# Presentation: 265848

## Radiometric Calibration

- Stuart F. Biggar, U of AZ
- Kurtis J. Thome, U of AZ
- Simon J. Hook, JPL

## Outline

- Preflight – Biggar
- Characterization
- Calibration
- On-board calibration systems
- Sensor artifacts
- In-flight VNIR and SWIR – Thome
- In-flight TIR - Hook

## Calibration (absolute)

- Why do we calibrate?
- Understand sensor performance
- L1 data in physical units (at sensor)
- Radiance in Watts/m2-sr-micrometer (or similar)
- Determine sensor linearity
- Derived units
- Reflectance
- Geophysical units (temperature, etc)
- Comparison to other sensors
- Atmospheric correction
- Reasonable looking imagery

## Calibration (2)

- What do we need to be able to calibrate?
- Stable instrument
- Over time
- With changing environment
- Characterized instrument
- Spectral response
- Spatial response
- Noise
- Systematic errors
- Image artifacts
- Stray light
- Stable calibration source

## Characterization

- Piece part measurements
- Filter transmittance
- Lens and/or mirror transmittance and reflectance
- Mirror reflectance and scatter (BRDF)
- Lamp output, stability, longevity
- Detector Quantum efficiency, spectral response, noise, temperature dependence

## Filter spectral characteristics

## Normalized Response

## Characterization (2)

- Assembly level
- Alignment
- Relative spectral response (of telescope)
- Noise – amplifier assembly for example
- Field-of-view
- Stray light

## Characterization (3)

- Sensor level
- Relative spectral response
- Field-of-view
- Modulation transfer function
- Noise
- Random
- Coherent
- Stray light

## ASTER design “features” (1)

- Long linear array detectors with long strip filters in VNIR and SWIR – “pushbroom”
- Good SNR due to longer integration time
- Detector and filter uniformity issues
- Many detectors (5000 in VNIR for example)
- Gain
- Offset
- Spectral response
- Large focal plane
- Post detector electronic chains (amps, A/D, etc)
- “Stripes” in images even of constant radiance scenes

## ASTER L1A,bands 1and 13

## ASTER design “features” (2)

- Short linear arrays (10 elements) with a scan mirror in the TIR sensor – “whiskbroom”
- Shorter integration time
- Larger pixels to improve the SNR
- Limited number of detectors
- Individual amplifiers
- “Stripes” in images but repeated along track

## ASTER design “features” (3)

- Three separate sensors
- 3 telescopes
- Individually pointed
- Alignment
- Rotation of images
- Focal plane distortions
- 3 manufacturers
- Different design practices
- Different measurement practices
- Different calibration methodologies and equipment

## Slide 14

## Slide 15

## Radiometric calibration

- Preflight in the laboratory
- Well characterized and controlled source
- Reasonable but not normal environment
- May (or may not) approximate normal imaging operation mode
- Scan mirror may not be operating
- Source is usually smaller angular extent than the earth
- “Uniform” source is not a normal image

## On-board calibrators

- VNIR
- Lamp based source with monitors
- SWIR
- Lamp based source with monitors
- TIR
- Blackbody that can be heated

## Philosophy

- Careful preflight calibration
- Determine absolute response so we can convert from DN to radiance
- At the same time, run the on-board systems to transfer a radiance value to them using the sensor (ASTER)
- Determine noise (random and coherent)
- Look for image artifacts
- Use preflight values after launch
- Update after launch as sensors usually change
- On-board
- Vicarious (ground reference and moon)

## Preflight source for VNIR/SWIR

- Large aperture integrating sphere
- 1 meter diameter sphere
- Round output port
- Multiple lamps
- Internal surrounding the port
- External through small ports
- High reflectance coating on the sphere wall
- Barium sulfate (BaSO4)
- Spectralon (sintered PTFE)
- Gold sandpaper (rough, diffuse surface)

## 40” BaSO4 integrating sphere

## Integrating sphere

- Advantages
- Uniform radiance across port (one % or so)
- Lambertian (radiance independent of direction)
- Multiple output levels
- Change number of lamps (MELCO)
- Change voltage/current of all lamps (voltage for NEC sphere)
- Change aperture size between external lamp and sphere
- Reasonably stable with good control of lamp (V or I)
- Repeatable if temperature is controlled
- Disadvantages
- Low output in blue relative to NIR and SWIR

