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

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”

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