Implementation of Implementation of Vicarious Calibration Vicarious Calibration for High Spatial for High Spatial Resolution Sensors Resolution Sensors Stephen J. Schiller Stephen J. Schiller Raytheon Space and Airborne Systems El Raytheon Space and Airborne Systems El Segundo, CA Segundo, CA Collaborators: Collaborators: Dennis Helder- South Dakota State University Dennis Helder- South Dakota State University Mary Pagnutti and Robert Ryan - Lockheed Martin Space Operations, Mary Pagnutti and Robert Ryan - Lockheed Martin Space Operations, Stennis Space Center Stennis Space Center Vicki Zanoni – NASA Earth Scinece Applications Directorate, Vicki Zanoni – NASA Earth Scinece Applications Directorate, Stennis Space Center Stennis Space Center
Transcript
Implementation of Vicarious Implementation of Vicarious Calibration for High Spatial Calibration for High Spatial
Resolution SensorsResolution SensorsStephen J. SchillerStephen J. Schiller
Raytheon Space and Airborne Systems El Segundo, CARaytheon Space and Airborne Systems El Segundo, CA
Collaborators:Collaborators:Dennis Helder- South Dakota State University Dennis Helder- South Dakota State University Mary Pagnutti and Robert Ryan - Lockheed Martin Space Operations, Stennis Space Mary Pagnutti and Robert Ryan - Lockheed Martin Space Operations, Stennis Space CenterCenterVicki Zanoni – NASA Earth Scinece Applications Directorate, Stennis Space CenterVicki Zanoni – NASA Earth Scinece Applications Directorate, Stennis Space Center
OverviewOverview• Calibration Considerations for Absolute Radiometry• Vicarious Calibration and its application to High Spatial
Resolution Sensors• Design of Ground Targets and Ground Truth
Measurements• Top-of-Atmosphere Radiance Estimates Using
MODTRAN and Considering:– BRDF Effects– Adjacency Effect – Aerosol Modeling– Evaluating Model Radiance Accuracy
• Absolute Calibration establishes the link to physical parameters and processes recorded in the remote sensing image.
• Multiple paths to SI units are necessary to evaluate systematic errors in calibration coefficients.
• Vicarious calibration provides a known at-sensor radiance independent of on-board calibration sources
• Goal of this presentation is to outline the process for not just obtaining a gain estimate at a single radiance level but to generate a Vicarious Calibration Curve over the operational dynamic range of the sensor
Reflectance Based Vicarious Reflectance Based Vicarious Calibration MethodologyCalibration Methodology
• Measure surface/atmospheric optical properties at the site containing one or more uniform targets
• Constrain input parameters in a radiative transfer model (MODTRAN 4) to match surface and atmospheric conditions at the time of the sensor overpass
• Predict the top-of-atmosphere spectral radiance for the ground target (hyperspectral resolution)
• Extract target signal from sensor data for each band• Integrate the at-sensor radiance spectrum with the
sensor’s relative spectral response for each band • Calculate the gain and bias for each band• Method provides an absolute calibration established
relative to the solar spectral constant
Ground-Based Vicarious Calibration of Sensor Gain Ground-Based Vicarious Calibration of Sensor Gain and Biasand Bias
Wavelength (nm)
Re
flect
an
ce
500 1000 1500 2000 2500
0.0
0.1
0.2
0.3
0.4
0.5
Average Surface Reflectance Spectrum of Grass Target Recorded between 11:49 and 12:10, June 30, 2000
Average of 108 Spectra1 Std. Dev. Limits of Reflectance Variability
Air mass
ln I
rra
dia
nce
(D
ete
cto
r D
N)
1 2 3 4
10
.01
0.2
10
.41
0.6
June 30, 2000 Evening Langley Plot at 521 nm Linear Fit is Between 2 and 4 Air Mass
Extinction Equation ln I = 10.8376 - 0.2115(Air mass)
S = (dS/dL) L +B Sensor Gain and Bias
Solar Spectral Constant
Radiative Transfer Calculation of At-Sensor Radiance (L) Sensor Signal (S) of Ground Targets
Measure Target
Reflectance
Monitor Atmospheric Transmittance,
Diffuse/Global Ratio
Traditional Approach to Vicarious Traditional Approach to Vicarious Calibration of Remote Sensing Systems Calibration of Remote Sensing Systems
June 10, 2000 Blue Band
Typical approach has been to characterize a large bright uniform target at a desert site to provide a known top-of-atmosphere radiance level.
