectroradiometer
– Use ground truth data and geometry that models an ASD FieldSpec® FR (Full Range) 
spectroradiometer measuring a 99% reflectance Spectralon panel as input to MODTRAN to 
predict radiance
– Compare MODTRAN-calculated radiance to actual radiance measured from Spectralon 
panel to verify the atmospheric model
– After panel Bi-directional Reflectance Distribution Function (BRDF) correction and 
radiometric calibration with NIST-calibrated integrating sphere, the expected panel radiance 
measurement uncertainty is ~2%
Issues
• The scientific community needs geometrically and radiometrically accurate products from the 
present and future “constellation” of spaceborne and airborne systems
– Insight into the system construction, calibration, and performance will be limited in many 
cases
– Most systems will not have any onboard radiometric calibration
• Cal/Val (vicarious calibration) will be essential
– Multiple approaches are desirable
– Ground-based reflectance radiometric methods have the greatest utility because all
systems image the ground
• Ground-based radiometric calibrations currently require teams of trained staff taking coincident 
data at the time of overpass and analysts to estimate Top-of-the-Atmosphere (TOA) radiance
– Costly
– Significant coordination is required between the imagery provider and the calibration team
– A variety of sites is needed
• Improved TOA radiance estimates are needed
– Level of confidence in ground truth data is limited because robust instrument Cal/Val is 
lacking
– Level of confidence in radiative transfer modeling is limited because independent validation 
methods are lacking
• Robust automated systems are clearly needed to effectively calibrate and validate products 
from such a large number of systems
– Several years away
– Concerted, well-funded projects will be need to be established
Ground-based Radiometric Cal/Val Needs
• Near-term
– Increased confidence through independent validation of ground truth and modeling
– Measurement techniques that reduce or at least do not increase staff
– Simpler and more accurate calibration approaches 
• Mid-term
– Development of techniques that are compatible with autonomous measurements 
• Long-term
– Fully autonomous vicarious calibration techniques and sites 
SSC Near-Term Cal/Val Development Goals
• Improved accuracy and higher confidence in TOA radiance estimates
– Radiative transfer modeling validation
– Alternative sun photometer calibration and validation
– Low-cost, simple, in-field, NIST-traceable radiometric calibration source
Autonomous approaches will evolve from improved, traditional,
labor-intensive radiometric calibrations
Calibration and Characterization of 
ASD FieldSpec Spectroradiometers
• NASA SSC maintains four ASD FieldSpec FR spectroradiometers
– Laboratory transfer radiometers
– Ground surface reflectance for V&V field collection activities
• Radiometric Calibration
– NIST-calibrated integrating sphere serves as source with 
known spectral radiance
• Spectral Calibration
– Laser and pen lamp illumination of integrating sphere
• Environmental Testing
– Temperature stability tests performed in environmental 
chamber
Environmental Chamber
with ASD
Desired In-field Radiometric Calibration Source
• Radiance level comparable to sea-level solar radiance values off terrestrial targets over 
the solar reflective region
• Radiometric stability equal to or better than 1%
• Capable of operating over a wide temperature range (10–40 °C)
• Spatially uniform light field over at least a 25 mm diameter aperture
• Spectrally stable
• Capable of operating for a continuous period of 8 hours without a line source
• Single-person portable
2)  Spectralon panels with traditional Tungsten-
Halogen lamps (irradiance source)
1)  Integrating spheres 
(Not easily field deployable or reliable)           
Typical Laboratory Radiometric Sources
LED-based Radiance Source
LED-illuminated integrating sphere radiometric 
calibration source with photodiode stabilization
• LED-based Approach
– Exploit recent developments in high-power LED sources
– Utilize integrating sphere to create uniform light field
– Use light-stabilization control to achieve radiometric stability
– Test and characterize system with environmental chamber and independent 
spectroradiometer
400 450 500 550 600 650 700 750
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
λ (nm)
L
 
(
W
/
m
2
·
n
m
·
s
r
)
Spectral Response
