Implementation of Vicarious Calibration for High Spatial Resolution Sensors

1
Implementation of Vicarious Calibration for High
Spatial Resolution Sensors

• Stephen J. Schiller
• Raytheon Space and Airborne Systems El Segundo,
  CA
• Collaborators
• Dennis Helder- South Dakota State University
• Mary Pagnutti and Robert Ryan - Lockheed Martin
  Space Operations, Stennis Space Center
• Vicki Zanoni NASA Earth Scinece Applications
  Directorate, Stennis Space Center

2
Overview

• 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
• Error Propagation Model

3
Sensor Absolute Calibration

• 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

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Reflectance Based Vicarious Calibration
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 sensors 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

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Ground-Based Vicarious Calibration of Sensor Gain
and Bias
Measure Target Reflectance
Solar Spectral Constant
Radiative Transfer Calculation of At-Sensor
Radiance (L)
Sensor Signal (S) of Ground Targets
Monitor Atmospheric Transmittance,
Diffuse/Global Ratio
S (dS/dL) L B Sensor Gain and Bias
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Traditional Approach to Vicarious Calibration of
Remote Sensing Systems
(Developed for Large Footprint Sensors Requiring
Natural Targets)
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
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Improvement is to Generate a Calibration Curve
Over the Sensors Dynamic Range using Multiple
Sites IKONOS
White Sands
Six Deployments
Lunar Lake Playa
Railroad Valley Playa
Vegetation Cover (Brookings)
Deep Dense Vegetation or Water Bodies (zero
reflectance)
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Calibration 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?
9
No! Not Yet. More Data shows There are
Systematic Variations in Gain Estimates Between
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.

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Enhanced application applied to high spatial
resolution sensors

• Generate a Vicarious Calibration Curve covering
  the sensor dynamic range in a single image
• Same atmospheric effects, scattered light levels,
  adjacency effect, sensor responsivity conditions
• Evaluates both gain and bias.
• Potential to evaluate non-linear responsivity.
• Potential to reduce cost compared to multiple
  campaigns

11
Enhanced 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

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DN Histogram Of A Pre-Campaign Image Of The
Target Area Provides Data To Identify A Surround
Spectral Sample Location
Histogram of 11 km by 11 km IKONOS image of
Brookings
Arrow indicates DN value of the surround target
area
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Vicarious Calibration Curve Generation for Push
Broom Sensors 1
Detector Array

• 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

Uniform cloud or ground scene
Side slither image
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Vicarious Calibration Curve Generation for Push
Broom Sensors 2

• 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

TOA Radiance (Watts/m2-ster)
Slope
abs
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Achieving Accurate Top of Atmosphere Radiance
Estimates 1

• Radiative transfer model (MODTRAN) must account
  for all major atmospheric effects

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Achieving Accurate Top Of Atmosphere 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

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Achieving Accurate Top Of Atmosphere 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)

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Comments on MODTRAN Model Characterization

• BRDF Considerations
• Adjacency Effect
• Aerosol Vertical Profile

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BRDF Knowledge of calibration panel and ground
targets is essential
BRDF effects are reduced with higher
diffuse-to-global ratio
20
Multi-angle images should be collected to verify
atmospheric and BRDF model
Tz7o
Tz19o
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Comments on MODTRAN Model Characterization

• BRDF Considerations
• Adjacency Effect
• Aerosol Vertical Profile

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Measuring Atmospheric Parameters To Characterize
The Adjacency Effect Is Critical
MODTRAN
Target spectrum surround spectrum
Modeling adjacency effect is required to
reproduce measured upwelling radiance off ground
targets
Grass spectrum used for surround
23
Surround Spectrums Influence On Sky Path Radiance
Red edge of vegetation observed in the
downwelling sky path radiance
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Comments on MODTRAN Model Characterization

• BRDF Considerations
• Adjacency Effect
• Aerosol Vertical Profile

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Aircraft Measurements Of Extinction At The
Boundary Layer Improve Aerosol Model
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).
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Analysis Designed To Uses Multiple Paths to SI
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

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Comparison of MODTRAN and Measured Upwelling
Radiance Grass Target
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Comparison of MODTRAN and Measured Sky Path
Radiance
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TOA 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

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Error Propagation Equation Deriving the
Uncertainty in the TOA Radiance

• 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,

Described in Technique for estimating
uncertainties in top-of-Atmosphere radiances
derived by vicarious calibration, S.J. Schiller,
SPIE vol. 5151, 2003
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Conclusion

• 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 parameters
• Ground 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 lt3 absolute accuracy from
  environments consistent with operational use