Provides a gain value based on a single radiance level
Uncertainty is estimated to be ~ +/- 3% (RSS estimate of measurement and modeling errors)
IKONOS Image of Lunar Lake, Nevada
(Developed for Large Footprint Sensors Requiring Natural Targets)
White Sands
Railroad Valley Playa
Lunar Lake Playa
Vegetation Cover (Brookings)
Improvement is to Generate a Calibration Curve Over the Sensor’s Dynamic Range using Multiple Sites – IKONOS
Deep Dense Vegetation or Water Bodies (~zero reflectance)
Six Deployments
Calibration Curve Generation From A Single Field CampaignCalibration Curve Generation From A Single Field Campaign(Using Man-made Targets)
Does the tight linear fit imply a better gain estimate?
Does the data resolve detector non-linearity?
No! Not Yet.No! Not Yet. More Data shows There are Systematic Variations in Gain Estimates More Data shows There are Systematic Variations in Gain Estimates
Between Sites and DatesBetween Sites and Dates
However, we now be seeing differences due to:
• Stray light,
•Out of band leakage,
•Temperature variations of focal plane and readout electronics,
•Limitations of ground truth data and atmospheric modeling.
Enhanced application applied to Enhanced application applied to high spatial resolution sensorshigh spatial resolution sensors
• Generate a Vicarious Calibration Curve covering the sensor dynamic range in a single image
• Evaluates both gain and bias.• Potential to evaluate non-linear responsivity.• Potential to reduce cost compared to multiple
campaigns
Enhanced Ground Target DesignEnhanced Ground Target Design• Lay out six to eight targets covering ~
0% to 85% reflectance
• Targets include - Spectrally flat (gray toned targets for
calibration curve generation)
- Strong spectral contrast (evaluate effects of spectral banding)
- Sample of surround spectrum (location where image DN for each band is near the average of the entire image)
• Reflectance of each target is measured at the site close to the time of the sensor overpass
• Use a site that is similar to image sites collected in operational use. (reproduce scattered light and out-of-band leakage effects)
- Ocean/coastal, vegetation, desert
Vicarious Calibration Curve Vicarious Calibration Curve Generation for Push Broom Sensors 1Generation for Push Broom Sensors 1
• Assumes a flat field image has been acquired for relative calibration of all detector channels on the focal plane (i.e. cloud, ice or desert scenes , side slither image)
• Relative gain for each channel is derived from its response in terms of the average response of all the channels
plane focal
flatbandchan,
flatbandchan,rel
bandchan,DN
DNg
Uniform cloud or ground scene
Detector Array
Side slither image
•Next, apply the relative gain to the vicarious calibration image
•Raw signal (DNraw ) of calibration targets are converted to relative signal (DNrel) and average over the target area
•Weighted least-squares regression of TOA Radiance, , vs relative signal gives absolute gain, with respect to average responsivity of focal plane,
This relation defines the vicarious calibration curve
•Absolute gain of each channel, Gchan,band,,is given by
Achieving Accurate Top of Achieving Accurate Top of Atmosphere Radiance Estimates 1Atmosphere Radiance Estimates 1• Radiative transfer model (MODTRAN) must account for
Achieving Accurate Top Of Achieving Accurate Top Of Atmosphere Radiance Estimates 2Atmosphere Radiance Estimates 2
• Requires extensive set of field data obtained with well calibrated radiometers and reference panels.– BRDF (Bi-directional Reflectance Distribution
Function) of calibration panels and targets– Atmospheric transmittance, upwelling radiance,
diffuse/global ratio, almucantor scans of sky path radiance (if possible - hyperspectral resolution)
– Verticle profiles of water vapor and aerosols (altitude of boundary layer)
– radiosonde / lidar / aircraft based measurements
Achieving Accurate Top Of Achieving Accurate Top Of Atmosphere Radiance Estimates 3Atmosphere Radiance Estimates 3