White 5W LED;I = 700ma
12" Sphere; ASD Lmax
Spectralon Panel
MODTRAN ρ=.3 ρbkg = 1
White light LED source in 8” sphere produces radiance levels 
comparable to brightly illuminated scenes in the visible
• LED-based Radiance Source Characteristics
– Temperature-stabilized white light LED
• Spectral range: 420–750 nm
• Other LEDs would increase the spectral 
range
– Temperature-stabilized photodiode and 
feedback loop stabilize integrating sphere 
radiance level
– Short term lab drift <0.2%
– Short term drift <0.5% over temperature range 
10–40 °C and over large spectral range
• Comparison of LED Integrating 
Sphere with Traditional Sources
– MODTRAN calculations for 30 
degree solar zenith, mid-
latitude summer atmosphere, 
23 km visibility and rural 
aerosol
– TOA radiance levels calculated 
for 30% reflectance targets and 
1 m above Spectralon panel
– 12” sphere data for 3200 K 
Tungsten lamp 
Shadowband Sun Photometer Measurements
• Total irradiance is equal to the sum of the direct component and the 
diffuse component
• The direct component of irradiance can be written in the following terms:
– Extraterrestrial irradiance (E0)
– Atmospheric optical thickness 
(tau)
– Relative air mass (m)
• Solving for tau 
• Diffuse-to-global ratio (D2G) used to determine molecular scattering can 
be defined as:
diffusedirecttotal EEE +=
diffuse
m
total EeEE +=
− )cos(00 θτ
m
EE direct )ln()ln( 0
0
−
=τ
totaldiffuse EE
• Knowing the reflectance factor             
as a function of zenith angle and 
azimuth angle
)cos(θρ
π direct
direct
LE =
• Spectralon can be considered an
irradiance-to-radiance converter if the 
spectroradiometer is radiometrically 
calibrated
diffusetotaldirect LLL −=
ρ
Spectroradiometer
Spectralon
Panel
Diffuse-to-global measurement setup
Ground-coupled 
Radiation
Scattered Direct 
Radiation
Direct 
Beam
Removable 
Baffle
Fiber Head
Alternative Sun Photometer Implementation
• 10 narrow bands
• 10 nm bandwidth
• Direct solar irradiance
Typical Sun Photometer Measurements
• Ground-level irradiance
– Direct Normal (e.g., Arizona Solar Radiometer)
– Total, Diffuse, and Derived Normal (e.g., Multifilter Rotating Shadowband Radiometers)
• Solar atmospheric transmission 
– Optical depth
– Note: TOA radiance estimates are very sensitive to atmospheric transmission 
measurements.
• Most sun photometers support measurements in several bands to separate key molecular 
and aerosol scattering and absorption bands
• 6 narrow bands
• 10 nm bandwidth
• Total and diffuse solar 
irradiance
University of Arizona 
Automated Solar 
Radiometer (ASR) 
“Reagan”
Multifilter Rotating 
Shadowband Radiometer 
(MFRSR)
Traditional In-field Calibration of Sun Photometers
• Langley plot calibration requires many clear days with 
stationary atmospheres
– Not practical in many locations
– 5% or more errors in transmission are quite possible
• Built-in irradiance source or laboratory irradiance calibration 
could improve accuracy in areas where stationary 
atmospheres are difficult to achieve
Test Case Evaluations
Alternative Sun Photometer Summary
• Differences between the alternative and traditional sun photometer data (tau, D2G) are 
relatively small in most cases (<0.02)
– Additional analysis shows that in certain cases, the prototype may produce more 
accurate measurements than the traditional method in a Stennis-like environment for lack 
of sufficient Langley plots
– Improves confidence in all sun photometer measurements
• Utilizes existing commonly used vicarious calibration equipment
– Spectralon panel and calibrated spectroradiometer
• The need for early deployment to catch many sunrises and sunsets can be minimized
• Current configuration takes hyperspectral measurements
– Current processing uses spectral synthesis to generate bands for either MFRSR or ASR
• Spectroradiometer calibration critical to success
– High-quality, in-field calibration could be extremely beneficial
Background
• Over the next several years, more than 50
optical satellite imaging systems with 39 m or 
better resolution will be in orbit
– 13 countries presently have imaging 
satellites in orbit; 20 countries will have 
imaging satellites in orbit by 2010 
– 30 imaging systems are in orbit; 25 imaging 
systems are planned by 2010
• These figures do not include the large number of 
advanced airborne multispectral imaging 
systems
Source: Stoney, W.E., 2006. Guide to Land Imaging Satellites. February 2, p. 10. 