## TIR preflight cal source

- Large area blackbody
- Measure the temperature precisely and accurately
- Measure (or compute) emissivity
- Compute radiance output
- Operated in a vacuum chamber with the sensor
- Vary temperature of source to get multiple radiance values
- Absolute calibration
- Linearity correction (output usually fit to something other than a gain and offset – quadratic is common)

## Calibration chain

- NRLM (Japanese equivalent of NIST) calibrated a set of fixed point blackbody simulators
- Calibration transferred to portable, variable temperature blackbody simulators
- Calibration transferred to the ASTER calibration sources (Spheres and TIR calibration blackbody)
- VNIR and SWIR spheres were measured at NEC and MELCO with a set of transfer radiometers from NIST, NRLM, GSFC, and University of Arizona
- NRLM (now AIST) and NIST collaborate to ensure that their scales are consistent

## Are preflight values usable?

- Sometimes ...
- Launch may cause a shift in a sensor performance
- Operating temperatures may be different
- Something may have moved
- On-board lamps may change output
- Convection inside the lamp may be different
- Aging of the lamp
- Emissivity of the on-board blackbody may change
- Mirror reflectance may change
- Filters/detectors/amplifiers may change
- There may have been unexpected “features”

## Unexpected “features”

- VNIR
- On-board calibrator monitor is “off-scale”
- Change in output is more than expected so dynamic range of A/D is too small
- SWIR
- Unexpected stray light causes “crosstalk”
- Present during preflight calibration and in all in-flight data

## VNIR OBC

- VNIR OBC has two monitors for each of two redundant calibrators
- One monitor diode at lamp
- One monitor diode at output
- They track but at different rates
- We really want the output monitor to determine how well the on-board calibrator is working but we have only the lamp monitor “on-scale”

## VNIR OBC output monitor

- NEC selected an expected output range based on preflight measurements, OPS experience with a similar calibrator, and desire to maximize resolution
- Output has fallen to below the lowest expected monitor output
- Telemetry value for monitor is now a flat line (signal – offset < zero)

## SWIR crosstalk

- SWIR has multiple spectral bands
- 6 linear PtSi detector arrays
- Spectral selection filters over the arrays
- Not all light hitting the detector is absorbed
- Light hitting between the detectors is reflected
- Some reflected light is reflected back down by the filters
- Optical “crosstalk”

## Slide 30

## Slide 31

## Stretched RGB of a Japanese Island, SWIR bands 4,5, & 9, 400x400 pixels

## SWIR Crosstalk

- Present in all images
- Preflight calibration
- In-flight calibration
- “normal” images
- Visible in images with strong contrast
- Not “visible” but present in others
- Currently _**NOT**_ corrected for in normal processing of L1 data

## Crosstalk correction

- If we know the amount of light leaking from one pixel to each other pixel in all the SWIR bands, we can correct for it
- Preflight data for MTF determination would probably contain much of the needed information but it was recorded only for the band under test
- Scan a line source across the array in both directions
- Scan a point (pixel size or less) in both directions
- It is possible to infer the correction from images with strong contrast (coast lines, islands, moon, and similar), however it is difficult and incomplete

## SWIR correction

- Japanese team has developed a “beta” level, Windows (Win32) based, correction program.  It does one scene at a time operating on a L1B HDF input file and writing a corrected L1B HDF file.
- US team is developing a program that is run as part of L2 processing.  It starts with L1A data.  For example, you will be able to order atmospherically-corrected, crosstalk-corrected surface reflectance at L2.

## “Corrected” image, SWIR RGB with 4,5, & 9

## SWIR Crosstalk Correction

- Qualitatively improves image
- Largest effect is band 4 into 5 and 9
- However, any band should leak into all others (including itself) with the strongest effect on adjacent bands on the focal plane
- Band 4 has higher typical radiance than the others
- Japan correction has only band 4 into others
- There is interplay between crosstalk and water vapor absorption, especially in band 9

## Conclusions (preflight)

- ASTER was calibrated preflight
- VNIR accuracy was probably within spec
- SWIR accuracy is poorer due to crosstalk
- TIR was probably within spec
- Preflight calibration is probably not appropriate at this point
- Change in sensor (VNIR and some TIR bands)
- Crosstalk in SWIR
- Calibration is being updated