• Requires MODTRAN parameters to be established via user supplied inputs (using a default atmosphere or surface reflectance is not adequate)– Target and surround reflectance spectrum
(hyperspectral resolution, user supplied BRDF)– Wavelength characterized aerosol extinction known
below and above the boundary layer (user supplied from sun photometry)
– Surface Range in the boundary layer (adjusted to reproduce observed transmittance)
– Aerosol scattering phase function ( adjust H-G asymmetry factor or input user-supplied)
Comments on MODTRAN Model Comments on MODTRAN Model CharacterizationCharacterization
• BRDF Considerations
• Adjacency Effect
• Aerosol Vertical Profile
BRDF Knowledge of calibration panel BRDF Knowledge of calibration panel and ground targets is essentialand ground targets is essential
Wavelength (nm)
Re
flect
an
ce
500 1000 1500 2000 2500
0.8
50
.90
0.9
51
.00
1.0
5
SDSU Spectralon Panel Absolute Reflectance Calibration Year 2000 Season
SolarAngle1015202530354045505560657075
Solid line represents the Labsphere hemispherical diffuse calibration
VNIR values are modified to remove systematic shift between VNIR and SWIR reflectance
Wavelength (nm)
Re
flect
an
ce
500 1000 1500 2000 25000
.00
.10
.20
.30
.40
.5
Changes In Average Surface Reflectance Spectrum of Grass Target Between 11:12 and 12:00, June 30, 2000
Average Reflectance Spectrum Recorded Between 11:00 and 11:25Average Reflectance Spectrum Recorded Betwen 11:49 and 12:10
BRDF effects are reduced with higher diffuse-to-global ratio
Multi-angle images should be collected to verify atmospheric and BRDF model
Θz=7o
Θz=19o
Comments on MODTRAN Model Comments on MODTRAN Model CharacterizationCharacterization
• BRDF Considerations
• Adjacency Effect
• Aerosol Vertical Profile
Rad
ianc
e ASD RadianceAverage ASD RadianceMODTRAN Radiance
0.4 0.5 0.6 0.7 0.8 0.9 1 1.1 1.20
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
Wavelength (m)
Radiance for Spectralon Panel
Measuring Atmospheric Parameters To Characterize The Adjacency Effect Is Critical
TargetTarget
Surround
Direct
Solar
Adjac
ency
EffectM
ultip
le Sca
tterin
g
TargetTarget
Surround
Direct
Solar
Adjac
ency
EffectM
ultip
le Sca
tterin
g
MODTRAN
Rad
ianc
e
0.4 0.5 0.60.6
70. 0.8 0.9 1 1.1 1.20
0.005
0.01
0.015
0.02
0.025
0.03
0.035
0.04
0.045
0.05
Wavelength (m)
Radiance of Spectralon PanelModeling adjacency effect is required to reproduce measured upwelling radiance off ground targets
Target spectrum= surround spectrum
Grass spectrum used for surround
Surround Spectrum’s Influence On Surround Spectrum’s Influence On Sky Path RadianceSky Path Radiance
Red edge of vegetation observed in the downwelling sky path radiance
Comments on MODTRAN Model Comments on MODTRAN Model CharacterizationCharacterization
• BRDF Considerations
• Adjacency Effect
• Aerosol Vertical Profile
Aircraft Measurements Of Extinction At The Aircraft Measurements Of Extinction At The Boundary Layer Improve Aerosol Model Boundary Layer Improve Aerosol Model
Wavelength (nm))
Tra
nsm
itta
nce
400 600 800 1000
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Vertical Transmittance Above the Boundary Layer Sept. 14, 2000 Comparison Between Reagan Measurements and Modtran Model
Vertical Transmittance Measured Using Reagan Sunphotometer From An Aircraft at 3200 m Modtran Model Vertical Transmittance Using Default 1976 Standard Atmosphere Aerosol Profile From 3200 m
Wavelength (nm))
Tra
nsm
itta
nce
400 600 800 1000
0.4
0.5
0.6
0.7
0.8
0.9
1.0
Vertical Transmittance Above the Boundary Layer Sept. 14, 2000 Fit of Adjusted Modtran to Reagan Measurements
Vertical Transmittance Measured Using Reagan Sunphotometer From An Aircraft at 3200 m Modtran Model Vertical Transmittance Using a Scaled 1976 Standard Atmosphere Aerosol Profile From 3200 m
Solar radiometer observations at the top of the boundary layer (altitude defined in the MODTRAN model) revealed a significantly higher transmittance than available with MODTRAN model atmospheres. The 1976 standard atmosphere was scaled to fit the observations.