http://www.asprs.org/news/satellites/ASPRS_DATABASE_020206.pdf
Stennis Verification & Validation (V&V) Site
Total Sky
Imagers
Water 
Surface 
Temperature 
Sensor
Multifilter
Shadowband
Radiometer
Total
Sky
Imager
Surface
Meteorology
Stations
NASA GSFC
AERONET’Node
Network of 
Ground 
Control 
Points
GPS, Spectroradiometer, and 
Upper Atmosphere Surveys
Radiometric Tarps
Two 20 x 40 m
Edge Targets and 
130 m Radial Target
Atmospheric
Modeling
(MODTRAN)
Sandmeier Field Goniometer 
for Bi-directional Reflectance 
Measurement
Radiative Transfer Validation Example
MODTRAN Radiance vs. Average Spectralon Radiance from ASD 
Background Reflectance Selected by 
Average Region of Interest Reflectance
Background Reflectance Set Equal to Target 
Reflectance
Adjacency as well as other radiative transfer modeling is validated
TOA radiance values for selected targets on two days. Radiance values generated with alternative 
sun photometer optical depth are compared to radiance values generated with the traditional method.
0.24 %40.19840.295SWIR
-0.48 %143.70143.02NIR
0.33 %182.68183.29Red
0.36 %161.79162.37Green
DG (Sand)
-0.56 %11.31411.251SWIR
1.30 %130.32132.03NIR
-2.68 %21.55420.992Red
-1.10 %41.17940.732Green
BL (Veg.)
84 / 6166 / 64/27/05
3.56 %9.24789.5891NIR
2.90 % 16.32216.809Red
2.40 %26.64927.305Green
2.02 %38.41839.209Blue
3.5%
0.08 %55.71055.754NIR
0.13 %81.36281.465Red
0.39 %96.88397.263Green
0.69 %107.29108.04Blue
34%
-0.93 %91.68890.845NIR
-1.12 %128.65127.23Red
-1.06 %145.33143.80Green
-0.84 %153.10151.83Blue
52%
184 / 1119 / 11/10/04
Percent DifferenceNew RadianceOriginal RadianceBandsTargets
New Vis/ 
IHAZE
Original Vis/ 
IHAZEDate
Differences in TOA radiance between the two methods are negligible in most cases. 
Traditional Calibration Source:  
Tungsten-Halogen Lamps
• Advantages
– Smooth spectral curve
• Disadvantages
– Expensive calibration ($12000 at NIST)
– Low energy efficiency
– Filament non-uniformity
– Short valid calibration period (less than 
100 hours)
– Large power requirements
New Calibration Approach:  
High-Intensity LEDs
• Advantages
– Extreme long life 50–100 thousands 
of hours to 50% of initial output
– Reduced maintenance costs
– Energy efficient
– Small footprint
– Solid state (no filament to break)
• Disadvantages
– Narrow spectrum (white phosphors 
help; sometimes an advantage)
Illumination Sources
https://ntrs.nasa.gov/search.jsp?R=20070038224 2019-08-29T18:37:20+00:00Z


Emerging Techniques for Vicarious Calibration of Visible Through Short Wave Infrared Remote Sensing Systems

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