Aerosol vertical profile plays a significant role in modeling the adjacency effect and extinction as a function of wavelength (composition varies with height).
Solar radiometer observations at the top of the boundary layer (altitude defined in the MODTRAN model) revealed a significantly higher transmittance than available with MODTRAN model atmospheres. The 1976 standard atmosphere was scaled to fit the observations.
Aerosol vertical profile plays a significant role in modeling the adjacency effect and extinction as a function of wavelength (composition varies with height).
Analysis Designed To Uses Multiple Paths to Analysis Designed To Uses Multiple Paths to SI Units for Accuracy AssessmentSI Units for Accuracy Assessment
• MODTRAN parameterization achieved with input of unitless quantities ties TOA radiance only to solar spectral constant– Transmittance– Reflectance– Diffuse/global ration– Assymetry factor
• Ground truth validation data from calibrated radiometers is traceable to NIST standards– Upwelling radiance at surface – Sky path radiance
• Direct comparison of MODTRAN predicted and measured upwelling radiance and sky path radiance evaluates systematic errors
Comparison of MODTRAN and Measured Comparison of MODTRAN and Measured Upwelling Radiance: Grass TargetUpwelling Radiance: Grass Target
Comparison of MODTRAN and Comparison of MODTRAN and Measured Sky Path RadianceMeasured Sky Path Radiance
TOA Error Propagation ModelTOA Error Propagation Model• Apply error propation analysis to the following
radiative transfer equation from ground to sensor.
• is the upwelling target radiance at ground level• is the transmittance along the path between the target and
the sensor• is the sky path radiance contribution as seen from the sensor
when viewing the target (the signal produced if looking at a surface of zero reflectance)
• Each component is directly related to calibrated ground measurements of which their uncertainty is known based on the measurement errors of the spectroradiometer and sunphotometer
TOApsen
upt
TOAs LTLL
uptL
senT
TOApL
Error Propagation Equation: Deriving the Error Propagation Equation: Deriving the Uncertainty in the TOA RadianceUncertainty in the TOA Radiance
sun
senm
m
ModsenT ,ModsunT .
2/1
2
,
2
,
2,
2
,
2
.
,,
2
,,
)(
TOAModpground
measpModsun
meassun
Modsun
Modsen
sun
senupModtup
meastModSens
TOAs
LBLTAT
T
Tm
mLLT
L
meassunT ,
upmeastL ,
groundmeaspL ,
ModsunTA ,
TOAModpLB ,
• Ratio of air mass from ground to sun and sensor
•MODTRAN calculated transmittance to sun and sensor
•MODTRAN calculated upwelling radiance at the ground
•Measurement uncertainty in transmittance from ground to sun
•Measurement uncertainty in upwelling radiance from target
•Measurement uncertainty in in sky path radiance from ground observation
•Uncertainty in estimating aerosol extinction at the MODTRAN input wavelengths from solar radiometry. A is a fraction of the total transmittance.
• uncertainty in TOA sky path radiance using the H-G scattering phase function characterized with ground measurements. B is a fraction of the TOA path radiance,
upModtL ,
Described in “Technique for estimating uncertainties in top-of-Atmosphere
ConclusionConclusion• Progress made in vicarious claibration techniques for high spatial
resolution sensors.– Natural targets to grey-toned deployed targets– Single radiance levels at different sites & dates to multiple levels evaluated
in a single campaign event.• Goal is to generate a vicarious calibration curve over the operational
dynamic range of EO sensors (Vis to SWIR)• Atmospheric model (i.e. MODTRAN) must be characterized using “user
supplied” parametersGround truth must address: – BRDF properties of targets– Adjacency effect (knowledge of surround spectrum)– Aerosol vertical profile– Radiometric accuracy knowledge of ground truth data for TOA radiance
uncertainty estimates• Working toward <3% absolute accuracy from environments consistent
